Phosphorus leaching from non-acidic and/or non-sandy

soils: evaluating the P saturation protocol

Promoter: Prof. Dr. ir. Stefaan De Neve (LA12)

Tutor: ir. Sara De Bolle (LA12)

Tutor(s) :

«Promotor_1»

«Promotor_2»

Master dissertation submitted in partial fulfillment of the requirements for the degree of Master of Science in Physical Land Resources

By Mechelle B. Ranises (Philippines)

Academic Year 2012-2013

INTERUNIVERSITY PROGRAMME

IN

PHYSICAL LAND RESOURCES

Ghent University Vrije Universiteit Brussel

Belgium

i

This is an unpublished M.Sc dissertation and is not prepared for further distribution.

The author and the promoter give the permission to use this Master dissertation for

consultation and to copy parts of it for personal use. Every other use is subject to the

copyright laws; more specifically the source must be extensively specified when

using results from this Master dissertation.

Gent,

The Promoter, The Author,

Prof. Dr. ir. Stefaan De Neve (LA12) Mechelle B. Ranises

ii

ACKNOWLEDGEMENTS

My deepest and heartfelt appreciation to those people who in one way or

another helped me in making this piece of work possible. Firstly, I would like to

take this opportunity to thank the Flemish Interuniversity Council (VLIR-UOS),

Flemish Government for granting me the scholarship to undertake a master

degree at Gent University.

Secondly, I would like to express my profound gratitude to my promoter

Prof. Dr. ir. Stefaan De Neve for his exemplary and continued guidance and

support throughout the course of my thesis work. This piece of work would not

have been possible without his constant follow ups, valuable suggestions and

expertise.

Thirdly, I would also like to thank my supervisor Ir. Sara De Bolle for her

constructive comments and guidance that led me in completing this task through

various stages. Likewise, Engr. Acita Kargosha for her practical contribution in

setting up the whole experiment. In addition, the technicians in the soil fertility

laboratory namely; Luc, Marteen, Mathew, Anime, Trees, Sophie,Tina, Mieke and

Hield also deserved a similar merit for making my lab life more enriching.

Fourthly, I am obliged to my family and love ones, and PLR 2011-2013

batch mates for being there through ups and downs, constant encouragement,

unconditional love and support, and prayers. There is no way my MS life would

have gone as meaningful and fun as it did without you guys – Thank you very

much.

Ultimately, I gratefully acknowledge our Heavenly Father for His bountiful

blessings. I praise you and I thank you my Saviour and Creator. To God be the

Glory.

iii

Table of Contents

ACKNOWLEDGEMENTS ...................................................................................................... ii

Table of Contents .................................................................................................................. iii

List of Figures ......................................................................................................................... v

List of Tables ........................................................................................................................ vii

List of Abbreviations ............................................................................................................ viii

SUMMARY ............................................................................................................................ ix

CHAPTER 1. INTRODUCTION ............................................................................................. 1

CHAPTER 2. LITERATURE REVIEW ................................................................................... 3

2.1. P-Cycle in Soil ......................................................................................................... 3

2.2. Effect of soil pH on the formation of different P compounds .................................... 4

2.3. P Reactions in Soils ................................................................................................ 6

2.3.1. Dissolution and Precipitation of P ......................................................................... 6

2.3.2. Sorption and Desorption ....................................................................................... 7

2.3.3. Mineralization and Immobilization ......................................................................... 8

2.4. Effect of soil type on P leaching............................................................................... 9

2.5. P Leaching from acidic and non-acidic soils ...........................................................10

2.6. P Saturation in Flanders .........................................................................................13

CHAPTER 3. MATERIALS AND METHODS ....................................................................16

3.1. Site Selection, Soil Sampling and Pretreatments .......................................................16

3.1.1 Site Selection .......................................................................................................16

3.1.2. Soil Sampling and Preparation ............................................................................17

3.2. Physical and Chemical Characterization of the Soils ..................................................17

3.2.1. Total P and Available P .......................................................................................18

3.2.2. Phosphorus Saturation Degree ............................................................................18

3.3. Phosphorus Leaching Experiment..............................................................................19

3.3.1. Leaching Column Preparation and experimental Set-up ......................................19

3.3.2. Leaching Experiment ...........................................................................................22

3.3.3. P Analysis in Leachates .......................................................................................22

3.5 Statistical Analysis ......................................................................................................23

CHAPTER 4. RESULTS ......................................................................................................24

4.1 General Soil Properties ..........................................................................................24

4.2 P status of the soils used in the study.....................................................................25

4.3 P leaching from non-acidic and/or non-sandy soils .................................................28

iv

4.4 Relationship between PSD in soil and P concentration in leachate: an evaluation of

P saturation protocol in non-acidic and/or non-sandy soil .................................................36

4.5 Correlation and relationship between soil parameters and various P forms in the

leachate ............................................................................................................................39

CHAPTER 5. DISCUSSION .................................................................................................42

5.1 General Soil Properties ..........................................................................................42

5.2 P status of the soils used in the study.....................................................................42

5.3 P leaching from non-acidic and/or non-sandy soils .................................................45

5.4 Relationship between PSD in soil and P concentration in leachate: an evaluation of

P saturation protocol in non-acidic and/or non-sandy soil .................................................48

5.5 Correlation and relationship between soil parameters and various P forms in the

leachate ............................................................................................................................49

CHAPTER 6. CONCLUSION AND RECOMMENDATION ...................................................51

REFERENCES .....................................................................................................................52

v

List of Figures

Figure 1. Phosphorus cycle in soils (Adapted from Pierzynski, 1994) .................................... 3

Figure 2. The concept of P retention by CaCO3 (Schoumans and Lepelaar, 1995). ............... 5

Figure 3. Soil pH and the solubility of some phosphate minerals (Adapted from Lindsay,

1979). The concentration of H2PO4- and HPO4

2- is given in moles expressed as log

function. ................................................................................................................. 7

Figure 4. Relationship between PSD and orthophosphate concentration in the soil solution

from light textured acidic sandy soil (Van der Zee, 1990)......................................14

Figure 5. Block (500 m) estimation of PSD for Soils in Flanders (Adapted from Van

Meirvenne et al. 2008). .........................................................................................14

Figure 6. Map showing the various sampling sites (red point star) and reference site (blue

point star) from 3 provinces in Flanders. ...............................................................16

Figure 7. Schematic overview of the column leaching experiment. .......................................19

Figure 8. The actual experimental set-up..............................................................................21

Figure 9. Total P, P extracted by ammonium oxalate (POX) and available P (PLAC) (mg kg-1

soil) from the upper 30 cm of the soil profile in the selected field and reference soil

respectively. .........................................................................................................25

Figure 10. Ammonium-oxalate extractable Al, Fe and P (Pox) concentrations (mmol kg -1 soil,

0-30 cm) in selected soils used in the study..........................................................26

Figure 11. Phosphorus sorption capacity (PSC) in the upper 0-30 cm of the soil from 13

sampling sites and reference soil. .........................................................................27

Figure 12. Phosphorus saturation degree (PSD) in the upper 0-30 cm of the soil from 13

sampling sites and reference soil. ......................................................................27

Figure 13. Cumulative amounts (mg P/soil column) of different P form (a. TP b. TDP) in

leachates after 55 d leaching period. ..................................................................29

Figure 14. Cumulative amounts (mg P/soil column) of different P form (a. TRP b. DRP) in

leachates after 55 d leaching period. ..................................................................30

Figure 15. Leaching potential of various P forms (mg P mm-1) plotted against leaching

events. ...............................................................................................................32

Figure 16. Leaching potential of various P forms (mg P mm-1) plotted against leaching

events. ...............................................................................................................33

Figure 17. Average volume-weighted concentration of total P (TP), total dissolved P (TDP)

and total particulate P (TPP) during 11 leaching events. Bars reflect the standard

error of the mean. ...............................................................................................34

Figure 18. Average volume-weighted concentration of total reactive P (TRP), dissolved

reactive P (DRP) and particulate reactive P (PRP) during 11 leaching events.

Bars reflect the standard error of the mean. .......................................................35

vi

Figure 19. Relationship between total P (TP) in leachates (mg L-1) and PSD (%) from the

various soils. ......................................................................................................36

Figure 20. Relationship between total dissolved P (TDP) in leachates (mg L-1) and PSD (%)

from the various soils. ........................................................................................37

Figure 21. Relationship between total reactive P (TRP) in leachates (mg L-1) and PSD (%)

from non-acidic and/or non-sandy soil. Black curve indicates the relationship

between ortho-P and PSD in acidic sandy soil used in the protocol. ...................37

Figure 22. Relationship between dissolved reactive P (DRP) in leachates (mg L-1) and PSD

(%) from non-acidic and/or non-sandy soil. Black curve indicates the relationship

of ortho-P and PSD for acidic sandy soil in used in the protocol. ........................38

vii

List of Tables

Table 1. The different P forms in water (Hens and Schoumans, 2002). ................................11

Table 2. Main soil characteristic of the selected sites. ..........................................................24

Table 3. Correlation matrix (r-values) of various soil parameters. .........................................39

Table 4. Pearson´s correlation coefficient between various P forms in the leachates and

selected soil parameters. ........................................................................................40

Table 5. Relationship between selected soil physicochemical properties and average

volume-weighted concentrations of the various P forms in the leachate. ................41

viii

List of Abbreviations

AlOX Oxalate extractable Aluminium

Al-P AlPO4

Ca-P Calcium phosphates

CaHPO4 Dicalcium phosphate

Ca(H2PO4)2 Calcium dihydrogen phosphate

Ca3(PO4)2 Tricalcium phosphate

CaCO3 Calcium carbonate

DOP Dissolved organic phosphorus

DRP Dissolved reactive phosphorus

Feox Oxalate extractable iron

Fe-P FePO4

HClO4 Perchloric acid

H2PO4- Dihydrogen phosphate

HPO42- Phosphoric acid

H2SO4 Sulfuric acid

ICP inductively coupled plasma

KCl Potassium Chloride

K2Cr2O7 Potassium dichromate

OC organic carbon

PE polyethylene tubes

Pi Inorganic phosphorus

PLAC Ammonium lactate extractable phosphorus

PO4- Phosphate

Pox Oxalate extractable phosphorus

PSC Phosphate sorption capacity

PSD Phosphate saturation degree

PRP Particulate reactive phosphorus

PSC Phosphorus sorption capacity

PSD Phosphorus saturation degree

PVC polyvinyl chloride

RP Reactive phosphorus

SPSS Statistical Package for Social Sciences

TDP Total dissolved phosphorus

TC Total carbon

TIC Total inorganic carbon (carbonates)

TOC Total organic carbon

TP Total phosphorus

TPP Total particulate phosphorus

TRP Total reactive phosphorus

USDA United State Department of Agriculture

ix

SUMMARY

The problems related to excess nutrients in soil have been attributed to long term

overfertilization that eventually resulted to tremendous build-up of P in the soil. Enrichment of

P leads to a high P saturation degree which increases the risk of P losses in the soil profile to

ground and surface waters. In this study, Flanders was chosen as a study site due to its

known P leaching problems and intensive agricultural practices with high application rates of

animal manure predominate. The potential for P leaching from these soils specifically from

non-acidic and/or non-sandy soil remains in question and up to date no known research has

been done so far particularly in this area. Thus, this study was conducted to investigate the P

leaching potential from intensively managed cropland in non-acidic and/or non-sandy soils

and to evaluate the validity P saturation protocol.

There were 13 sites selected from different agricultural fields in 3 provinces in the Flemish

region which are known for high P status and had non-acidic and/or non-sandy soil property.

A reference soil was also taken as representative for acidic sandy soils with large P content

where the P saturation protocol is widely used. The leaching experiment was done in the

laboratory using leaching columns in 3 replicates with a total of 42 leaching columns. The soil

was irrigated with demineralized water every 3 days and the leachates were collected every

5 days. A total of 11 batches of leachates were collected, and the leaching experiment lasted

for 10 wks. The leachate was analyzed for various P forms such as TP, TDP, TRP and DRP.

The orthophosphate concentration was plotted against PSD to evaluate the P saturation

protocol. Correlation and multiple regression analyses were performed to determine the

effect of the selected soil properties in the concentration of the various forms of P in the

leachates.

The current P status (total P, PLAC and POX) are very high reflecting the high intensity of

agricultural production in Flanders. The sorption sites in the soils are largely dominated by

extractable FeOX which had 50% higher than the AlOX concentration. The PSD values in the

soils ranged from 30 to 95% owing it to the very high total P content of the soil. The dissimilar

pattern and concentration of cumulative P leached in these soils were affected by the

variability in their physicochemical properties. The highest and the increasing pattern of P

released at various forms towards the end of the leaching event were probably affected by

the long period of partly saturation of the soil leading to temporary anaerobicity soil condition.

The average volume-weighted concentration of orthophosphate loss was dominated by high

proportion of inorganic P constituted to 72-99% of total P.

x

There was a positive non-linear relationship between orthophosphate concentration and PSD

in non-acidic and/or non-sandy soils. A shift from the fitted curve adopted from the existing

protocol was observed in the soil used in this study which demonstrates that the P saturation

protocol was incomparable with non-acidic and/or non-sandy soil. Result from multiple

regression analysis revealed no interaction effect within the soil properties in relation to

various forms of P in the leachate which further indicate that many of such soil properties are

statistically independent of each other. Al+Fe (OX) concentration and PSC had a significant

negative correlation to TRP and DRP indicating that the increased amorphous Al and Fe in

soils enhanced the capacity of P fixation and somehow limiting the downward transport of P

in the soil via leaching. Lastly, it is important to consider that the P saturation protocol is valid

for the entire soil profile with large number of soils whereas, this study focused only on the

potentiality of the surface soil. Nevertheless, this finding can provide valuable information

when trying to investigate the validity of the P saturation protocol in non-acidic and/or non-

sandy soil in a field scale bases with the needed soil profile.

Introduction

1

CHAPTER 1. INTRODUCTION

In many developed countries, issues concerning environmental quality have been studied

and monitored intensively to insure sustainability and to reduce risks on human and animal

health problems. The problems related to excess nutrients in soil have been attributed to

long term overfertilization in both organic and inorganic forms. Frequent and continued

overapplication of fertilizers over the years increased the risk for nutrient losses that

accelerate eutrophication process and resulted in deteriorating water quality (Serrano et al.,

2011 and Djodjic et al., 2004). Eutrophication is thought to be caused primarily by surplus of

nitrogen (N), however, phosphorus (P) has also been found to be one of the essential

contributing factors (Schindler, 1977). In fact, P causes one of the major water

contaminations in most part of the lowland areas of Europe (European Environment Agency,

1999).

In Belgium and The Netherlands, more than 60% of the agricultural lands have high P status

(Chardon and Schoumans, 2007). The intensive agricultural activities involving application of

animal manure and inorganic fertilizers have resulted in accumulation of P in soil particularly

in Flanders (De Smet et al., 1996). Enrichment of P in such areas leads to a high P

saturation degree (PSD) which may contribute to P losses in the soil profile to shallow

ground waters and surface waters (Sims et al., 1998; Koopmans et al., 2007). In West-

Flanders for example, a study conducted in the acidic sandy soils showed that phosphate

sorption capacity is limited. The P status of the soil ranged from high to very high and more

than half of the soil profiles examined are phosphate saturated based on the critical

phosphate saturation degree of 40% (De Smet et al. 1996).

In most of the agricultural areas in Flanders, the topography is nearly flat wherein P losses

from erosion is unlikely to occur and the dominant transport pathway is much more related to

leaching (Van Den Bossche et al., 2005). In the said protocol developed by Van der Zee et

al., (1990), its applicability is more specific to non-calcareous sandy soils and thus the use of

the (PSD) is restricted to these soils only. However in some areas in Flanders, intensive

agricultural practices have been recorded as such having non-acidic and/or non-sandy soil

property. The potential for P leaching from these soils remains in question and up to date no

known research has been done particularly in Flanders. Thus in this study, more emphasis

will be given to leaching pathways of P from intensively managed cropland of a non-acidic

and/or non-sandy soils as an evaluation for P saturation protocol.

Introduction

2

Essentially, investigation of the potential risks for phosphate leaching from intensively-

managed cropland in Flanders is crucial for environmental safety. The result of the study will

provide useful information for current soil P status for crop production, and provide a better

understanding of P behavior in this soil in relation to environmental issues. In this study,

Flanders will be chosen as a study site due to its known P leaching problems and since

intensive agricultural practices with high application rates of animal manure predominate.

This also implies that phosphate leaching may occur in non-acidic and/or non-sandy soils in

these areas. In particular, the study aims to:

1. Assess the current status of P in non-acidic and/or non-sandy soils from

intensively managed cropland,

2. Investigate the phosphate leaching potential of non-acidic and/or non-sandy soils

from intensively managed crop land,

3. Evaluate the P saturation protocol in non-acidic and/or non-sandy soil samples

4. Formulate alternative measurements in predicting P losses in non-acidic and/or

non-sandy soils

Literature Review

3

CHAPTER 2. LITERATURE REVIEW

2.1. P-Cycle in Soil

Phosphorus (P) is present in all terrestrial environments originating from P containing

minerals such as apathite and phosphorite. The P present in the minerals is gradually

released into the soil through weathering processes. In agricultural systems, various sources

of P are added to the soil such as commercial fertilizers, plant residues, agricultural wastes,

municipal and industrial by-products (Figure 1). Available P in the soil solution is then taken

up by the plants and removed from the soil at harvest (Saroa and Biswas, 1989). Also, P is

subjected to various transporting factors such as runoff, erosion especially on sloping lands,

and leaching. These P transporting factors that occur in the soil depends on watershed

hydrology, and they are considered very critical as they translate P sources into actual loss

from a field to a watershed, streams, lakes and groundwater (Sharpley et al., 2001).

Figure 1. Phosphorus cycle in soils (Adapted from Pierzynski, 1994)

Literature Review

4

Various inorganic transformations in the soil affect the availability of P for the plants including

sorption and desorption, precipitation, dissolution and organic transformations mainly by

mineralization and immobilization. In addition, P cycle is also controlled by biological

processes (Frossard et al. 1995). Organic P in soil from manures and crop residues are

important source of P for plant growth which can be vulnerable to land-use management

(William and Haynes 1990; Papanikolaou et al. 2010; Jouany et al. 2011).

Moreover, soil P is also divided into 3 major pools that are in dynamic equilibrium. These

pools are labile, slowly cycling active P and occluded P (Stevenson and Cole 1999; Richter

et al., 2006). The labile P pool is believed to be available for fast biological uptake (Tiessen

and Moir 2008), and consists of phosphate in the soil solution or phosphate that can rapidly

desorb or mineralize from inorganic or organic soil compounds (Vandecar et al., 2009). The

slowly cycling active P is the pool that can be easily converted to labile P (Richter et al.,

2006). Lastly, occluded pool is represented by inorganic compounds that are believed to be

resistant to microbial processes like mineralization in the soil (De Schrijver et al., 2012).

2.2. Effect of soil pH on the formation of different P compounds

The availability and transformation of different P compounds in the soil is predominantly

controlled by soil pH among other factors. For instance, at low pH (<4), Al and Fe-(hydr)

oxides may dissolve, causing high concentration of Al and Fe in the system (Koopmans et

al., 2003). The presence of Al and Fe with P results in the formation of hydroxyphosphate

and precipitates mainly as Al and Fe phosphates.

In acidic soil, the overall reaction of inorganic P with Al and Fe hydroxides is the result of a

fast adsorption reaction in the surface sites and a slow reaction involving the diffusion

through the solid phase or micropores of Al and Fe hydroxide. As the reaction progresses,

precipitation or adsorption inside the aggregates occurs (van Riemsdijk and Lyklema, 1980a,

b; van Riemsdijk and de Haan, 1981; Barrow, 1983; van Riemsdijk et al., 1984a; Bolan et al.,

1985; Madrid and De Arambarri, 1985; Willet et al., 1988). The fast reaction can be

associated as a ligand exchange reaction between phosphate anions and OH-- or H2O-

groups at the surface of Al and Fe hydroxides (van Riemsdijk and Lyklema, 1980a).

Increasing adsorption of PO4- ions in soil was found out with decreasing pH below 6

(Gustafsson et al. 2012). In general, as soil pH increases the sorption of P decreases due to

the deprotonation of functional groups on the surface of clay and metal oxides resulting in the

more negative charge in the colloidal surfaces (Pierzynski et al., 2005). In addition, P can be

free from its binding with ferric complexes due to the competition between hydroxyl ions and

the bound P ions (Anderson, 1974). The available form of orthophosphate for the plants is

Literature Review

5

H2PO4- which is abundant at pH below 7.20. Above this pH range, its availability decreases

due to the fact that solubility of calcium (Ca) phosphate decreases with an increasing pH

(Lindsay, 1979).

In calcareous soils (pH >7.5), the predominant forms of phosphate ions in solution are

HPO42- and H2PO4

-. These ions can principally form into CaHPO4 and possibly Ca(H2PO4)2 in

the soil solution. P in the form of orthophosphate reacts readily with soil components, but the

capacity of soil to bind P is limited (Del Campillo et al., 1999). The equilibrium between outer

bulk solution and the diffuse ionic atmospheric surrounding charged surface particles in

calcareous soils leads to such complexes occurring in considerable concentration within this

ionic atmosphere. The phosphate ions adsorbed by the displacements of CO32- ions by

HPO42-. When adsorption process is completed along with increasing equilibrium

concentration of phosphate ions, formation of multilayer begins and probably initiating at

potential energy minima on the adsorbing surfaces. The formation of multilayer is a precursor

to the formation of dicalcium phosphates crystallites (Figure 2). These crystallites are

subjected to hydrolytic reactions in the alkaline environment of calcareous soil which

changes rapidly to a more basic calcium phosphate (Ca3(PO4)2) (Greenland and Hayes

1981).

Figure 2. The concept of P retention by CaCO3 (Schoumans and Lepelaar, 1995).

Literature Review

6

In Swedish calcareous agricultural soil, Gustafsson et al. (2012) found out that the minimum

solubility of PO4- was at pH 7.5. Below this pH, there was a rapid increase in the solubility of

PO4-. This could be due to desorption from hydroxide surfaces and weathering of apatite-like

compounds (Gustafsson et al. 2012). In addition, the decomposition of organic materials can

also increase soil pH which can further decrease P adsorption, particularly on oxide mineral

surfaces (Barrow, 1984; Jiao et al., 2007). The application of lagoon liquids from dairy and

swine with relatively high pH of 7.4 could also increase the solution pH which decreases P

sorption capacity and thereby enhancing P movement (Kang et al., 2011).

2.3. P Reactions in Soils

Behaviour of P in soil is generally dynamic and rather complex. The excessive buildup of this

nutrient in agricultural soils is a significant environmental problem. Thus, in order to manage

P for optimum plant growth and productivity while minimizing the risk of P movement to

surface and groundwater, it is necessary to understand the reactions that control its

availability and solubility. Because of these chemical and biological reactions, P can be

released to soil solution and become mobile.

2.3.1. Dissolution and Precipitation of P

The solubility of P containing minerals is greatly affected by soil pH, and the concentrations

of Al, Fe, Ca in the soil solution as well as other soil properties (e.g. temperature and

moisture). In general, solubility of P is highest when soil pH range from 6.0 to 6.5 making it

available for plants and moves through surface runoff or leaching waters (Pierzynski, 1994)

(Figure 3). As a reverse process of dissolution, precipitation is the formation of insoluble

compounds in soils that usually exceeded the solubility products (Pierzynski, 1994). At higher

pH the precipitation of phosphates with Ca is expected to occur forming a number of Ca-P

minerals such as amorphous Ca phosphate, octa-Ca phosphate and apatite

(hydroxyapatite), (Gustafsson et al. 2012).

Literature Review

7

Figure 3. Soil pH and the solubility of some phosphate minerals (Adapted from Lindsay,

1979). The concentration of H2PO4- and HPO4

2- is given in moles expressed as log function.

2.3.2. Sorption and Desorption

Sorption and desorption are the most important mechanisms concerning the retention and

release of inorganic P in soil. Desorption is the release of P from solid phase into soil solution

(Pierzynski et al., 2005). This process is one of the series of processes that controls P

uptake and utilization for plant growth (Elkhatib and Hern 1988). It also involves sorption of

PO4- to the surfaces of Fe and Al hydroxides, in clay minerals, and to the carbonates

minerals (Gustafsson et al. 2012; Karathanasis et al. 2009; Yagi and Fukushi, 2011). The

surfaces of the colloidal particles are composed of permanent charges arising from

isomorphous substitution and variable charges which are dependent on the pH of the soil

solution.

P sorption reactions are kinetically biphasic which initially involves a rapid reaction followed

by a slow reaction that can last for weeks. During the initial reaction, it involves nonspecific

adsorption and ligand exchange on mineral edges or even in the amorphous oxides and

carbonates. On the other hand, the slow reaction includes surface precipitation or

polymerization on mineral surfaces and diffusion of adsorbed P to the interior of soil solid

phases (Pierzynski et al., 2005). In calcareous condition, free calcium carbonate (CaCO3)

Literature Review

8

can sorb P ions at low solution P concentration without the precipitation of Ca3(PO4)2

(Freeman and Rowell 1981).

Desorption of bound phosphate is necessary for plant availability as well as for predicting P

losses through leaching (Hartikainen et al., 2010). This happens when plant uptake, runoff or

leaching depletes soil solution P, or in an aquatic environment with low P where the

sediment-bound P interacts (Pierzynski et al., 2005). Therefore, desorption is related to soil P

concentration which is strongly influenced by P fertilization either inorganic or organic

substances (McDowell and Sharpley 2001). As reported by McDowell and Sharpley (2003),

desorption of P increases with increasing amount of Olsen-P in the soil. Also, the addition of

organic matter with time increases desorption (McDowell and Sharpley 2001). Organic

matter increases the net negative charge thereby reducing phosphate adsorption capacity

(Jiao et al. 2007) and enhances the rate of P desorption by blocking the adsorption sites

(Shariatmadari et al. 2006).

Many studies about phosphate desorption in soils observed that the rate of reaction is very

rapid during initial stage and slowly declines as it approaches equilibrium. The initial fast

desorption of P is characterized by more labile P, while the latter fraction is believed to be

less mobile (Kuo and Lotse 1973). The rapid initial desorption was attributed to the rapid

dissolution of poorly crystalline forms such as octo-Ca and hydroxyl appatite (Evans and

Jurinak 1976; Griffin and Jurinak 1974). However, the second slower reaction corresponds to

the desorption of surface labile P and slow dissolution of the crystalline phosphate

compounds in the soil (Toor and Bahl 1999).

2.3.3. Mineralization and Immobilization

Mineralization is a decomposition process of organic compounds to soluble forms and the

release of inorganic forms of nutrients into the soil solution that plant roots can absorb (Brady

and Weil, 1999). Although the mechanisms by which organic P is mineralized still remain

unsolved, it is believed that the extracellular phosphates enzymes produced by plant roots,

soil microorganisms, and mycorhizza are involved in the hydrolysis of organic P to inorganic

P (Pierzynski et al., 2005). For instance, the higher amounts of desorbed P in manure-

amended soil were found to be caused by the contribution of mineralized P from the manure

(Fekri et al., 2011).

The process of mineralization is also controlled by many factors such as the amount of

carbon (C) in the organic substrate and temperature. C act as the energy source for the

decomposing microorganisms which stimulates microbial growth that consumes available P.

Literature Review

9

Low C:P ratio results in excess soluble P available for plant uptake, runoff and leaching

(Pierzynski, 1994). Increased temperature accelerates P cycling by enhancing plant growth

that will result in the increase of nutrient demands, microbial biomass as well as root turnover

and exudation (Sardans et al. 2006 and Raghothama 1999).

Immobilization is the conversion of mineral P by soil microorganisms into biochemical

compounds for microbial metabolisms or into microbial biomass. Biological fixation of

phosphates takes place as immobilization of available phosphates by microorganisms into

their cellular materials (Tan, 2011). When the C:P ratio in the soil rises above 100, P is

immobilized by the microorganisms especially bacteria which require relatively high P for

about 1.5 to 2.5 % by dry weight compared to plants (White, 2006). The process of microbial

immobilization of P contributes to the lower amounts of desorbed P in soil with organic

amendments particularly with compost (Fekri et al. 2011) and causes a decrease in soluble

inorganic P concentration in the soil solution (Tan, 2011). Although biologically fixed

phosphate is considered biologically stable, it is less insoluble than the Al- and Fe-phosphate

compounds. This can also be released more easily than inorganically fixed phosphate,

because organic matter is always subject to decomposition and mineralization processes. In

addition, organic phosphates also exist in soluble forms (Tan, 2011).

2.4. Effect of soil type on P leaching

Many studies found that the various soil types have different responses in the amount of P

leaching due to its retention capacity and affinity of phosphate ions. It is therefore important

to realize the behavior of P with various soil types in the soil system. Soil texture is one of the

most static soil properties that are more or less unaffected by any external variable and are

typically permanent attributes of the soil material (Hillel, 1998). This can be grouped as sand,

silt and clay. Sand particles usually consist of quartz, fragments of feldspar, mica, and

occasionally heavy minerals such as zircon, tourmaline and hornblende (Mack and Leistikow,

1996). This type of particle tends to be loose, well-drained and easy to cultivate and are

commonly referred as light textured soil. On the other hand silty particles generally resemble

as sand particles having greater surface area per unit mass and are often coated with clay.

Clay is considered as the most active fraction in the soil. This is due to the fact that clay

particles have greater surface area than other soil particles and its resulting physicochemical

activity. Most importantly, this soil carries negative charge. Clay typically exhibits plastic

behavior and sticky when moist, forms cracks, tight and cohesive when dry (Hillel, 1998).

Additionally, clay tends to absorb and retain much more water at field capacity.

Literature Review

10

As mentioned previously, heavy and light textured soil materials greatly affect the movement

and transport of water in the soil system and therefore soil texture is an important attribute to

consider when studying leaching of ions in the soil. Many studies found that light textured soil

loses considerable amount of P than heavy textured soil, with respect to the degree of P

saturation, sorption capacity and fertility status. One of the studies supporting this

observation was conducted in Sweden and researchers found lower P leaching losses in

sandy soil due to high sorption capacity in the profile while higher P losses was observed in

sandy soil with high amounts of extractable P and low sorption capacity resulting in higher

degree of P saturation (Andersson et al, 2013). Similarly, Ulén (2006) revealed that coarse

textured, tile-drained soils with a high degree of P saturation and low P sorption capacity

were prone to P leaching. Moreover, the sandy soil in the river delta of north-western Europe

with excessive nutrient input of P was found to be positive for subsequent P leaching that

resulted to an increased concentration of P in surface water (Chardon and Schoumans,

2007). Similarly, most of the soils in Flanders were considered highly vulnerable to leaching

because of its acidic sandy characteristic and high fertility status (Van Den Bossche et al.,

2005). In terms of total P in the soil, a very clear influence of the texture was observed. In

East Flanders for instance, soils with >82.5% sand had significantly larger total P

concentrations than soils with 82.5 to 50% sand content (Van Den Bossche et al., 2005).

In the case of light to heavy textured soils like clay, the degree of P saturation is lower but

the sorption capacity is higher than in sandy soils (Andersson et al., 2013). In addition, the

presence of continuous macropores in clay soils enables water and solutes to be rapidly

transported through the soil, leading to short contact time between the percolating water and

the soil matrix resulting in the increasing P losses with increasing clay content, despite low P

saturation (Djodjic et al., 2004). In this case, chemical soil properties were found to be less

important for P leaching in soils with macropores than in soils without (Andersson et al.,

2013). Moreover, considerable P leaching losses was found in clay soils in the Netherlands

through preferential flow resulting from the presence of cracks especially on dry periods

(Chardon and Shoumans 2007).

2.5. P Leaching from acidic and non-acidic soils

Phosphate leaching is the downward movement of soil P in both dissolved and particulate

forms which is associated with soil colloids and organic matter. This has become a

widespread problem in some countries in Europe and USA (Culley et al. 1983; Beauchemin

et al. 1998; De Bolle et al. 2013. Djodjic et al. (2004) concluded that the most important

factors causing P leaching are the potential of P release to the surface horizon, the capacity

of subsoil to adsorb or release P and the water transport mechanisms. At slow water

Literature Review

11

transport with prolonged contact between soil particles and percolating water, P leaching is

low. However, when water transport increases rapidly in the subsoil, larger leaching loads

will occur. The concept of vertical transport of P through the soil profile between sandy and

clayey soils differs with respect to their capacity of water absorption. Saturated flow occurs in

sandy soil while mostly preferential flow in clay soil. P in soil solution can sorbed in deeper

soil layer at the occurrence of saturated flow. On the other hand, during preferential flow

large volume of water is transported rapidly through a small part of the soil (Simard et al.,

2000). Important P leaching process also includes the preferential flow from cracks due to

shrinking of clay, and the influence of other soil organisms which create holes and channels

resulting in high P losses (Chardon and Schoumans 2007; Djodjic et al., 2004).

In the leachates, the different P forms can be divided into fractions which provide better

understanding for potential P transfer (inorganic versus organic P transfer). As proposed by

Hens and Shoumans (2002), P forms in water can be divided into physical (size) and

chemical fractionation (Table 1). Physical fractionation relates to the concept of dissolve and

particulate P forms which is usually separated by recommended filter size i.e 0.45 µm.

Chemical fractionation of P involves the concept of total P (TP) and inorganic P. TP and

inorganic P can be measured using the Inductively Coupled Plasma (ICP) and molybdate

blue color reaction of Murphy and Riley (1962) respectively. According to Haygarth and

Sharpley (2000), P measured by molybdate color reaction does not exclusively represent

inorganic P because of the possible hydrolysis of organic P due to acid molybdate solution,

resulting in an overestimation of the inorganic P. Because of this, the concept of reactive P

(RP) has been proposed for the P measured according to Murphy and Riley (1962) (Hens

and Schoumans (2002).

Table 1. The different P forms in water (Hens and Schoumans, 2002).

Chemical

fractionation

Physical fractionation by difference

Unfiltered water

sample

Filtered water

sample

Total P (TP) total P

(TP)

total dissolved P

(TDP)

Total particulate P

(TPP)

Reactive P (RP) total reactive P

(TRP)

dissolved reactive P

(DRP)

Particulate reactive P

(PRP)

Literature Review

12

Looking at the effect of soil management practices on P leaching, Kang et al. (2011) reported

that with continuous organic matter application, the total amount of P from organic materials

may be subjected to leaching. Thus, the total P content from organic sources and the rate of

P removal by the crops should be monitored. Furthermore, results revealed that high P

leaching potential is directly related to the duration and length of vegetable cropping (Qin et

al. 2010). In fact, according to Djodjic et al. (2004) the long-term continuous farming indicates

greater possibility of P leaching even in the areas where fertilizer had not been applied for

the past four decades. This further implies that there are some soils still susceptible to P

leaching at smaller external inputs.

Kolahchi and Jalali (2013) recently observed that there is an increased leaching of P in

calcareous soils following the application of acid rain. This occurrence may be related to the

desorption of surface labile P and slow dissolution of crystalline phosphate compounds and

decomposition of organic matter in the soil. Interestingly, the P concentration in leachate was

greater at pH 7.0 (0.30 mg l-1) which is much greater than the recommended guideline (0.10

mg P L-1) used in the Netherlands as an environmental upper limit for P in shallow

groundwaters. Furthermore, significant nonlinear correlation was observed between the

amount of P leached and the simulated acid rain. This indicates that with increasing pH, the

amount of P leaching increased up to a critical pH value (5.0) and decreased as pH

increased to above 7.0 (Jalali and Naderi 2012).

In the study conducted by Jalali and Karamnejad (2011) on P leaching in calcareous soil

treated with plant residue and inorganic fertilizer, revealed that the concentration of P in the

leachate from soils amended with water-soluble P added with organic residue were higher

than soil with inorganic fertilizer alone. The values for P concentrations in the leachates vary

between 8 and 220 mg L-1 in water-soluble-P treatment, 0.80 and 230 mg L-1 in organic

residue + water soluble P treatment.

In a calcareous soil amended with organic matter, the release of P was observed to be

initially rapid and then became slower upon reaching equilibrium. This quick initial reaction

was attributed to faster dissolution of poor crystalline or amorphous phosphates, while the

second reaction was due to desorption of surface labile P and slow dissolution of the

crystalline phosphate compounds in the soils (Fekri et al., 2011). This may become a

problem because organic matter may also affect nutrient mobility and the use of organic

residue in soils may cause leaching of certain ions (Esteller et al., 2009). In a study

conducted in Canada, for example, it was revealed that manure application decreases the

soil´s phosphate retention and also increases phosphate release into the soil solution due to

Literature Review

13

the increase in the net negative charge thus reducing the phosphate adsorption capacity.

Moreover, it was also found out that phosphate was weakly bound in manured soils (Jiao et

al., 2007).

2.6. P Saturation in Flanders

P saturation in the soil refers to the amount of sorbed P in a soil to its P sorption capacity

(PSC), or its maximum ability to adsorb/precipitate P. Accumulation of P in the soil decreases

the difference between sorbed P and PSC which loses its ability to remove additional P from

the soil solution (Van Den Bossche et al., 2005). The build-up of sorbed P relative to PSC

increases equilibrium solution of P concentrations to the extent where P can be readily

removed through runoff and leachate (Kleinman et al., 1999). In acidic soils PSC is controlled

by non-crystalline Al and Fe minerals (Van Der Zee et al., 1990).

Many phosphate saturation parameters were developed to estimate the susceptibility of soils

to leaching and the risk of causing eutrophication. This includes the concept of phosphate

saturation degree (PSD). The determination of PSD is based on ammonium oxalate

extraction of non-crystalline Al and Fe compounds in acidic soils (Van der Zee et al. 1990b).

The PSD of the soil is determined as (Van der Zee et al. 1990):

PSD = [POX/α (FeOX + AlOX)]

where

POX = sorbed P fraction extracted with ammoniumoxalate-oxalic acid (mmol P kg soil-1)

α (Feox + Alox) = PSC = P sorption capacity (mmol kg soil-1)

α = the degree of saturation = 0.5 (Van der Zee & Van Riemsdijk, 1988)

Feox = ammonium-oxalate-oxalic acid extractable Fe (mmol Fe kg soil-1)

Alox = ammonium-oxalate-oxalic acid extractable Al (mmol Al kg soil-1)

The general PSD for the profile is determined by calculating the average Pox and the average

PSC, Alox and Feox, to a depth of 90 cm, or to the groundwater table boundary if it is less than

90 cm.The relationship between the phosphate saturation of the profile and the concentration

of orthophosphate in the groundwater in acidic light textured soils is given by Van der Zee et

al. (1990) (Figure 4). At concentration of 0.1 mg L-1 from the base of the soil profile is

considered as lower limit for eutrophication and Psat should be less than 24% of the whole

soil profile. In Flanders, protocol for Psat was set to a total profile depth of 90 cm and Psat of

35% in the agricultural regions to be considered as saturated (VLM, 2007).

Literature Review

14

Figure 4. Relationship between PSD and orthophosphate concentration in the soil solution

from light textured acidic sandy soil (Van der Zee, 1990).

In Flanders huge application of manure and inorganic fertilizers over the years increases the

general fertility status of the soil (Van Den Bossche et al., 2005). As mentioned previously,

many soils of this area are sandy, making them more susceptible to P leaching (VLM 1997).

It can be seen in Figure 5 that large part of sandy areas for about 4404 km2 used for

agriculture is already P saturated with estimated PSD of more than 35 %.

Figure 5. Block (500 m) estimation of PSD for Soils in Flanders (Adapted from Van

Meirvenne et al. 2008).

Literature Review

15

Aside from long term application of nutrients that farmers practice the intensive animal

husbandry resulting to a massive application of slurry has also been considered as a main

cause for increased P saturation in this region. In agricultural lands in West Flanders for

example, the mean PSD in the top 0 to 30 cm was 57 % much higher than the critical value

(De Smet et al., 1996).

The study conducted in different organic farms in East Flanders having sandy to silt loam

texture revealed that the average value of the ammonium-oxalate – oxalic acid extractable P

over the total profile (0-90) is 14 mmol P kg-1 soil which was slightly higher than the average

value found in agricultural soils of East Flanders which is about 12.6 mmol P kg-1 soil (VLM

1997). The resulting values for average P sorption capacity was about 37 mmol kg -1 with the

Psat of 37 %. The result also showed that a large part of the organic farms still had a Psat

higher than that of the critical value of 35 %, which indicates P saturation and potential risk of

P leaching to groundwater (Van Den Bossche et al., 2005).

A recent study comparing the PSD in the recent past (2001-2005) with the current (2010) to

investigate whether there are changes up to 2009 in legislation with respect to the addition of

P fertilizers and impact on the P status of these soils. It was revealed that PSD from

intensively managed acid sandy soils in Flanders continued to increase over the past 5-10

years despite the strict legislation. The strongest increase in PSD was observed in the

deeper soil layers (30-60 cm) and can be explained by P redistribution to deeper soil layers.

This suggests that P leaching in these acid sandy soils is probably faster than before and

can be considered as a potential major threat to groundwater quality (De Bolle et al., 2013).

Materials and Methods

16

CHAPTER 3. MATERIALS AND METHODS

3.1. Site Selection, Soil Sampling and Pretreatments

3.1.1 Site Selection

There were 13 sites selected from different agricultural fields in 3 provinces in the Flemish

region namely Brabant, West Flanders and East Flanders (Figure 6). These sites represent

agricultural lands in Flanders which have been managed intensively for the production of

mostly horticultural crops. The chosen fields were characterized by non-acidic or non-sandy

soils where P leaching problems were not fully known (i.e. where the P saturation protocol

has not been validated). Soil samples were taken from the 13 sites selected in the above-

mentioned provinces: 8 soil samples from West Flanders, 3 from Flemish Brabant and 2

representative fields from East Flanders. Another soil sample was also taken in West

Flanders to serve as reference for acidic sandy soils with large P content where the P

saturation protocol is widely used.

Figure 6. Map showing the various sampling sites (red point star) and reference site (blue

point star) from 3 provinces in Flanders.

Materials and Methods

17

3.1.2. Soil Sampling and Preparation

In each of the sampling sites, approximately 30 kilograms (kg) soil of an about 20 to 40

augerings was collected in a zigzag way over the entire field to a depth of 30 cm. The

collected soil samples were bulked and homogenized to obtain composite samples, and

visible debris, roots, and other non-soil materials were removed by hand. The samples were

air dried at room temperature and sieved (2 mm sieve) prior to analysis of soil the

characteristics and leaching experiment.

3.2. Physical and Chemical Characterization of the Soils

The particle size distribution was determined using the pipette method (Gee & Dani, 2002).

Exactly 20 g of air dried soil that had passed through 2 mm mesh sieved was prepared

including a reference soil and placed to 2 L beaker. Dispersion of the clay fraction was done

by removing the cementing materials e.g. CaCO3, organic matter and oxides. The fine

fractions (silt and clay) were separated from the sand fraction by wet sieving on a sieve with

an aperture of 50 µm. The clay (<2 μm) and silt fraction (2-50 μm) were separated by

pipetting with a pipette Robinson-Köhn after sedimentation at a constant temperature and

fixed settling, according to Stokes' law. All fractions were weighed after drying at 105 °C and

the results were expressed as a percentage.

The organic C content was determined using the Walkley and Black (1934) method. Exactly

1 g of air-dried soil was placed in a 500 ml Erlenmeyer flask (EF). The soil sample was

oxidized by adding 10 mL of 1 N K2Cr2O7 and then swirled gently to dispense the solution.

Ten ml of concentrated H2SO4 was added immediately and then the flask was gently swirled

for at least 1 minute. The mixture was allowed to react under the exhaust hood for 30

minutes before adding 150 ml of distilled water. Afterwhich, 10 ml of concentrated H3PO4 and

four drops of ferroine indicator were added to the solution accordingly before doing final

titration with 1N FeSO4. The endpoint was reached when the colour changes from green to

reddish brown.

The amount of CaCO3 present was measured by titration using the method of Gee and Dani,

(2002). In a 500 ml EF, 1 g of air dried soil sample was placed before adding 25 ml of 0.5 N

H2SO4. It was diluted with 150 ml distilled water and heated at 80 °C for 1 h. The samples

were allowed to cool down before titrating with 0.5 N NaOH until the endpoint was reached,

and the color changes turns from green to pinkish.

Materials and Methods

18

Total inorganic carbon (TIC) or the carbonates content in soil solid was calculated by

obtaining the difference between total carbon (TC) and total organic carbon (TOC) measured

using TOC analyzer (TOC SSM-5000A, Shimadzu, Japan).

The soil pH was measured potentiometrically in 1:2.5 ratio (soil:KCl extract). The samples

were mixed thoroughly to bring the soil into suspension. The mixture was allowed to stand at

room temperature for 10 min, thereafter, all the samples were agitated before measuring with

a pre-calibrated pH meter (Thermo Orion Model 420+).

3.2.1. Total P and Available P

The total P in soil was measured by digestion with perchloric acid (HClO4) (Olsen and

Sommers 1982) followed by colorimetric determination by the Molybdenum blue method

(Scheel 1936) at 700 nm using spectrophotometer (model Cary 50, UV-visible

spectrophotometer).

Available P in ammonium lactate extraction (PLAC) was carried out following the method of

Egnér et al., (1960). After 4 h shaking 5 g of the soil in 100 ml PLAC solution, filtering was

done using 589/3 ash less Whatman filter paper. The P in filtrates was measured using ICP

(ICAP 6000 series; Thermo scientific). P oxalate extraction (POX) was measured following the

method of Egnér et al., (1960). Exactly 5 g of soil was used and a 100 ml Pox extracting

solution was added. After 2 h of shaking, filtering was done using Whatman filter paper 589.3

to collect the leachates. Then the P concentration was measured using ICP.

3.2.2. Phosphorus Saturation Degree

PSD was determined according to the protocol of Van der Zee et al., (1990).

PSD = [POX/α (FeOX + AlOX)]

Exactly 5 g of dried, sieved soil samples taken from the upper 30 cm of the soil profile were

extracted with acid ammonium oxalate in a dark polyethylene bottles and shaken for 2 h

(Schwertmann 1964). The soil suspensions were filtered (Whatman filter paper 589.3) and

the POX, FeOX and AlOX concentrations were measured using ICP. The resulting values (mg

kg-1) were converted to mmol kg-1 soil. Finally, PSD was calculated and expressed in

percentage.

Materials and Methods

19

3.3. Phosphorus Leaching Experiment

3.3.1. Leaching Column Preparation and experimental Set-up

The columns were prepared in the lab using polyethylene tubes (PE) with an inner diameter

of 10 cm and 30 cm long. PE tubes were used specifically for this study because this

material is non-reactive with respect to P. Other material such for example polyvinyl chloride

(PVC) can absorb P that might cause problem during the leaching experiment. Endcaps with

holes at the center were carefully fitted at the bottom of the tubes to allow the flow of

leachates during the experiment (Figure 7). Prior to that, the bottom of the tubes were

covered with nylon mesh (0.31 μm) to allow permeability during leaching experiment and to

maintain the contact with the column and the endcap. The endcap along with the nylon mesh

was carefully glued and sealed to avoid entry or leakage of air and water during the leaching

period. Glass funnel was also glued at the bottom of the prepared column and finally fitted to

the Buchner filter flask using rubber stopper. On the other hand, a PVC pipe that was also

fitted to the Buchner was linked to the vacuum pump.

Figure 7. Schematic overview of the column leaching experiment.

Materials and Methods

20

The moisture content of the soil was determined by oven drying to a temperature of 105 °C

for 24 h. About 2 kg of air dried soil that passed through 2 mm mesh sieved were brought to

18 % field moisture content to facilitate soil packing and water transport within the soil

column during the leaching experiment. The soil was carefully mixed before packing into the

tube. The pre-wetted soil was added in fractions equivalent to 5 cm while simultaneously

tapping the walls of the tube with a wood to achieve uniform packing. Tapping was continued

until the height of 20 cm from the bottom of the column to attain the desired bulk density of

1.35 kg m-3.The Buchner flasks were covered with aluminum foil to avoid unwanted reactions

and algal growth. The soil above the column was covered with filter paper (Whatman 2) to

minimize the impact of water drops during irrigation. Filter papers were replaced every two

weeks to avoid further decomposition. The entire surface was covered with parafilm to

minimize evaporation and finally suction was applied prior to the start of the experiment. The

final set-up is shown in Figure 8.

Materials and Methods

21

Figure 8. The actual experimental set-up.

1

.

2

3 4

5

Materials and Methods

22

3.3.2. Leaching Experiment

The prepared soil samples in the columns were allowed to stand for 2 d at room temperature

before initiating the leaching experiment. Demineralized water was used to irrigate the soil

until saturation. Every 3 d, an equal amount of demineralized water (20 mm=157ml) was

carefully added on the surface of each column. On the other hand, the leachate was

collected every 5 d and preserved inside the fridge before the analysis. The amount of water

added was calculated based on the fraction of water leached down the soil after rainfall. The

annual precipitation in the study area is approximately 800 mm per year and about 300-450

mm fraction of water being leached out. The pressure gauge was set to -300 mbar

(equivalent to field capacity potential for non-sandy soil) to allow leaching until field capacity

and prevent anaerobic condition. The leaching experiment was conducted in 3 replicates with

a total of 42 leaching columns. Furthermore the leaching experiment set up was done in two

batches since there is limited space in the lab. Each of the batches contained 21 columns. A

total of 11 batches of leachates were collected, and the leaching experiment last for 10 wks.

Thereafter, the leachates were captured and analyzed.

3.3.3. P Analysis in Leachates

The leachates were collected in a beaker and the volume was measured using graduated

cylinders prior to analysis. The collected leachates were divided into 4 aliquot samples for

total reactive P (TRP); dissolved reactive P (DRP), total P (TP) and total dissolved P (TDP).

Two aliquot samples were filtered with 0.45 μm filter paper for TDP and DRP. Unfiltered

samples were analyzed for TRP and TP. The 0.45 μm filter paper is used to separate the

dissolved and particulate P forms. The analytes for both TRP and DRP were analyzed for

molybdate-reactive P following the methods of Murphy and Riley (1962). Exactly 5 ml of

leachates was placed in 50 mL volumetric flask. 4 ml of mixture was added and diluted with

distilled water up to the mark. The sample was mixed the solution carefully. It was allowed to

stand for 30 minutes to attain maximum color reaction. Thereafter, the P concentration was

measured colorimetrically using spectrophometer (Varian 50 Conc UV- Visible

Spectophometer). Total P (TP) and total dissolved P (TDP) of the leachates were analyzed

using (ICP-MS) in the unfiltered and filtered samples respectively. Total particulate P was

calculated by obtaining the difference between the TP and TDP while the particulate reactive

P (PRP) was calculated as the difference of the TRP and DRP.

Materials and Methods

23

3.5 Statistical Analysis

Correlation analysis was used to investigate the relation between selected physicochemical

properties and different forms of P in the leachates using the statistical software PASW 19

package (SPSS version PASW 18; SPSS Inc., USA). To investigate the relationship between

the average volume-weighted concentrations of the different P forms in the leachates and

selected soil physicochemical parameters, multiple linear regressions was performed using

Spotfire (S+8.2). Significant interaction and main effects between the different soil variables

were considered at p<0.05.

Results

24

CHAPTER 4. RESULTS

4.1 General Soil Properties

Generally, soil texture is dominated by large proportions of silt and sand ranging from 22% to

82% and 8% to 71.8% respectively, while sand fractions are less than 20% (ranging from 7%

to 14%). According to USDA classification, the soils are from sandy loam to clay representing

various soil texture in the area. The maximum measured value for pH-KCl was 7.3 in site 1,

with a minimum of 6.2 in site 10 and considered very slightly acid to very slightly alkaline.

The reference soil was characterized by acidic sandy loam with 71% sand and a low pH-KCL

value of 5. Highest organic carbon was observed in site 12 (2.08%) while the lowest

measured value was in site 13 (0.98%) averaging to 1.3%. Amount of CaCO3 ranged from 0

to 1.23% in site 7, 10, reference soil and site 9 respectively whereas, total inorganic carbon

(carbonates) ranges from 0 to 0.008%.

Table 2. Main soil characteristic of the selected sites.

Site Number

Texture (USDA

classification)

%Clay (0-2μm)

%Silt (2-50μm)

%Sand (50-200μm)

pH-KCl Corg (%)

CaCO3 (%)

TIC-solid (%)

1 Sandy loam 10.4 35.1 54.5 7.3 1.18 0.73 0.000

2 Sandy loam 10.6 22.9 66.5 6.7 1.18 0.85 0.008

3 Sandy loam 8.4 32.1 59.5 6.7 1.14 0.61 0.000

4 Sandy loam 7.9 29.0 63.1 6.7 1.18 0.49 0.000

5 Loam 14.0 41.4 44.7 7.1 1.45 0.61 0.000

6 silt loam 11.9 54.3 33.8 7.1 1.45 0.37 0.000

7 silt loam 10.4 74.7 14.9 6.3 1.04 0.00 0.000

8 silt loam 7.9 77.3 14.9 7.1 1.14 0.98 0.005

9 silt loam 6.9 78.6 14.5 7.0 1.00 1.23 0.007

10 silt 7.1 81.1 11.8 6.2 1.12 0.00 0.000

11 silt 7.3 82.0 10.7 6.8 1.02 0.61 0.000

12 clay 69.3 22.7 8.0 6.7 2.08 0.74 0.000

13 silty clay 46.7 43.9 9.4 6.8 0.98 0.86 0.000

Reference Soil

sandy loam 5.7 22.4 71.8 5.1 1.81 0.00 0.000

Results

25

4.2 P status of the soils used in the study

Total P, POX and PLAC The total P content of the studied soils ranged from 714 mg P kg-1 soil to 1258 mg P kg-1 soil

in site 6 and reference soil respectively, with an average value of 926 mg P kg-1 soil (Figure

9). The reference soil contained highest amount of total P although it is comparable with site

2, 3, and 12. Most of the sites were characterized by high total P content in the upper 30 cm

in the soil profile. Sites 6 to 11 have comparable values ranging from 714 mg P kg-1 to 843

mg P kg-1 soil. The amount of P extracted by ammonium oxalate in the studied soil was

higher than available PLAC except for site 2. The both methods follow the same trend wherein

sites 6 to 11 have lower amounts (255 to 341 mg kg-1 soil). Again, highest amount for POX

was found out in the reference soil (773 mg kg-1) followed by site 2 and site 12 respectively

(624 and 621 mg P kg-1). Available PLAC content in the soil was between 84 mg P kg-1 and

643 mg P kg-1 soil with an average value of 345 mg P kg-1. The maximum value for PLAC was

observed in site 2 while the minimum was in site 7.

Figure 9. Total P, P extracted by ammonium oxalate (POX) and available P (PLAC) (mg kg-1

soil) from the upper 30 cm of the soil profile in the selected field and reference soil

respectively.

0

200

400

600

800

1000

1200

1400

mg

kg-1

so

il

Site Number

Total P P Oxalate P Lactate

Results

26

Oxalate extractable P, Fe and Al

The values for ammonium-oxalate extractable P in the top soil (0-30 cm) averaged 13.7

mmol P kg-1 soil and ranged from 8.2 mmol P kg-1 soil to 25.0 mmol P kg-1 soil respectively

(Figure 10) Noticeably, the average concentration of Fe in the soil was 50% higher than Al,

except for site 12. Fe concentration ranged from 28.7 mmol P kg-1 soil to 54.2 mmol kg- soil

with the average of 36.7 mmol kg- soil while the Al content ranged between 10.3 mmol P kg-1

soil to 35.1 mmol kg- soil. Highest Fe and Al concentrations were observed in site 5 and site

12 while the lowest observed concentrations were at reference soil and site 1 respectively.

Remarkably, the ratio between Fe and Al is much less in the reference soil compared to soils

from non-acidic and/or non-sandy soils except for site 12.

Figure 10. Ammonium-oxalate extractable Al, Fe and P (Pox) concentrations (mmol kg -1 soil,

0-30 cm) in selected soils used in the study.

0

10

20

30

40

50

60

mm

ol

kg

-1

Site Number

Al Fe P

Results

27

P sorption capacity (PSC) and P saturation degree (PSD)

PSC was presented in Figure 11 with the highest value in site 5 (35.26 mmol kg-1 soil) and

the lowest value was observed in site 2 (21.72 mmol kg-1 soil). Both site 5 and site 12 had

comparable values which slightly higher among the others. The values of PSD from the

upper 0-30 cm of the soil ranged from 30% to 95% with an average value of 56% (Figure 12).

The highest value was clearly found in the reference soil. When comparing the reference soil

with the other soil type, the highest PSD value of 93% was observed in site 2, almost

comparable with that of the value from the reference soil. Lowest PSD was found out in site 7

(30%) which has also lowest POX content among all the soil samples. It can be clearly

observed from the graph of PSD that the highest percentage was in site 2 where it also has

the lowest PSC.

Figure 11. Phosphorus sorption capacity (PSC) in the upper 0-30 cm of the soil from 13

sampling sites and reference soil.

Figure 12. Phosphorus saturation degree (PSD) in the upper 0-30 cm of the soil from 13

sampling sites and reference soil.

0

10

20

30

40

PSC

(m

mo

l kg-1

)

Site Number

0

20

40

60

80

100

Psa

t (%

)

Site Number

Results

28

4.3 P leaching from non-acidic and/or non-sandy soils

Cumulative P leaching

The cumulative amount in different forms of P (mg P/soil column) leached from the soil

columns are presented in Figure 13 and 14. Interestingly, not all soils follow the same pattern

of P leaching as the reference soil for both total P (TP) and total dissolved P (TDP). In the

figure, three dissimilar patterns for TP release were found (Figure 13ab). During the 1st and

5th leaching event (5-25 d), a more gradual release was observed in the leachates, followed

by a rapid release between 25 to 35 d. Furthermore, a decline in the curve was observed for

the succeeding leaching event (35-45 d) and finally increases towards the end of the

leaching experiment. This kind of pattern was well pronounced in several soils, namely the

reference soil, site 3, 1, 4, 5, and 6. Second pattern had a more gradual P release towards

the end of leaching period which was recognized in site 2, 13, 12, 9 and 11. Lower rate of P

release was observed in the 3rd pattern where the curve is characterized by a slow to steady

state P release. This was clearly observed in site 10, 8 and 7 respectively. Maximum

cumulative amount of TP was perceived in site 3 (3.98 mg P/soil column) whereas the

minimum value was observed in site 7 (0.19 mg P/soil column). The same trend in the curve

was also observed for TDP as well as having comparable amount in the cumulative P

released. The amount of TDP leached varies between 0.22 mg P/soil column (site 7) and

3.77 mg P/soil column (site 3) after 11th leaching event (55 d).

In the case of TRP and DRP (Figure 14ab), almost all of the samples followed the same

trend except for site 3 and reference soil respectively. In site 3, a gradual release of P in the

leachates can be seen from 1st to 10th (5-50 d) and then rapidly increase during 10th to 11th

(50-55 d) leaching event. Whereas in the reference soil, a rapid release of TRP and DRP

was observed in the beginning of the leaching period (5-10 d), followed by a gradual release

during 2nd to 6th (10 to 30 d) and a final increase again from 6th (30 d) towards the end of the

leaching event (55 d). Maximum cumulative amount of total reactive P was observed in site 2

(0.62 mg P/soil column) and the minimum was observed at site 7 (0.02 mg P/soil column).

During the simulation period, no leachate was collected from one of the three replicates from

site 6 specifically from 3rd until the end of the leaching period. Same situation was also

observed in the case of reference soil wherein two replicates had no leachate from 2nd to 9th

leaching event.

Results

29

Figure 13. Cumulative amounts (mg P/soil column) of different P form (a. TP b. TDP) in

leachates after 55 d leaching period.

0

1

2

3

4

5

Cu

mu

lati

ve t

ota

l P le

ach

ed (

mg

P)

(a) Site 1

Site 2

Site 3

Site 4

Site 5

Site 6

Site 7

Site 8

Site 9

Site 10

Site 11

Site 12

Site 13

Reference

0

1

2

3

4

5

5 15 25 35 45 55

Cu

mu

lati

ve t

ota

l dis

solv

ed

P le

ach

ed (

mg

P)

Time/d

(b)

Site 1

Site 2

Site 3

Site 4

Site 5

Site 6

Site 7

Site 8

Site 9

Site 10

Site 11

Site 12

Site 13

Reference

Results

30

Figure 14. Cumulative amounts (mg P/soil column) of different P form (a. TRP b. DRP) in

leachates after 55 d leaching period.

0

0,2

0,4

0,6

0,8

Cu

mu

lati

ve t

ota

l re

acti

ve P

leac

he

d (

mg

P) (a) Site 1

Site 2

Site 3

Site 4

Site 5

Site 6

Site 7

Site 8

Site 9

Site 10

Site 11

Site 12

Site 13

Reference

0

0,2

0,4

0,6

0,8

5 10 15 20 25 30 35 40 45 50 55

Cu

mu

lati

ve d

isso

lve

d r

eac

tive

P (

mg

P)

Time/d

(b) Site 1

Site 2

Site 3

Site 4

Site 5

Site 6

Site 7

Site 8

Site 9

Site 10

Site 11

Site 12

Site 13

Reference

Results

31

P leaching potential

The relative amount of P leached was investigated to better understand the potential

behavior and release of various P forms at a given leaching event (Figure 15 and 16). This

can be expressed both units of leachate mm-1 or per kg-1 soil, since all columns had equal

surface area and equal amounts of soil. For the reference soil, the highest TP released in the

leachates was observed at the end of the leaching event (11th) with the value of 0.5 mg TP

mm-1. In the beginning of the simulation period, the highest concentration of TP in the

leachates was found in site 2 (0.02 mg TP mm-1) whereas the lowest was in site 6 (0.0006

mg TP mm-1). During 2nd leaching event, there was slight increase in the amount of TP

leached in the soils except for site 2. At 5th leaching event, soils from site 3, 2,1,4,5 and 6

follow the same curve with that of the reference soil. The highest amount in TP released from

these soils occurred at 7th leaching event. Notably, there was a rapid increase in TP in site 3

from 3rd to 7th leaching event and latter follows the same pattern with the reference soil as it

approaches towards the end. The opposite scenario was observed in site 13, 12, 9, 11 and 8

as it slightly decrease during 7th leaching event then begin to increase at 9th and finally

remain constant towards the end of leaching. Relatively constant shape of the curve from the

beginning until the end of leaching was observed in other soil samples such as in site 10 and

7. Site 7 had the lowest amount of TP leached ranging from 0.0010 mg TP mm-1 to 0.0014

mg TP mm-1 respectively during the entire leaching period which is almost negligible. In the

case of TDP, the highest concentration in the leachates from the reference soil had a

comparable value with TP (0.05 mg TDP mm-1). Nevertheless, the potential release of TP

and TDP with time appears to be different. There was a constant increase in the release of

TDP from the beginning until the end of the leaching event while a different pattern was

observed with that of TP. Apparently, a contrasting shape in the curve was observed

between the various soils during 7th leaching event. Site 3, 2,1,4,5, and 6 follows the same

trend with the reference soil whereas, site 12,9,11,8,10 and 7 follows constant shape from

the beginning towards the end of the leaching period.

The result for TRP form revealed that site 2 loses considerable amount P during the 1st and

2nd leaching period (0.0032 to 0.0031 mg TRP mm-1) and then decline from 2nd to 4th followed

by a constant release from 4th until the end of the leaching event. The lowest concentration of

TRP in site 2 was observed during 9th leaching event with a value of 0.0017 mg TRP mm-1.

However, a different case was observed in other soils wherein the highest concentration of

TRP leached occurs at different leaching event during the entire leaching experiment. In the

reference soil, highest concentration of TRP in the leachate was observed at 7th leaching

event with a value of 0.004 mg TRP mm-1 and subsequently decline as it approaches to a

constant pattern.

Results

32

Figure 15. Leaching potential of various P forms (mg P mm-1) plotted against leaching

events.

0

0,01

0,02

0,03

0,04

0,05

0,06 (

mg

P m

m-1

) Total P (TP) Site 1

Site 2

Site 3

Site 4

Site 5

Site 6

Site 7

Site 8

Site 9

Site 10

Site 11

Site 12

Site 13

Reference

0

0,01

0,02

0,03

0,04

0,05

0,06

1 3 5 7 9 11

(m

g P

mm

-1)

leaching events

Total dissolved P (TDP) Site 1

Site 2

Site 3

Site 4

Site 5

Site 6

Site 7

Site 8

Site 9

Site 10

Site 11

Site 12

Site 13

Reference

Results

33

Figure 16. Leaching potential of various P forms (mg P mm-1) plotted against leaching

events.

0

0,001

0,002

0,003

0,004

0,005 (

mg

P m

m-1

) Total reactive P (TRP)

Site 1

Site 2

Site 3

Site 4

Site 5

Site 6

Site 7

Site 8

Site 9

Site 10

Site 11

Site 12

Site 13

Reference

0

0,001

0,002

0,003

0,004

0,005

1 2 3 4 5 6 7 8 9 10 11

(mg

P m

m-1

)

leaching events

Dissolved reactive P (DRP)

Site 1

Site 2

Site 3

Site 4

Site 5

Site 6

Site 7

Site 8

Site 9

Site 10

Site 11

Site 12

Site 13

Reference

Results

34

Site 3, 4 and 5 had the same pattern as the reference soil for TRP release although it was

well pronounced in the latter part of the leaching experiment. In site 12 and 13, the highest

concentration in TRP was observed in 4th (0.0020 mg TRP mm-1) and 3rd (0.0017 mg TRP

mm-1) leaching event respectively. Site 1, 6, 8,9,10 and 11 follows a constant curve with a

very small release during the entire leaching simulation.

For DRP the same trend was observed for reference soil with that of the curve for TRP.

During the 7th leaching event, the highest amount of dissolved P released occurs with the

value of 0.004 mg DRP mm-1. Site 3 and 4 had the same pattern wilt the TRP however the

peak occurs during 9th leaching event with the value of 0.002 mg DRP mm-1 and 0.001 mg

DRP mm-1. There was a relatively straight pattern of curve was again observed for site

11,8,10, and 7 thought out the entire leaching simulation.

Average losses of different P forms

The highest average volume-weighted concentration of TP reached a value of 2.92 mg TP L-

1 and was observed in site 3 (Figure 17). This was followed by the reference soil that had a

value of 2.69 mg TP L-1. On the other hand, the lowest concentration was found in site 7 that

had a value of 0.13 mg TP L-1. On the contrary, the reference soil had the highest average

concentration in terms of TDP in the leachates with a value of 3.27 mg TDP L-1. It was

followed by site 3 (2.76 mg TDP L-1) which was slightly lower than the observed value for TP.

The lowest average concentration was again observed in site 7 (0.16 mg TDP L-1).

Figure 17. Average volume-weighted concentration of total P (TP), total dissolved P (TDP)

and total particulate P (TPP) during 11 leaching events. Bars reflect the standard error of the

mean.

0

0,5

1

1,5

2

2,5

3

3,5

4

volu

me

-we

igh

ted

P c

on

c. (

mg

L-1)

Site Number

TP TDP TPP

Results

35

For TPP in the leachates, the concentrations were 0.30, 0.15, 0.004, 0.137 and 0.143 mg

TPP L-1 in site 2, 3, 10, 12 and 13, respectively. Sites 1, 4-9, 11 and reference soil had

negative values for TPP in the leachates which were not expected to occur. The negative

values were assumed to zero as presented in the figure.

Similar to TDP, reference soil acquired the highest concentration for TRP amounting to 0.33

mg TRP L-1 followed by site 2 (0.28 mg TRP L-1), site 3 (0.26 mg TRP L-1), site 13 (0.18 mg

TRP L-1) (Figure 18). Essentially, among the 13 soil samples from various sites, site 6, 7, and

10 have values lower than 0.1 mg L-1. For the concentration of DRP in the leachates, the

maximum concentration was observed in reference soil that had a value of 0.30 mg DRP L-1

whereas, the minimum concentration was found in site 7 with the value of 0.004 mg DRP L-1.

Particulate reactive P (PRP) on the other hand had a lower values averaging to 0.01 mg PRP

L-1 wherein site 2 had the highest concentration (0.43 mg PRP L-1), while lowest was at site 6

(0.0001 mg PRP L-1).

Figure 18. Average volume-weighted concentration of total reactive P (TRP), dissolved

reactive P (DRP) and particulate reactive P (PRP) during 11 leaching events. Bars reflect the

standard error of the mean.

0

0,05

0,1

0,15

0,2

0,25

0,3

0,35

0,4

0,45

volu

me

-we

igh

ted

P c

on

c. (

mg

L-1)

Site Number

TRP DRP PRP

Results

36

4.4 Relationship between PSD in soil and P concentration in leachate: an evaluation of P saturation protocol in non-acidic and/or non-sandy soil

Total P (TP) and total dissolved P (TDP)

There was a positive non-linear relationship (r2=0.70) between TP concentration in the

leachate and PSD from non-acidic and/or non-sandy soils (Figure 19). The PSD value

ranged from 30% to 92% which corresponds to 0.1 to 2.9 mg L-1 TP in leachate. The

reference soil followed the same curve with that of the other soils used in this study.

Figure 19. Relationship between total P (TP) in leachates (mg L-1) and PSD (%) from the

various soils.

The relationship between PSD (%) and TDP (mg L-1) (Figure 20) appeared to follow the

same trend with TP previously mentioned (Figure 19). Nevertheless, the curve seemed

gradual compared to TP. The fitted curve in TDP followed polynomial pattern (r2 = 0.80),

while in TP followed exponential pattern. The minimum and maximum concentration of TDP

in the leachate ranged from 0.16 mg L-1 to 2.7 mg L-1 with the corresponding PSD value of

30% to 92 %.

y = 0,1599e0,0331x (r² = 0,7074)

0

0,5

1

1,5

2

2,5

3

3,5

4

0 20 40 60 80 100

Tota

l P (

mg

L-1)

PSD (%)

non-acidic and/ornonsandy soil

reference soil

Results

37

Figure 20. Relationship between total dissolved P (TDP) in leachates (mg L-1) and PSD (%)

from the various soils.

Total reactive P (TRP) and dissolved reactive P (DRP)

The relationship between orthophosphate concentration and PSD from acidic sandy soils

used in the protocol was plotted to demonstrate the applicability with other soil property

(Figure 21). A positive non-linear relationship (r2=0.90) was obtained when TRP (mg L-1)

plotted against PSD (%). There was a shift in the curve to the right in samples used in the

study from the curve of acidic sandy soil. At 30 to 42% PSD, concentration of TRP in the

leachates ranged from 0.006 to 0.05 (mg L-1) respectively which is lower than the value of 0.1

mg L-1 set in the protocol as lower limit for eutrophication.

Figure 21. Relationship between total reactive P (TRP) in leachates (mg L-1) and PSD (%)

from non-acidic and/or non-sandy soil. Black curve indicates the relationship between ortho-

P and PSD in acidic sandy soil used in the protocol.

y = -0,0002x2 + 0,0511x - 1,0287 (r² = 0,7967)

0

0,5

1

1,5

2

2,5

3

3,5

4

0 20 40 60 80 100

Tota

l dis

solv

ed

P (

mg

L-1)

PSD (%)

non-acidic and/or non-sandy soil

reference soil

y = 3E-05x2 + 0,0009x - 0,0218 (r² = 0,9038)

0

0,05

0,1

0,15

0,2

0,25

0,3

0,35

0,4

0,45

0 10 20 30 40 50 60 70 80 90 100

tota

l re

acti

ve P

(m

g L-1

)

PSD (%)

acidic sandy soil

non-acidic and/ornonsandy soilreference soil

Results

38

The same curve was observed in the relationship between DRP and PSD (Figure 22) with

that of TRP and PSD mentioned above. There was also, a significant non-linear relationship

between orthophosphate concentration and PSD (r= 0.87).The PSD value ranged from 30%

to 92% with DRP concentrations of 0.004 mg L-1 to 0.24 mg L-1 in soil samples from non-

acidic and/or non-sandy soil. Lower potential for DRP losses was observed via leaching

wherein the values are less than 0.1 mg L-1 when PSD was in between the ranged of 30% to

42%. Above this PSD range, considerable increased in DRP concentration in leachate was

observed. The reference soil also loses considerable amount of DRP in the leachate which

had a value of 0.30 (mg L-1) at PSD of 94%. In both TRP and DRP curve, the reference soil

follows the same curve with the other soil used in the study which was not expected to occur.

Figure 22. Relationship between dissolved reactive P (DRP) in leachates (mg L-1) and PSD

(%) from non-acidic and/or non-sandy soil. Black curve indicates the relationship of ortho-P

and PSD for acidic sandy soil in used in the protocol.

y = 2E-05x2 + 0,001x - 0,0191 (r² = 0,8677)

0

0,05

0,1

0,15

0,2

0,25

0,3

0,35

0 10 20 30 40 50 60 70 80 90 100

Dis

solv

ed

re

acti

ve P

(m

g L-1

)

PSD (%)

acidic sandy soil

non-acidic and/or nonsandy soil

reference soil

Results

39

4.5 Correlation and relationship between soil parameters and various P forms in the leachate

Correlation between soil parameters

Pearson correlation coefficients were calculated between soil pH, organic carbon, clay

content, total P, POX, PLAC, Al+Fe (OX), PSC and PSD in the soil (Table 3). Organic carbon

content was significantly correlated with clay content (r2=0.83; p<0.01). Total P in the soil

was also significantly and positively correlated with POX, PLAC and PSD. There were also a

strong significant positive relations between Al+Fe (OX) concentration in the soil and sorption

capacity (PSC) (r2=0.91; p<0.01). On the other hand, poor correlation was observed between

soil pH and other soil parameters.

Table 3. Correlation matrix (r-values) of various soil parameters.

Soil Parameters Correlation Coefficient

pH COrg Clay TP POX PLAC Al+Fe PSC PSD

Soil pH-KCl 1

COrg(%) 0.12 1

Clay Content (%) 0.19 0.83** 1

Total P (mg kg-1) 0.39 0.52 0.49 1

POX (mg kg-1) 0.41 0.42 0.52 0.93** 1

PLAC (mg kg-1) 0.52 0.36 0.44 0.95** 0.94** 1

Al+FeOX (mg kg-1) -0.14 0.03 0.01 -0.33 -0.12 -0.39 1

PSC (mmol kg-1) -0.18 0.38 0.29 -0.21 -0.09 -0.36 0.91** 1

PSD (%) 0.41 0.15 0.28 0.88** 0.91** 0.96** -0.45 -0.48 1

** Correlation is significant at the 0.01 level (2-tailed).

Correlation between soil parameters and various P forms in the leachate

Correlation coefficients were calculated between the average volume-weighted concentration

of the different P forms in the leachates, the soil pH, OC, clay fraction, total P, POX and PLAC

in the non-acidic and/or non-sandy soils (Table 4). The TP in leachates was significantly

correlated with the properties related to total P (p<0.05), POX (p<0.05), PLAC (p<0.01) and

PSD (p<0.01). A poor correlation was obtained between TP in the leachate with soil pH,

organic carbon and clay content. The same was true with other P forms in the leachate such

as TDP, TRP and DRP. For the TDP, a strong positive correlation was also found in P

content in the soil (total P, POX and PLAC) and PSD (p<0.01). Both TRP and DRP were also

best correlated with P inputs in soil and PSD (p<0.01). In addition, there was a significant

negative relation between TRP and DRP with Al+Fe (OX) concentration and PSC in the soil

(p<0.05).

Results

40

Table 4. Pearson´s correlation coefficient between various P forms in the leachates and

selected soil parameters.

Soil Parameters Correlation Coefficient

TP (mg L-1) TDP TRP DRP

Soil pH-KCl 0.39 0.14 0.02 0.04 Organic Carbon (%) -0.08 -0.04 0.01 -0.08 Clay Content (%) 0.06 0.10 0.25 0.21 Total P (mg kg-1) 0.66* 0.75** 0.86** 0.80** POX (mg kg-1) 0.65* 0.77** 0.81** 0.76** PLAC (mg kg-1) 0.74** 0.85** 0.92** 0.88** Al+Fe (OX) (mg kg-1) -0.43 -0.43 -0.56* -0.59* PSC (mmol kg-1) -0.55 -0.52 -0.56* -0.61* PSD (%) 0.78** 0.89** 0.94** 0.93**

*Correlation is significant at the 0.05 level, **. Correlation is significant at the 0.01 level

Relationship between soil parameters and various P forms in leachates

Multiple regression analysis was performed to investigate the main plus interaction effect of

the selected soil parameters which known to have better correlation on the average volume-

weighted concentrations of the various P forms in the leachate. Result revealed that there

was no significant interaction effect among the different variables investigated with various

forms of P in the leachate (result not presented), thus simple linear regression was done and

regression equation was developed (Table 5). There was a significant increase in the TP in

the leachate with increasing total P in the soil (TP= -0.39 + 0.004 TPsoil; p=0.001). A strong

significant positive relationship was also obtained between TP and PSD with a regression

equation of TP= -0.70 + 0.0353PSD (r2= 0.83). Same relationship was also observed for

TDP and TRP.

In the case of DRP mainly orthophosphate concentration in the leachate, the relationship

was more pronounced in soil parameters related total P content, Al+Fe (OX), PSC and PSD.

There was a significant increase in DRP concentration in the leachate with increasing total P

and PSD in soil (DRP= -0.25 + 0.0004 TPsoil;r2= 0.65 and DRP= -0.08 + 0.0034 PSD; r2=

0.85). Whereas, an increase in Al+Fe (OX) concentration and PSC values in the soil resulted

to a significant decrease in DRP concentration in the leachate (DRP= 0.39 - 0.0001 Al+Fe

(OX) ; r2= 0.35 and DRP= 0.41 - 0.0117 PSC ; r2= 0.38).

Results

41

Table 5. Relationship between selected soil physicochemical properties and average

volume-weighted concentrations of the various P forms in the leachate.

Soil Parameters Regression Equation r2 p-value

TP Total P (mg kg-1) TP= -0.39 + 0.004 TPsoil 0.62* 0.001 Al+FeOX(mg kg-1) TP= 3.44 - 0.0009 Al+Fe 0.19 0.136 PSC (mmol kg-1) TP= 3.90-0.1020 PSC 0.26 0.074 PSD (%) TP= -0.70 + 0.0353 PSD 0.83* 0.000

TDP Total P (mg kg-1) TDP= -1.94 + 0.0034 TPsoil 0.57* 0.003 Al+Fe OX (mg kg-1) TDP= 3.14 - 0.0008 Al+Fe 0.18 0.145 PSC (mmol kg-1) TDP= 3.67 - 0.0947 PSC 0.27 0.066 PSD (%) TDP= -0.51 + 0.0310 PSD 0.79* 0.000

TRP Total P (mg kg-1) TRP= -0.32 + 0.0005 TPsoil 0.74* 0.000 Al+FeOX (mg kg-1) TRP= 0.44 - 0.0001 Al+Fe 0.32 0.046 PSC (mmol kg-1) TRP= 0.44 - 0.0124 PSC 0.31 0.048 PSD (%) TRP= -0.10 + 0.0041 PSD 0.89* 0.000

DRP Total P (mg kg-1) DRP= -0.25 + 0.0004 TPsoil 0.65* 0.001 Al+FeOX (mg kg-1) DRP= 0.39 - 0.0001 Al+Fe 0.35* 0.032 PSC (mmol kg-1) DRP= 0.41 - 0.0117 PSC 0.38* 0.026 PSD (%) DRP= -0.08 + 0.0034 PSD 0.86* 0.000

*=P value < 0.05, significant at 5% level

Discussion

42

CHAPTER 5. DISCUSSION

5.1 General Soil Properties

The soil samples in this study were collected from different sites in Flanders (Figure 6) where

mixed land use (arable/vegetable) predominates. The arable land is mainly used for the

production of maize while horticultural cultivation mostly consists out of vegetable crops e.g.

cabbages. Soil textural analysis and pH measurements revealed that these soils represent

non-acidic and/or non-sandy soils in the area (Table 2). According to Haynes and

Mokolobate (2001) the decomposition of organic residue often causes transitory increase in

soil pH in the soil solid phase. In the case of the organic C content, the resulting values are in

between the range from soils in arable/vegetable East Flanders ranging from 0.7 % to 3.6 %

(Van Den Bossche et al., 2005). For the reference soil, the soil represents light textured,

acidic soil wherein the protocol for P saturation is generally applicable.

5.2 P status of the soils used in the study

Total P and PLAC The total P content in the soil appears to be very high, with an average value of 926 mg P

Kg-1 (Figure 9) slightly larger than organic farms in Flanders (Van Den Bossche et al., 2005)

and much larger than e.g. in arable land in Germany having a value of 599 mg P kg-1

(Godlinski et al., 2004). This implies high intensity of agricultural production in such areas

with the application of fertilizer and manure, resulting in excessive P build-up in soil (Van Den

Bossche et al., 2005). Looking at the textural characteristics in relation to the total P content,

it was revealed that a higher amount of total P corresponds with a higher sand content

except for site 12 which had higher clay content. The reason for the higher total P in clay can

be partly explained by the P released from the weathering of primary P-bearing minerals and

the addition of inorganic and organic fertilizers which combines primarily with clay fractions

resulting in higher P percentage in the clay than that of the coarser particles (Burnham

1982). High amount of total P was also found in the case of the reference soil which had

been classified as sandy loam. This is due to the fact that sandy and loamy sand soils in

Flanders are associated with heavy manure application and intensive vegetable production

(Van Den Bossche et al., 2005).

Discussion

43

The relative amount of plant available P extracted by ammonium lactate (PLAC) in the

samples varied between 84 and 643 (mg P kg-1 soil) which is lower than the values reported

in organic farms in East Flanders ranging from 83 to 814 mg P kg-1 soil (Van Den Bossche et

al., 2005). However these values are still considered very high resulting from long-term

agricultural practices thus increasing the total P content in the soil.

Oxalate extractable P, Fe and Al

The P that initially sorbed into the soil, which assumes to represent P sorbed to amorphous

forms of Al and Fe is measured as oxalate extractable P (Freese et al., 1992). Result from

this study revealed that the 50% of the total P in the soil can be attributed to P oxalic acid

extractable P (Figure 9). The values of the ammonium-oxalate-oxalic acid extractable P in

mmol P kg-1 soil (Figure 10) at the upper 30 cm of the profile averaged to 13.7, which is

comparable with the results found by Van Den Bossche et al. (2005) namely, 14 mmol P kg-1

soil in organic farms in East Flanders. Also, this result was slightly larger than that of the

calcareous soils in Minatoba, Canada averaging to 12.7 mmol P kg-1 soil (Ige et al. 2005).

The Feox ranged between 28.7 to 54.2 mmol P kg-1 soils while AlOX ranged from 10.3 to 35.1

mmol P kg-1 soil, respectively. These values are small when compared with the results

obtained by Igne et al., (2005) who reported FeOX in the range 4.1 and 92.5 mmol kg-1 and

AlOX 5.4 and 72 mmol kg-1 in calcareous soils in Canada. The sorption sites are largely

dominated by extractable FeOX which revealed 50% higher than the AlOX concentration

except in site 12. This is in contrast with what was found by Lookman et al. (1996) in acidic

sandy soil area in Northern Belgium which were dominated by AlOX. The high concentration

of FeOX is possibly related to the origin of specific soil wherein the light alluvial regions

coincided with extremely high FeOX and the coarse sandy soils coincide with very low iron

contents (Lookman et al., 1995).

In the case of site 12, the higher concentration of AlOX probably related to the higher organic

carbon content. Site 12 had the highest organic carbon content among all the soil which

corresponds to 4.16% organic matter. Lookman et al. (1996) found that AlOX is closely

correlated with organic carbon and total carbon content in the soil which indicates that AlOX is

strongly associated with organic matter. Higher concentration of AlOX and FeOX in the

reference soil can be attributed to its acidic sandy soil property.

Discussion

44

P sorption capacity (PSC) and P saturation degree (PSD)

The values for PSC (Figure 11) in the upper 0-30 cm were in a range of 21-35 mmol kg-1 with

an average sorption capacity of 27 mmol kg-1 , lower than the average value of the profile (0-

90 cm) in organic farms in East Flanders (37 mmol kg-1) (Van Den Bossche et al., 2005). The

relationship between clay content and sorption capacity was observed by Mozaffari and Sims

(1994), which revealed that high clay content corresponds to higher P sorption capacities.

Same result was reported in clay soils in Sweden wherein clay soils coincides with higher

sorption capacities and lower PSD ratios than sandy soils (Andersson et al., 2013). Indeed,

the result in this study revealed that clay soils (site 12) had also higher P sorption capacity

than sandy soils (site 1-4 and reference soil). According to Parfitt (1978) clay particles itself

can also possess anion-sorption capacities, as a result of a pH-dependent positive charge at

the platelets´ edges.

Among the recent method developed to predict P losses at various scales, P saturation

degree (PSD) which have been developed by Van der Zee (1990) is commonly used in

several countries in Europe. The method can be used to estimate the risk of P leaching from

noncalcareous sandy soils which postulates that if an orthophosphate concentration of 0.1

mg L-1 also considered as the upper limit for eutrophication, the P saturation of the whole

profile (0-90 cm) should be less than 24%. In The Netherlands, 25% of PSD was defined as

a critical value for sandy soils with shallow groundwater table (Breeuwsma et al. 1995;

Lookman et al., 1996) while in Flanders a higher critical value of 35% was used.

Based on the result from this study, the PSD values in the upper 0-30 cm varied from 30% to

95% with an average value of 55% which was slightly lower than that of 57% profile average

value (0-90 cm) found in agricultural land in West Flanders (De Smet et al., 1996) and

relatively higher than the 39% value for East Flanders (VLM, 1997). The result suggests that

generally, the soil surface have higher risk for P transfer into the underlying soil profile

through subsurface flow. In a more strict sense taking into account the critical value of 24%

(average value for the whole profile), the PSD of the surface soil in this study were

excessively high. However, Igne et al. (2005) found that the ratio of ammonium oxalate-

extractable P to (Al + Fe)OX which has been used to calculate PDS in acid soils, was not

suitable for neutral to alkaline soils. The reference soil indeed has the highest PSD value

(95%) which means that P can be readily removed in the soil and redistributed to the lower

profile.

Discussion

45

5.3 P leaching from non-acidic and/or non-sandy soils

Cumulative P leaching

The cumulative amount of TP in leachates from non-acidic and/or non-sandy soil varied

between a minimum value of 0.19 mg P/soil column (site 7) and a maximum value of 3.98

mg P/soil column (site 3). Physico-chemical properties revealed that soil from site 3 had high

sand content with pH value of 6.7 whereas, soil from site 7 belongs to silty loam and pH

value of 6.3. Moreover, soils from site 3 had higher total P content (1084 mg P kg-1) than that

of in site 7 (719 mg P kg-1). It was also found out that the PSD value in site 3 is much larger

than site 7 with a value of 81% and 30% respectively. The dissimilar patterns in P released

from various soil samples was suspected to be caused by the difference in the

physicochemical properties especially texture and total P content in the soil. Sandy soil had

larger cumulative amount of TP in the leachates whereas loamy to clay soil had smaller

amount. Also, a more gradual release in total P in the leachates was observed in soils with

apparently lower total P content. The rapid and slow release of TP and TDP could be due to

desorption and dissolution processes. Hansen and Strawn (2003) believed that the

dissolution of various P mineral phases, desorption of P from mineral surfaces, and release

of P from organic matter are the common P released mechanisms.

On the other hand, the cumulative amounts of TRP and DRP revealed similar pattern

throughout the leaching period except for site 3 and reference soil. According to Lang and

Kaupenjohann (1999), the change in the release rate may be related to the two-phase

release characteristic of a diffusion controlled process. In the case for the reference soil, the

fast initial and final slow P release is related to the desorption and diffusion-dissolution

reactions; respectively (De Smet et al., 1998).

P leaching potential

The result for P leaching potential in mg mm-1 from non-acidic and/or non-sandy soils as

mentioned in the results (Figure 15 and 16) follows dissimilar curve and the maximum

release of the different forms of P in the leachates doesn’t occur at the same time. In site 13,

12, 9, 11 and 8 the largest release in P was observed during the 5th leaching event which

was more rapid compared to the other soils. This rapid release of TP and TDP in the

leachates could be due to rapid initial desorption of P in the soil resulting from relatively

higher surface coverage of soil with P and the easy replacement of the adsorbed phosphate

ions (Fekri et al., 2011). The P-leaching loss that mainly occurred during 5th leaching

accounting for 25% to 40% of total P leached. Apparently, other soils such as in site 3, 2, 1,

4, 5, and 6 the largest TP and TDP released in the leachates occurred at 7th leaching event.

Discussion

46

McDowell and Sharpley (2003), assumed that P desorption and diffusion from inside of soil

particles possibly was the rate limiting steps in short- and long term P release reactions

respectively.

Moreover, gradual and almost negligible release of P was observed from site 7 and 10. This

gradual reduction in P release rate with time probably resulted from lower total P content in

the soil compared to the other soils and the relatively higher soil pH. In non-acidic soil,

surface adsorption and precipitation takes place causing P retention and depressing P

mobility (Pizzeghello et al., 2011). Moreover, there was a clear influence of texture on

different P release in the leachates. Soils from site 3, 2,1,4,5 and 6 which follow the same

curve with that of the reference soil had also sandy loam texture whereas other soils belong

to more heavy textured soil.

Looking at the potential release of TRP and DRP in the leachates, it follows distinct pattern

with that of TP and TDP. In the figure, a more visible peak can be observed showing the

release of P from the various soils at different leaching events. In case of site 2 rapid release

of TRP was observed during the first and second leaching event accounting to 12% of the

total leachates. This result further supports the findings of many studies for phosphate

desorption wherein a rapid rate is commonly observed at first leaching event, and then

declines slowly as the apparent equilibrium is reached. As stated by some authors, the initial

rapid desorption corresponds to the rapid dissolution of poorly crystalline or the amorphous

phosphates in the soil, which were metastable and ultimately converted to crystalline forms

such as octacalcium phosphate and hydroxyl apatite (Evans and Jurinak 1976; Griffin and

Jurinak 1974).

In contrast, reference soil followed different pattern wherein it rapidly increases at 7th

leaching event and then reached to apparent equilibrium towards the end of leaching event.

The highest P-leaching-loss occurs on the later part of leaching period. Probably, this can be

explained partly by saturation of the soil during the leaching period. Even though suction was

applied to each of the leaching columns at -300 mbar which is equivalent to field capacity

potential for non-sandy soil, ponding water on the top of the leaching columns were observed

that might result in temporary anaerobic condition. In some soil, ponding of water on the

surface persists from the day of irrigation until the next irrigation schedule (3 d). This is

probably due to the dispersion of clays resulting in the clogging of pores in the soil matrix.

This situation is very crucial since some of Fe3+ in the soil can be reduced resulting in the

concurrent release of metal cations and oxyanions (e.g. P) to the soil solution

(Ponnamaperuma, 1972; Lovley, 1991).

Discussion

47

Average losses of different P forms

Taking into account the possible differences in P leaching caused by the differences in

leachate volumes, volume-weighted concentrations were calculated by dividing the total

concentration (mg P) of different P forms by the total volume of leachates (ml). The means of

the volume-weighted concentrations are expressed in mg L-1 (Figure 17 and 18). TP and TDP

were measured by the ICP before and after filtration. TP in the leachates was considerably

higher in the reference soils, site 1, 2 and 3 resulting from higher total P content in the soil,

higher PSD and lower PSC. This result also confirms with most of the findings found in

literature that sandy soils are vulnerable to high leaching losses.

Surprisingly, TDP in the most of the soils were observed to be higher than the TP which is

physically impossible. As shown in Figure 17, the TPP which had negative values were

assumed to be zero. The negative values that were not expected were probably due to the

fact that great variation existed between replicates TP concentration in the leachates.

Apparently, Site 4 to 13 appeared to have comparable results which are lower than the four

soils mentioned above. The reason for this may correspond to the variability of its physico-

chemical properties. Negligible results were also found for most of the soils except for site 2,

3, 12 and 13 for the TPP. The relative lower concentration of particulate P in the leachates

was probably related to lower concentration of colloids and organic matter. The increase of

colloidal concentration increases the particulate P forms in the leachates which also increase

the migration of the TRP and RPP (reactive particulate P) (Zhang 2008).

Filtered and unfiltered samples which were analyzed for molybdate reactive P revealed that

the concentrations of TRP, DRP and PRP ranged from 0.0062 to 0.3348 (mg L-1), 0.0044 to

0.3051(mg L-1) and 0.0001 to 0.0428 (mg L-1), respectively. DRP was found dominant form in

the leachates constituted to 72-99 % of TRP. This can be explained by the high proportion of

inorganic P forms in the soil. Results from TRP are much lower than values found in the

leachates from sandy topsoil in Sweden which ranges from 0.14 mg L-1 to 0.57 mg L-1 (Liu et

al., 2012). The average concentration of TRP in all sites was 0.1263 mg L-1. This value was

slightly lower than the concentration of the clay soil in Sweden (0.1mg L-1) and from silty

loam soil in Florida (ranging from 0.16-0.23 mg L-1).

For DRP almost all the sites exceed current estimates of those required to promote surface

water eutrophication (0.1 mg L-1) except for site 4, 5 and 7 respectively. The results revealed

that the average concentration of DRP in the leachates was 0.11 mg L-1 lower than the value

from clay soil in Sweden (0.14 mg L-1) and in Florida ranging from 0.14 mg L-1 to 0.20 mg L-1

(Liu et al., 2012). In this study, it appears that the TRP and DRP concentration in the

leachate was controlled mainly by total P content in the soil and also with respect to the

Discussion

48

texture. Light textured soils losses considerable concentration of TRP and DRP than heavy

textured soils. McDowell and Sharpley (2001) suggested that the amount of P lost by

leaching is dependent on the concentration of P in the soil and how P saturated is the soil.

5.4 Relationship between PSD in soil and P concentration in leachate: an evaluation of P saturation protocol in non-acidic and/or non-sandy soil

The relationship between TP and PSD can be explained by an exponential equation

suggesting that at PSD of 30% the relative amount of TP loss in the leachates corresponds

to 0.16 mg L-1(Figure 19). The same curve was observed in TDP in the leachate as

presented in Figure 20. Figure 21 and 22 present the relationship between TRP and DRP in

the leachates plotted against PSD in non-acidic and/or non-sandy soil. Also, the relationship

between orthophosphate concentration and PSD which were used in the protocol was plotted

in order to investigate the applicability of the protocol to other soil type.

The right shift in the curve in non-acidic and/or non-sandy soils from the fitted curve of acidic

sandy was found suggesting that the corresponding amounts of orthophosphate leached

from these soils at a given PSD were much lower than the protocol. In the protocol, they

suggest that when PSD exceed 24% the soil is saturated and orthophosphate concentration

will believed to rise above 0.1 mg L-1 (Van der Zee et al. 1990). The result further

demonstrated that the P saturation curve in acidic sandy soil used in the protocol is not

exactly comparable with other soil type such in this case that had non-acidic and/or non-

sandy soils. In this study, a low potential P loss via leaching was found when the PSD was in

between 30% to 42% and there was a sharp increase in P leaching above this PSD value

(Figure 22). In contrast with the result obtained by Maguire et al. (2001) and Mcdowell and

Sharpley (2001a) where a low potential losses via leaching was found when PSD is less than

20% to 25%. In case of soils in UK where they studied the relationship between P release to

solution and PSD, it was revealed that there was a little desorption of P below values

equivalent to a PSD of 20% (Hooda et al. 2000).The lower concentration of both TRP and

DRP with the corresponding higher PSD with respect to the values used in the protocol

probably parameters used to describe PSD by oxalate extraction might overestimates the P

concentration that can be leached. According to Igne et al. (2005), the ratio of POX to (Al +

Fe)OX is not appropriate for neutral to alkaline soils. The lower concentration of TRP and DRP

in the leachates can also be attributed to high soil pH that will results in precipitation of P with

Ca and Mg and low availability to amorphous Al and Fe at higher pH ranges. Another reason

probably caused by the heavier textured soil which has larger capacity to bind P than coarse

textured soil.

Discussion

49

Nevertheless, it is important to remember that the PSD protocol refers to the average

orthophosphate concentration of an entire soil profile (0-90 cm); whereas in this study only

the surface soil was considered. The reference soil follows the same curve with non-acidic

and/or non-sandy soils which was not expected to occur. This is due to the fact that the

relationship of PDS with the corresponding orthophosphate concentration in groundwater

used in the protocol is a rough approximation which may not be applicable at all times.

5.5 Correlation and relationship between soil parameters and various P forms in the leachate

Correlation between soil parameters

The organic carbon content in the soil was positively and significantly correlated with clay

content (p<0.01), this due to the fact that OM is more physically and bio-chemically protected

from decomposition in fine texture soil than that of coarser particles. In agreement with this,

Rice (2002) stated that soils with higher clay content had greater potential to form

aggregates, hence physically protecting the organic matter from further mineralization. Also,

Prasad and Power (1997) reported that under same climatic condition, fine textured soil

contained 2-4 times more organic matter than in coarse texture soil.

The significant positive correlation between total P in the soil with POX, PLAC and PSD was

expected reflecting that indeed, an increase in total P in the soil would also increase the POX,

PLAC and PSD. A strong positive relation between Al+Fe (OX) with the measured PSC

suggesting that, PSC was controlled mainly by non-crystalline Al and Fe (OX). This is in

accordance with the findings of Maguire et al. (2001) which revealed that P sorption was

better correlated with AlOX and FeOX.

Correlation between soil parameters and various P forms in the leachate

Correlations were performed between average volume-weighted concentrations of TP, TDP,

TRP and DRP in leachates and the selected soil properties. The TP and TDP in leachates

were correlated with P inputs and PSD, indicating the influence of these soil parameters to

the consequent increased in the volume weighted concentration of TP. Contrasting result

was obtained by Djodjic et al. (2004) wherein a lack of general pattern was observed when

total P in the topsoil were plotted against TP concentrations in the leachates.

TRP had also a strong positive relation with P inputs whereas the increase in concentration

of Al+Fe (OX) and PSC values will result to a decrease in the TRP that can be leached or

redistributed. For the DRP mainly the concentration of orthophosphate in the leachates, there

was also a strong positive correlation with PLAC. , POX, total P and PSD content in the soil.

Discussion

50

Same result was obtained by Heckrath et al. (1995) wherein the elevated soil P content

correlates with leaching of the (DRP) in tile drains and in lysimeters (Hesketh & Brookes,

2000). However, there was no significant correlation among soil pH and OC with

concentration of various P forms in the leachates indicated that these soil properties were not

the major factors controlling the movement of different P forms in leachates from non-acidic

and non-sandy soils.

Relationship between soil parameters and various P forms in leachates

In order to investigate the significant effects of the soil physicochemical properties on the

various P forms in the leachates multiple regression was performed. Result suggests that

there are no interaction effects within the soil properties in relation to the various P forms in

the leachates which further indicate that many of such soil properties statistically

independent of each other.

The increase in total P and PSD in the soil resulted to an increase concentration of TP, TDP,

TRP and DRP in the leachates. Whereas, an increase in measured Al+Fe (OX) concentration

in the soil and PSC value will result to a decrease in DRP in the leachate. Al+Fe (OX) content

had a significant negative relationship indicating that the increased amorphous Al and Fe in

soils enhanced the capacity of P fixation and somehow limiting the downward transport of P

in the soil. In general, the concentrations of the various P forms in the leachates such as TP,

TDP, and TRP were mainly controlled by P content in the soil and PSD. However the DRP

concentration in the leachate was controlled by P content, Al+Fe (OX), PSC and PSD. This is

in accordance with most of results of leaching studies (McDowell and Sharpley 2001; Zhang

2008 etc.)

GeneralDiscussion and Conclusion

51

CHAPTER 6. CONCLUSION AND RECOMMENDATION

The current P status (total P, PLAC and POX) are very high reflecting the high intensity of

agricultural production in Flanders. In general, the sorption sites in the soils are largely

dominated by extractable FeOX. The results demonstrated that the phosphate leaching

potential from non-acidic and/or non-sandy soils followed dissimilar pattern and the release

of the various P forms in the leachate occurred at different leaching event. This can be

explained by the different physico-chemical properties and the influence of the texture.

Higher concentration of various P forms in leachate was observed in light textured soils than

heavy textured. The average volume-weighted concentration of orthophosphate loss was

dominated by high proportion of inorganic P constituted to 72-99%. Although this result is

only for surface soil, this will be sorbed and redistributed in the deeper soil profile and can be

a source of direct input in case of e.g. drained fields.

There was a right shift in the curve in non-acidic and/or non-sandy from the fitted curve of

acidic sandy and a lower corresponding amount of orthophosphate leached at a given PSD.

Therefore, it can be concluded that the P saturation protocol is not exactly comparable with

other soil type such in the case of non-acidic and/or non-sandy soil and that relationship is a

rough approximation of the corresponding orthophosphate concentration in the leachate at a

given PSD. This result is not directly applicable because in this study only the surface soil

was used and not the entire soil profile. However, these findings can provide valuable

information for the potential of the surface soil to release considerable amount of

orthophosphate. The results from regression analysis revealed no interaction effects within

the soil properties in relation to various P forms in the leachates which further indicate that

many of such soil properties are statistically independent to each other. Correlation analysis

confirmed that the leaching of different P forms is significantly related to P status, PSD, PSC

and Al+Fe (OX) concentration in the soil. Lastly, since the result demonstrated that the P

saturation protocol is not exactly comparable with other soil type, an alternative

measurements in predicting P losses in non-acidic and or/ non-sandy soils can be proposed.

However, it is also important to consider that this study was conducted in a more controlled

environment and thus variability on a field scale is always possible. Most importantly the P

saturation protocol is valid for the entire soil profile with larger number of soils however this

study focused only on the potentiality of the surface soil. Nevertheless, this finding can

provide valuable information when trying to investigate the validity of the P saturation

protocol in non-acidic and/or non-sandy soil on a field scale bases using the needed soil

profile.

References

52

REFERENCES

Andersson, H., Bergström, J., Djodjic, F., Ulén, B. and Kirchmann, H. 2013. Topsoil and subsoil properties influence phosphorus leaching from four agricultural soils. Journal of Environmental Qual. 42:455-463

Anderson, G., and R.E. Malcolm. 1974. Nature of alkali-soluble soil organic phosphates.

Journal of Soil Sci. 25:282–297. Barrow, N.J. 1983. On the reversibility of phosphate sorption by soils. Journal of Soil Sci. 34:

751-758 Barrow, N.J., 1984. Modelling the effects of pH on phosphate sorption of soils. Journal of Soil

Sci. 35(2), 283-297. Beauchemin, S., Simard, R.R. and Cluis, D. 1998. Forms and concentration of phosphorus in

drainage water of 27 tile-drained soils. Journal of Environmental Qual. 27:721_728.

Bolan, N.S., N.J. Barrow and A.M. Posner. 1985. Describing the effect of time on sorption of

phosphate by iron and aluminum hyoxides. Journal in Soil Sci. 36:187-197 Brady, N.C., and R.R. Wiel.1999. The Nature and Properties of Soils, 13th ed.

Prentice-Hall, Inc. A simon and Schurfer Comp., New Jersey. USA

Breeuwsma, A., J.G.A Reijerink, and O.F. Schoumans. 1995. Impact of manure on accumulation and leaching of phosphate in areas of intensive livestock farming. p. 239-249. In K. Steele (ed) Animal waste and the land-water interface. CRC Press, Boca Raton, FL.

Burnham, C.P. and Lopez-Hernandez, D.1982. Phosphate retention in different soil

taxonomic classes. Soil Sci., 134: 376-380.

Chardon, W.J. and O.F. Schourmans. 2007. Soil texture effects on the transport of

phosphorus from agricultural land in river deltas of Northern Belgium, The Netherlands and Norh-West Germany. Soil Use and Management 23:16-24.

Chen G.C., He Z.L., Stoffella P.J., Yang X.E., Yu S., Calvert D. 2006. Use of dolomite

phosphate rock (DPR) fertilizers to reduce phosphorus leaching from sandy soil. Environmental Pollution 139:176-182. DOI: 10.1016/j.envpol.2004.12.016.

Culley, J.L.B., E.F. Bolton, and V. Bernyk. 1983. Suspended solids and phosphorus loads

from a clay soil: I. Plot studies. Journal in Environmental Qual. 12. 493-498. De Bolle, S., De Neve., S. and Hofman, G. 2013. Rapid redistribution of P to deeper soil

layers in P saturated acid sandy soils. Soil Use and Management, (Suppl. 1), 76-82

Del Campillo, M.C., Van der Zee, S.E.A.T.M. & Torrent, J. 1999. Modelling long-term

phosphorus leaching and changes inphosphorus fertility in excessively fertilized acid sandy soils. European Journal of Soil Science, 50, 391–399.

References

53

De Schrijver, A., Vesterdal, L., Hansen, K., De Frenne, P., Augusto, L., Achat, L.D., Staelens,J., Baeten, L., De Keersmaeker, L., De Neve, S. and Verheyen, K. 2012. Four decades of post-agricultural forest development have caused major redistrbution of soil phosphorus fractions. Oecologia, 169:221-234.

De Smet, J., Vanderdeelen, J. & Hofman, G.1998. Effect of soil properties on the kinetics of

phosphate release. Communications in Soil Science and Plant Analysis, 29(11-14), 2135-2147.

De Smet, J., G. Hofman, J. Vanderdeelen, M. VanMeirvenne, and L. Baert. 1996. Phosphate

enrichment in the sandy loam soils of West-Flanders, Belgium. Fertilizer Res. 43:(3)209-215.

Djodjic, F., Börling, K., Bergström, L., 2004. Phosphorus leaching in relation to soil type

and soil phosphorus content. Journal of Environmental Quality 33, 678–684. Egnér, H., H. Riehm, and W. Domingo. 1960. Untersuchungen über die chemische

Bodenanalyse als Grundlage für die Beurteiling des Nährstoff zustandes der Böden: II. Chemische Extraktionsmethoden zur Phosphor und Kaliumbestimmung. Lantbrukshoegsk. Ann. 26:199–215.

Elkhatib, E.A. and Hern, J.L. 1988.Kinetics of phosphorus desorption from Appalachian soils.

Soil Sci 145:222–229 Esteller, M.V., Martinez, V.H., Garrido, S., and Uribe, Q. 2009. Nitrate and phosphate

leaching in a Phaeozem soil treated with biosolids, composted biosolids and inorganic fertilizers. Waste Manage. 29:1936–1944.

European Environment Agency.1999. Nutrients in European ecosystems. Environmental

Assessment Rep. 4. European Environment Agency, Copenhagen. Evans, R.L. and Jurinak, J.J. 1976. Kinetics of phosphate release from a desert soil. Soil Sci.

121: 205-211. Fekri, M., Gorgin, N., and L., Sadegh. 2011. Phosphorus desorption kinetics in two

calcareous soils amended with P fertilizer and organic matter. Environmental Earth Science, 64:721-729

Freeman, J.S., and D.L. Rowell. 1981. The adsorption and precipitation of phosphate onto

calcite. Journal of Soil Sci 32:75-86. Freese, D., Van Der Zee, S.E.A.T.M. and Van Riemsdijk, W.H. 1992. Comparison of different

models for phosphate sorption as a function of the iron and aluminum-oxides of soils. Journal of Soil Science, 43, 729–738.

Frossard, E., Brossard, M., Hedley, M.J., and Metherell, A. 1995.Reaction controlling the

cycling of P in soil. In: Tiessen H (ed) Phosphorus in the global environment. Wiley, Chichester, pp 107–138

Gee, G. W. and Dani, O. 2002. Particle-size analysis. 2nd ed ed. In Methods of soil analysis.

,eds. J.Dane & G. Topp, SSSA. Madison, WI, pp. 255-293. Godlinski, F., Lienweber, P. Meissner, R., and J. Seeger. 2004. Phosphorus status of soil

and leaching losses: results from operating and dismantled lysimeters after 15 experimental years. Nutrient Cycling in Agroecosystem, 68:47-57.

References

54

Greenland, D.J., and M.H.B. Hayes. 1981.The Chemistry of Soil Processes. John Wiley &

Sons, Ltd. (printed at the Pitman Press, Bath, Avon.

Griffin, R.A., and Jurinak, J.J.1974. Kinetics of the phosphate interaction with calcite. Soil Sci

Soc Am Proc 38:75–79 Gustafsson, J.P., Luhuvilo, M., and Kergoat, K., 2012. The pH dependence of phosphate

sorption and desorption in Swedish agricultural soils. Geoderma 189-190, 304-311

Hansen, J.C. and Strawn, D.G. 2003. Kinetics of phosphorus release from

manure-amended alkaline soil. Soil Sci 168:869–879 Hartikainen, H., Rasa, K. And Withers, P.J.A. 2010. Phosphorus exchange properties of

European soils and sediments derived from them. European Journal of Soil Science, 61:1033-1042.

Haygarth, P. M. and Sharpley, A.N. 2000. Terminology for phosphorus transfer. Journal of

Environmental Qual. 29, 10–15. Haynes, R.J., and M.S. Mokolobate. 2001. Amelioration of Al toxicity and P defi ciency in

acid soils by additions of organic residues: A critical review of the phenomenon and the mechanisms involved. Nutr. Cycling Agroecosyst. 59:47–63.

Heckrath, G., Brookes, P.C., Poulton, P.R. and Goulding, K.W.T.1995. Phosphorus leaching

from soils containing different phosphorus concentrations in the Broadbalk experiment. Journal of Environmental Qual, 24, 904–910.

Hens, M., and O.F. Schoumans. 2002. The role of soil processes: Synthesis of actions, In

W.J. Chardon and O.F. Schoumans (eds.) Phosphorus losses from agricultural soils: Processes at field scale. COST Action 832. Alterra, Wageningen, the Netherlands.

Hesketh, N. and Brookes, P.C. 2000. Development of an indicator for risk of phosphorus

leaching. Journal of Environmental Qual, 29,105–110. Hillel, D. 1998. Environmental Soil Physics. Academic Press. California, USA. Hooda, P.S., A.R. Rendell, A.C. Edwards, P.J.A. Withers, M.N. Ait- ken, and V.W. Truesdale.

2000. Relating soil phosphorus indices to potential phosphorus release to water. Journal of Environmental.Qual., 29:1166–1171

Ige, D.V., Akinremi, O.O, D.N., Flaten, Ajiboye, B., and M.A., Kashem. 2005. Phosphorus

sorption capacity of alkaline Manitoba soils and its relationship to soil properties. Canadian Journal of Soil Sci., Downloaded from pubs.aic.ca by UNIVERSITEIT GENT on 07/29/13

Igne, D.V., Akinremi, O.O. and Flaten D.N. 2005. Environmental index for estimating the risk

of phosphorus loss in calcareous soils of Manitoba. Journal of Environmental Qual. 34:1944–1951

Jalali, M. and Karamnejad, L. 2011. Phosphorus leaching in a calcareous soil treated with

plant residues and inorganic fertilizer. Journal of Plant Nutrition and Soil Sci 174:220–228

References

55

Jalali, M. and Naderi, E. 2012. The impact of acid rain on phosphorus leaching from sandy

loam calcareous soil of western Iran. Environ Earth Sci. 66:311-317 Jiao, Y., Whalen, Y.J.J.K. and W.H., Hendershot 2007. Phosphate sorption and release in a

sandy-loam soil as influenced by fertilizer sources. Soil Science Society of American Journal, 71:118-124.

Jouany, C., Cruz, P., Daufresne. T., and Duru, M. 2011. Biological phosphorus cycling in

grasslands: interactions with nitrogen.Phosphorus in action. Soil Biol 100:275–294.

Kang, J., A. Amoozegar, Hesterberg, D. and Osmond, D.L. 2011. Phosphorus leaching in a

sandy soil as affected by organic and inorganic fertilizer sources. Geoderma 161: 194-201.

Karathanasis, A.D., Shumaker, P.J., 2009. Organic and inorganic phosphate interactions

with soil hydroxy-interlayered minerals. Journal of Soils and Sediments 9, 501–510.

Kleinman, P.J.A., Bryant. R.B. and Reid, W.S. 1999. Development of pedotransfer functions

to quantify phosphorus saturation of agricultural soils. Journal of Environmental Quality 28, 2026–2030.

Kolahchi, Z. and Jalali M. 2013. Phosphorus movement and retention by two calcareous

soils. Soil & sediment contamination. 22:21-38. Koopmans, G.F., Chardon, W.J. & McDowell, R.W. 2007. Phosphorus movement and

speciation in a sandy soil profile after longterm animal manure applications. Journal of Environmental Quality, 36:305–315.

Koopmans, G.F., W.J. Chardon, and C. van der Salm. 2005. Disturbance of water-

extractable phosphorus determination by colloidal particles in heavy clay soil from the Netherlands. Journal of Environmental Quality, 34:1446-1450.

Koopmans, G.F., W.J. Chardon, J. Dolfing, O. Oenema, P. van der Meer, and W.H. van

Riemsdijk. 2003. Wet chemical analysis and phosphorus-31 nuclear magnetci resonance analysis of phosphorus speciation in a sandy soil receiving long-tern fertilizer or animal manure applications. Journal of Environmental Qual. 32:287-295.

Kuo, S., and Lotse, E.G. 1973. Kinetics of phosphorus adsorption and desorption by

hematite and gibbsite. Soil Sci., 116:400-406. Lang, F., Kaupenjohann, M., 1999. Molybdenum fractions and mobilization kinetics in acid

forest soil. Journal in Plant Nutrition and Soil Sci. 162: 309-314. Liu, J., Aronsson, H., Ulen, B., and L., Bergstrom. 2012. Potential phosphate leaching from

sandy topsoils with different fertilizer histories before and after application of pig slurry. Soil Use and Management, 28:457-467.

Lindsay, W. L.1979. Chemical equilibria in soils. John Wiley & Sons, New York, USA.

References

56

Lookman, R., Freese, D., Merckx, R., Vlassak, K. and Vanriemsdijk, W. H.1995. Long-Term Kinetics of Phosphate Release from Soil. Environmental Science & Technology, 29(6), 1569-1575.

Lookman, R., Vandeweert, N., Merckx, R. and Vlassak, K. 1995. Geostatistical Assessment

of the Regional Distribution of Phosphate Sorption Capacity Parameters (Fe-Ox and Al-Ox) in Northern Belgium. Geoderma, 66(3-4), 285-296.

Lookman, R., Jansen, K., Merckx, R. and Vlassak, K. 1996. Relationship between soil

properties and phosphate saturation parameters - A transect study in northern Belgium. Geoderma, 69(3-4), 265-274.

Lovley, D. R.: 1991, Microbiol. Rev. 55, 259. Mack, W.N., and Leistikow, E.A. 1996. Sands of the world. Sci Am. 275, 44-49. Madrid, L., and P. De Arambarri. 1985. Adsorption of phosphate by two iron oxides in relation

to their porosity. Journal of Soil Sci. 36:523-530. Maguire, R.O., R.H. Foy, J.S. Bailey, and J.T. Sims. 2001. Estimation of the phosphorus

sorption capacity of acidic soils in Ireland. European Journal of Soil Scinece, 52: 479-488.

McDowell, R.W., and Sharpley, A.N. 2003. Phosphorus solubility and release kinetics as a

function of soil test P concentration. Geoderma, 112:143-154. McDowell, R.W., and Sharpley, A.N. 2001. Phosphorus losses in subsurface flow before and

after manure application to intensively farmed land. Sci. Tot. Environ. 278:113–125.

McDowell, R.W., and A.N. Sharpley. 2001a. Approximating phosphorus release from soils to

surface runoff and subsurface drainage. Journal of Environmental Qual. 30:508–520.

Mozaffari, M. and Sims, J.T. 1994. Phosphorus availability and sorption in an Atlantic coastal

plain watershed dominated by animal-based agriculture. Soil Sci., 157: 97-107. Murphy, J. and Riley, J.P.1962. A modified single solution method for the determination of

phosphate in natural waters. Anal Chim Acts 27:31–36. Olsen, S.R. and Sommers, L.E.1982. Phosphorus. In: Methods of soil analysis. Part 2:

Chemical and microbial properties, ed Al Page, ASA & SSSA Madison WI USA

pp 403-430

Papanikolaou, N., Britton, A.J., Helliwell, R.C., and Johnson, D. 2010. Nitrogen deposition,

vegetation burning and climate warming act independently on microbial community structure and enzyme activity associated with decomposing litter in low-alpine heath. Global Change Biol.16:3120–3132.

Parfitt, R.L. 1978. Anion adsorption by soils and soil materials. Advances in Agronomy

Journal, 30: 1-50. Prasad, R. & Power, J.F. 1997. Soil fertility management for sustainable agriculture. New

York, USA, Lewis Publishers. 356 pp.

References

57

Pierzynski, G. M. , Sims, J.T., and Vance, G.F.2005. Soils and environmental quality, 3rd ed.,

Lewis Publishers, Boca Raton, FL. Pierzynski, G. M. 1994. Soils and environmental quality. Lewis Publishers, Boca Raton, FL. Pizzeghello, D., Berti, A., Nardi, S., and Morari, F. 2011. Phosphorus forms and P-sorption

properties in three alkaline soils after long-term mineral and manure applications in north-eastern Italy. Agriculture, Ecosystem and Environ., 141: 58-66.

Ponnamaperuma, F.N.1972. The chemistry of submerged soils. Adv Agronomy 24:29-96. Qin H.L., Quan Z., Liu X.L., Li M.D., Zong Y., Wu J.S. and Wei W.X. 2010.Phosphorus

Status and Risk of Phosphate Leaching Loss From Vegetable Soils of Different Planting Years in Suburbs of Changsha, China. Agricultural Sciences in China 9:1641-1649. DOI: 10.1016/s1671-2927(09)60261-3.

Raghothama, K.G. 1999. Phosphate acquisition. Annu Rev Plant Physiol Plant Mol Biol

50:665–693. Rice, C.W. 2002. Organic matter and nutrient dynamics. In: Encyclopedia of soil science, pp.

925–928. New York, USA, Marcel Dekker Inc. Richter, D.D., Allen, H.L., Li, J., Markewits, D., and Raikes, J. 2006. Bioavailability of slowly

cycling soil phosphorus: major restructuring of soil P fractions over four decades in an aggrading forest. Oecologia 150:259–271.

Sardans, J., Penuelas, J., and Estiarte, M. 2006. Warming and drought alter soil

phosphatase activity and soil P availability in a Mediterranean shrubland. Plant Soil 289:227–238.

Sarao, G.S., and Biswas, C.R. 1989. A semi-descriptive model for predicting residual-P from

fertilizer P applications. Fertilizer Research, 19: 121-126. Scheel, K.C. 1936. Colorometric determination of phosphorus acid in fertilizers with the

Aulfrich photometer. Zeitschrift fur Analytische Chemie 105, 256-259. Schindler, D. W.1977. Evolution of phosphorus in lakes. Science (Washington D.C.)

195:260-262 Schoumans, O.F., and P. Lepelaar.1995. Emissions of pesticides and nutrients in the flower

bulb culture. (In Dutch.) Rep. No. 387.1, Staring Centre, Wageningen, the Netherlands.

Schwertmann, U. 1964. Differenzierung der Eisenoxide des Bodens durch Extraktion mit

Ammoniumoxalat-Losung. Zeitschrft Pflanzenerzarung, Duning und Bodenkunde, 105, 194–202.

Serrano, J.M., Peca, J.O., da Silva, J.R.M. Shahidian, S. and Carvalho, M. 2011.

Phosphorus dynamics in permanent pasture: differential fertilizing and animal effect. Nutrient Cycling in Agroecosystems 90:63-74. DOI: 10.1007:s10705-010-9412-2.

References

58

Shariatmadari, H., Shirvani, M. and Jafari, A. 2006. Phosphorus release kinetics and availability in calcareous soils of selected arid and semiarid toposequences. Geoderma, 132: 261-272.

Sharpley, A.N., R.W., McDowell and P.J.A., Kleinman. 2001. Phosphorus loss from land to

water: integrating agricultural and environment management. Plant and Soil. 237: 287-307.

Simard, R.R., S. Beauchemin, and P.M. Haygarth. 2000. Potential for preferential pathways

of phosphorus transport. Journal of Environmental Qual. 29:97-105. SPSS Inc., 1999. SPSS, Version 19.0. Chicago, IL. Sims, J.T., Simard, R.R., Joern, B.C., 1998. Phosphorus loss in agricultural drainage:

Historical perspective and current research. Journal of Environmental Qual., 27(2), 277-293.

Stevenson, F.J. and Cole, M.A.1999. Cycles of soil: carbon, nitrogen, phosphorus,

sulfur,,micronutrients. Wiley New York. Tan, K.H. 2011. Principles of Soil Chemistry. 4th ed. Taylor & Francis Group, New York, USA

(362 pp). Tiessen, H., and Moir J.O. 2008. Characterization of available P by sequential extraction. In:

Carter MR (ed) Soil sampling and analysis, 2nd edn. Can Soc Soil Sci/Lewis), New York, pp 293-306.

Toor, G.S., and Bahl, G.S. 1999. Kinetics of phosphate desorption from different soils as

influenced by application of poultry manure and fertilizer phosphorus and its uptake by soybean. Bioresour Technol 69:117-121.

Turner, B.L., M.A. Kay, and D.T. Westermann. 2004. Colloidal phosphorus in surface runoff

and water extracts from semiarid soils of the western United States. Journal of Environmental Quality. 33:1464-1472.

Ulén, B. 2006. A simplifi ed risk assessment for losses of dissolved reactive phosphorus

through drainage pipes from agricultural soils. Acta Agric. Scand. B 56:307–314. Vandecar, K.L., Lawrence, D., Wood, T., Oberbauer, S.F., Das; R. Tully, K., and

Schwendenmann, L. 2009. Biotic and abiotic controls on diurnal fluctuations in labile soil phosphorus of a wet tropical forest. Ecology 90:2547–2555.

Van Den Bossche, A., De Neve, S. and Hofman G., 2005. Soil phosphorus status of organic

farming in Flanders: an overview and comparison with the conventional management. Soil Use and Mgmt. 21, 415-421.

Van der Zee, S.E.A.T.M., Van Riemsdijk, W.H. and De Haan, F.A.M. 1990. The protocol for

phosphate saturated soils. Part I: Explanation. Department of Soil and Plant Nutrition Agricultural UniversityWageningen. [In Dutch]

Van der Zee, S.E.A.T.M., Van Riemsdijk, W.H. and De Haan, F.A.M., 1990a. Het protokol

fosfaatverzadigde gronden. Deel I: Toelichting. Vakgroep Bodemkunde en Plantenvoeding, Landbouwuniversiteit Wageningen.

References

59

Van der Zee, S.E.A.T.M., Van Riemsdijk, W.H. & De Haan, F.A.M. 1990b. Het protokol fosfaatverzadigde gronden. Deel II :Technische uitwerking. Vakgroep bodemkunde en plantenvoeding.Landbouwuniversiteit Wageningen, Wageningen, the Netherlands.

Van der Zee, S.E.A.T.M. and Van Riemsdijk, W.H. 1988. Model for long-term phosphate

reaction-kinetics in soil. Journal of Environmental Quality, 17, 35–41. VLM (Vlaamse Land Maatschappij [Flemish Land Agency]). 2007. Afbakening van de

fosfaatverzadigde gebieden in Vlaanderen op basis van een kritische fosfaatverzadigingsgraad van 35%.

VLM (Vlaamse Land Maatschappij [Flemish Land Agency]) 1997. Phosphate

saturation in sandy soils in Flanders. VLM Meise. [In Dutch] Van Meirvenne, M., Tariku, M., De Neve, S., Hofman, G.,Salomez, J. & De Bolle, S. 2008.

Afbakening van de fosfaatverzadigde gebieden in Vlaanderen op basis van een kritische fosfaatverzadigingsgraad van 35%. Ghent University, Ghent.

van Riemsdijk, W.H., L.J.M. Boumans, and F.A.M. de Haan. 1984a. Phosphate sorption by

soils: I. A model for phosphate reaction with metal-oxides in soil. Soil Science Society of American J. 48:537-541.

van Riemsdijk, W.H., and F.A.M. de Haan. 1981. Reaction of orthophosphate with a sandy

soil at constant supersaturation. Soil Science Society of American J. 45:261-266. van Riemsdijk, W.H., and J. Lyklema. 1980a. The reaction of phosphate with aluminum

hydroxide in relation with phosphate bonding in soils. Colloids Surfaces 1:33-44. van Riemsdijk, W.H., and J. Lyklema. 1980b. Reaction of phosphate with gibbsite (Al(OH)3)

beyond the adsorption maximum. Journal of Colloid Interface Sci., 76:55-66. van Riemsdijk, W.H., A.M.A. van der Linden, and L.J.M. Boumans. 1984b. Phosphate

sorption by soils: III The P diffusion-precipitation model tested for three acid sandy soils. Soil Science Society of American J. 48:545-548.

Walkley, A., and Black, I.A.1934. An examination of Degtjareff method for determining soil

organic matter and a proposed modification of the chromic acid titration method. Soil Science, 37: 29-37.

Whalen, Y.J.J.K. and W.H., Hendershot. Phosphate sorption and release in a sandy-loam

soil as influenced by fertilizer sources. Soil Science Society of American J. 71:118-124.

White, R.E. 2006. Principles and practice of Soil Science. 4th ed. Blackwell Science Ltd.,

Oxford, UK. 354 pp. Willet, I.R., C.J. Chartress, and T.T. Nguyen. 1988. Migration of phosphate into aggregated

particles of ferrihydrite. Journal of Soil Sci. 39:275-282. Williams, P.H., and Haynes, R.J.1990. Influence of improved pastures and grazing animals

on nutrient cycling within New Zealand soils. New Zealand Journal of Ecol 14:49–57.

References

60

Yagi, S., Fukushi, K., 2011. Phosphate sorption on monohydrocalcite. Journal of Mineralogical and Petrological Sciences, 106, 109–113.

Zhang, M.K. 2008. Effects of soil properties on phosphorus subsurface migration in sandy

soils. Pedosphere. 18(5): 599-610.