5. Buffering capacity

• Soils high in SOM and clay minerals are more resistant to change in pH

• Sandy soils and highly weathered soils are least buffered

• Base Saturation = exchangeable bases CEC

BS = (exch Ca + Mg + Na + K)

(exch Ca + Mg + Na + K + Al + H)

6. Lime Requirement

• Amount of CaCO3 needed to increase the pH of the soil to an optimum pH

• Depends on soil mineralogy, % clay fraction, % OM, cultivation practices (leaching, fertilization, etc)

• Variety of liming materials

• Only practical to raise pH to ~6 (KCl-extractable acidity is ~0)

Lime material

• CaCO3 calcic limestone• CaMg(CO3)2 Dolomite• CaO: Quick lime• CaOH calcium hydroxide• Byproducts: ground shells, cement factory

waste• Consume H+ and provide an alternative

cation for the exchange phase (Ca or Mg)

• Lime characteristics cost purity speed of effect (fine ground vs coarse) ease of handling

• Lime requirement depends on pH, CEC and buffer capacity of the

soil

• Lime Application: small amounts split and incorporated into the soil

Liming to increase soil pH

http://wwwlb.aub.edu.lb/~webeco/SIM215acidsoilsandlimimg_files/image002.gif

To increase pH in a well-buffered soil requires much more lime than in a sandy or weathered soil; more lime required to go from 6 to 7 than

from 4-5

• a major threat to agricultural productivity in arid regions

• One-third of the world’s irrigated land is salinized

• More than one million hectares affected

• Salts cause both osmotic effects and specific ion toxicity

Natural causes:

• Weathering of parent material with little or no

leaching

• more salinity in hot, dry regions (climate + irrigation)

• Accumulation of salts in enclosed drainage basins

• Coastal spray and inundation

• High water tables (capillary rise brings salts to the

surface)

Sources of Soil Salinity

Anthropomorphic causes of Salinity

• Irrigation Not just with poor quality water Inadequate leaching and drainage

• Acid rain (enhances weathering; salt production)

• Application of fertilizers, manures, biosolids, composts which are often saline

Salt-impacted agricultural soils

Measurement of Salinity• Electrical Conductivity (EC) is an measure of the flow of electricity

through a material • Saline soils and salty water conduct more electricity than nonsaline

soils or pure water. It is the ions that pass or conduct electricity from one ion to the next. As salt concentration increases, EC increases. Acidic or low pH solutions also exhibit high EC Expressed in dS/m (SI units) or mmhos/cm (old unit) dS/m = mmhos/cm

• Use an EC ‘bridge’ or meter to measure how well water extracted from soil can conduct electricity:

Dissolved ions and two metal plates

Voltage is applied & ions move toward oppositely charged plates

EC values for common waters (dS/m)

• Deionized water: 0.0005 to 0.002

• NMSU tap water: 0.5 to 1.0 (rarely this high)

• Seawater: 40 to 55

• Good irrigation water: < 0.7 Rio Grande N of Las Cruces is good Quality decreases (EC increases) downstream

• Poor quality irrigation water: > 3

• Saturated Paste Extract EC of saline soils: ≥ 4

Relate I and EC

• Ionic strength is a parameter that estimates the interaction between ions in solution.

• Salts are ionic solids that dissolve in water

• Empirical relationships: I = 0.0127 EC (in arid and semi-arid regions) I = 0.014 EC (in humid regions) Easier to calculate because you don’t need full

composition of solution

Instruments to measure ECConductivity meter Electromagnetic induction

Time Domain Reflectometry

Measurement of Salinity – TDS

TDS – Total dissolved solids Cations + anions + anything <2 microns Good quality water has <500 mg/L or ppm TDS measure using gravimetry or EC

• Evaporate water off and accurately weigh the residue• Problematic due to hydration and volatilization

EC (dS/m) x 640 ≈ TDS (mg/L) • TDS ‘meters’ are really EC meters with conversion factor

Osmotic potential (OP)• That portion of the Total Soil Water Potential due to

the presence of solutes in soil water• Salts reduce the water potential by inhibiting the

movement of water molecules• OP (kPa) ≈ -0.40 x EC (dS/m)

Pure Water OP = 0

Water Diluted by a Solute (Red Spheres) OP is negative

http://www.genomestudy.com/BIO196/Lab4/osmosis.gif

Water moves from regions of higher water potential (pure water = 0) to regions of lower water potential

(saline water = -x) across a semi-permeable membrane (e.g., plant roots)

http://www.uic.edu/classes/bios/bios100/lecturesf04am/changesinwaterpotential.jpg

Atm = -20,000 kPa

Leaf = -500 kPa

Root = -70 kPa

Soil = -50 kPa

ESP > 15% Dispersed soil

ESP < 15% Flocculated soil

Good soil structure

Na saturated Ca saturated

Sodium Hazard – dispersion and Na toxicity

Sodicity Measurement

SAR = [Na+] [(Ca+2 + Mg+2) / 2]½ units = mmolc/L

(old units = meq/L)

The concentration of cations in the soil saturated paste extract (solution phase)

• Sodium Adsorption Ratio = “SAR”

SAR = [Na+] units = mmol/L [Ca+2 + Mg+2]½

Sodicity Measurement• Exchangeable sodium as a percent of the total CEC

= “ESP”

ESP = exchangeable Na X 100 units = cmolc/kg soil CEC (old units = meq/100g)

The concentration of cations on the soil exchange phase

http://www.terragis.bees.unsw.edu.au/terraGIS_soil/images/ec-ncps-soil_solution-4.jpg.jpg

Low ESP High ESP

Nomogram for estimating ESP to/from SAR (Handbook 60, U.S. Salinity Lab, 1954)

Saline and Sodic Soils

Halophytes are plants which tolerate or even demand sodium chloride concentrations in the soil water they absorb.

Saline Soils

• Most common salt problem and the easiest to correct • EC > 4.0 dS/m; SAR < 13 or ESP < 15 • May be called ‘white alkali’ because of the accumulation

of salts on the surface

• Typical ions: Ca+2, Mg+2, K+, Na+; SO4-2, Cl-, HCO3

-

Soil Chemistry of Saline Soils • pH is usually 7.8 - 8.2 but can also be acidic

Soil Physical Condition • Soil physical condition is generally good (well

aggregated with good internal fluid movement)• Crusting may be a problem

Plant Growth Problems • Osmotic potential contributes significantly to total

water potential; inability of plant to extract water is the major plant growth problem on saline soils.

• Toxic ions can be a problem (Na+, Cl-, HCO3-)

• Plants differ in their tolerance to salt

Increasing NaCl concentration

http://www.isb.vt.edu/news/2001/news01.dec.html

Chile pepper response to salinity

Sodic Soils

• Less common problem and much more expensive to correct.

• EC < 4.0 dS/m; SAR > 13; ESP >15 • May be called black alkali because of the

accumulation of humic material (black color) on the surface (Na causes organic matter to disperse)

• Too much Na is the problem

• Typical ions are Na+, Ca+2, Mg+2, K+ ; Cl-, SO4-2,

HCO3-, CO3

-2

Soil Chemistry of Sodic Soils • pH is usually 8.5 or greater because Na is high (Na is a

strong base-forming cation)

Soil Physical Condition • Soil physical condition is poor (Na disperses the colloids

resulting in the loss of aggregation) • Very slow or no fluid exchange

Plant Growth Problems

• Poor aeration and standing water

• Toxic ions (Na+, Cl-, and HCO3-) can be a problem

• Some plants may be tolerant to poor fluid exchange and high Na

Saline-Sodic Soil

• EC > 4; SAR > 13 ESP > 15

• Combination of problems found in saline and sodic soils

• Soil physical condition is more like a saline soil in that drainage is normal

15 year old pecans south of Las Cruces that are stunted by sodium

This saline-sodic soil near Vado is one of the worst in the Mesilla Valley. A heavy clay layer keeps water from freely draining.

The SAR of this soil is about 25 and the EC is about 15 dS/m.

The white salt is mainly NaCl and Na2SO4.

Reclamation of Saline and Sodic Soils

Saline Soils• Leach with good water • The leaching requirement (LR) can be used

Sodic Soils

• Exchange Na with Ca and leach. CaSO4, So, H2SO4 are used. The H2SO4 dissolves CaCO3 in the soil to produce Ca+2 and the So is converted into H2SO4 in the soil by microorganisms.

• Leach with good water• Growth of plants (barley, triticale, halophytes) that can

withstand poor aeration and high levels of Na. • Can take several years.

Problems caused by Salinity and Sodicity

• Osmotic effects: by lowering the osmotic potential and making it difficult for plant to extract water

• Specific Ion effect: Na, Cl, H4BO4, HCO3 can be toxic and can cause imbalances in the uptake and utilization of other cations

• Soil structure deteriorates and aeration decreases• Plants get stunted and exhibit small dark bluish green

leaves

Leaching Requirement:

Amount of water needed to remove excess salts from saline soils

LR = ECiw/ECdw

ECiwis EC of irrigation water

ECdw is maximum acceptable salinity of the soil solution

If root zone needs 15 cm of water to be fully wetted, then amount of water to be leached = 15*0.4= 6 cm

So supply 15 + 6 or 21 cm of water total to irrigate and leach

Example: if EC of irrigation water is 2.5 dS/m and crop can tolerate an EC of 6 dS/m. What is LR?

LR = 2.5/6 = 0.4