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Nitrogen and Sulfur

Economy of Soils

Introduction

• Nitrogen and Sulfur share important

characteristics

• Essential plant nutrients ·

• Both usually found in organic forms

• Present in soils and plants as anions (negative charge)

• Can cause serious environmental problems

Nitrogen in Soils

Usually the most limiting plant nutrient

Most N in soils is found in organic matter

• 2-3% of organic N released each year by mineralization

as NH4+ (ammonium}

• N returned to organic form by immobilization

• these processes are carried out by microbes

• extent to which process(es) occur is dependent on conditions

favoring overall microbial activity

Nitrogen in Soils

• Difficult to store large quantities of applied N

. in soils from year to year

• removed by plants, leaching, volatilization, etc

• Ratio of N in to N out affects potential biomass

accumulation

• Organic N materials provide slower release of N but

require larger applications to meet plant needs

Additions of Nitrogen to Soils

• Atmospheric Nitrogen

• 78% N in atmosphere - 2000 times CO2

amount

• In the form of N2

• Joined with triple bond so very stable

• Inert gas

• Is made available (to plants) through ...

• Lightning

• Atmospheric N compounds in precipitation

• Industrial fixation {fertilizer production)

• Biological fixation

Fates of N Added to Soils

N in soil may come from residues, fertilizers, manures, precipitation

• As fertilizer mainly in NH4+

form, some as N03-

• When added to soils, N may be:• appropriated by microorganisms (immobilized)

• Mineralized by microorganisms

• used directly by plants

• fixed in interlayer of clay minerals (vermiculite) or OM0 lost as gas to atmosphere by volatilization (NH4

+) or lost in

runoff (leaching)

• Undergo the process of nitrification

• Undergo denitrification

The

Nitrogen

Cycle

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of NO, NO�. NJO • .:md NH, S)mbio.tic biological

...,.w,;��s-,.-r-----�..---.J N ftXation

HNOi NitriC.:l(id

NH� Ammonia

gas

C .9 .f: X

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Anirnals

1.eachiog loss

Erosion and

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The5Nitrogen Cycle

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fixation

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.A.nimal

rnanur-es

an,j bio::.olicJs

t •

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lndu:3trial fo:ation

(rnmrnercial fertilizers)

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Runoff and

erosion

Denitrif1cation

Leaching

Negative effects of excess N

e Nitrate pollution problems

• derive from presence of more nitrate in soil than canbe used by plants

• high levels in drinking water (10 ppm) can be dangerous(metahemoglobinemia)

• nitrates leach below root zone during fall and winter

• excess nitrate from excessive manure or fertilizerapplication, organic matter

Nitrogen Fixation

Biological nitrogen fixation

Certain organisms can convert atmospheric N2

gas to N-containing organic compounds

• Symbiotic fixation with legumes and bacteria

• Host plant provides bacteria with carbohydrates forenergy, bacteria provides fixed-nitrogen compounds

• Major biological source of N in agricultural soils

Nitrogen Fixation

Symbiotic fixation with non-legumes

• Soil actinomycetes and 200+ species of non

legumes

Cyanobacteria and certain non-legumes

Nonsymbiotic nitrogen fixation

• Carried out by certain free-living soil organisms

Nitrogen Immobilization

• N immobilization: conversion of inorganic N

(N03- and NH4+ ) into organic forms

• Occurs when plant material added to soils

• N in residue utilized by microorganisms to build body

tissue, thus it becomes unavailable to plants

Nitrogen Mineralization

• N mineralization: release of organically bound Nto inorganic mineral forms

inorganic forms= N03- and NH4+

• soil organisms simplify and hydrolyze organic Ncomponents ultimately producing N03- and NH4+

Net mineralization (over immobilization) releases

free (plant available) N into soil system

Net Nitrogen Mineralization or

Immobilization?

• Depends mainly on the C:N ratio of the residue ...

e C: N < 20 ... net mineralization

• C:N > 30 ... net immobilization

... and the availability of C and N in the residue

• Highly decomposed organic material may have afavorable C:N ratio but neither C nor N are readily

available

Process of Nitrification

Nitrification: converts NH4+ to N03-

• process is an oxidation reaction - aerated soils

Nitrosomonas spp. bacteria convert ammonium to nitrite (NH4

+ to N02-)• 2 NH4+

+ 3 02

7 2 N02

- + 4 H+

+ 2 H20 + energy

• Nitrobacter spp. bacteria convert nitrite to nitrate (N02-

. to N03-)• 2 N02- + 02 7 2 N03- + energy

• Very common reaction in soils, so most free N in soils is N03-

• Nitrite is undesirable, toxic to plants and animals

Conditions Affecting

Nitrification Process

Oxygen required

• need good aeration and drainage

Temperature

• cool soils will slow process

Moisture

• optimum moisture for plants is also optimum for

process

N volatilization ( as ammonia)

• Chemical equilibrium reaction

• Reaction is in equilibrium at pH 9.3

• NH4

+ is the ammonium ion (commonly added in

this form in N fertilizers)

• NH3 is ammonia (a gas)

Factors Affecting NH3 Volatilization

Type of N fertilizer, application rate and mode

of application

• High NH4+ or urea content, high rate and

surface application increase losses

• Hot and windy weather increase losses

Soil properties

• High pH, dry soils and low CEC increase losses

100

Effect of I

Deni1rification , I

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Soil > so

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Moisture •

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Nitrification _.· . � . ' C .

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Processes � • . r•· .• ' . . ' •

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in the Soil 0 JO 40 60 so 100

Percent. o( soil pore volume filled with water

Nitrates and Denitrification

0 Nitrate rather easily lost from soil system

• by leaching• has negative charge (N03-) so not adsorbed by dominantly

negatively charged colloids in soil

• by denitrification

• reduction of nitrates (and nitrite) to N-oxides and

elemental N0 often carried out by facultative anaerobes, sometimes by

chemical reaction

• process returns N to atmosphere in gaseous forms

Nitrates and Denitrification

0 Denitriflcation

• Occurs under saturated conditions (wetlands, riparianareas)

• Performed by a large number of microorganisms

• Cleans up nitrate laden water from home/industry

• Process speeds up with increase of usable carbon insoils

• Carbon needed/ used as food source by microorganisms

Effect of Soil Moisture on Rates of Denitrification

Measured rates of

denirrification (kgN/ha/yr)

Developed upland

Nitrate in

Soil drainage classes

MWD = moderately well drained SPD = somewhat poorly drained

38.5

21.4

15.9

7. l

MWD SPD PD VPD

... 1 : ....

Stream

PD = poorly drained VPD = very poorly drained

16.3

6.3 5.7 4.8

VPD PD SPD MWD

Riparian >! zone

ground water 3 I 10 7 <I <I <I

(mg/L)

0 5 10 15 20 25 30 35 40 45 50

Horizontal scale-meters

Forested upland

Unsaturated

Saturated zone

Natural Sources of Sulfur

• Organic matter

• Most sulfur present in organic forms• Over time, microorganisms convert this to soluble inorganic

forms (typically sulfate)

Soil minerals

• Sulfates present naturally in some soils• Clay fraction of soils high in kaolinite and Fe and Al oxides

Natural Sources of Sulfur

Atmospheric sulfur

• Comes down to earth as dust and solids in dry

deposition

Come down to earth mixed with precipitation in wet

deposition

• Atmospheric sulfur in either wet or dry form can be

absorbed directly by plants or soil

The

Sulfur

Cycle

Er�'}rillf

Volal'ilizalfon

Sulfid� S<'il 1:ni:n, ....... r.s

Sr,Tf 13 1�a..-s SO: H.S COS

© Sullur (S}

Sulf�1('$ .adsorbed "J1da�,s

ThesSulf ur Cycle

Atrnospt-1eric

deposition

Animal manures

an1j td1J·3olid'3

Plant

re:3i1jues

, •

bacteria fungal l assim11at1on -\ ·

'9>Mi&H

t·,.� in era I

fertilizers

Loss from :;;,)ii

Sulfur in the Soil

• Mineralization

0 Organic forms of S must be mineralized by organisms

to forms available to plants

Immobilization

., Occurs when low sulfur, energy rich organic material

added to soils • Microbial growth is stimulated and inorganic sulfates are

assimilated into microbial tisssue

Sulfur in the Soil

Oxidation (analogous to nitrification process)

• Acidic process

• Can be carried out by chemical reaction or through action of

several species of bacteria

• Sulfates formed can be beneficial or detrimental

0 Lowers pH, useful for reclaiming sodic soils, can produce toxic acid

sulfates

Sulfur in the Soil

Reduction (analogous to denitrification)

• Sulfate ions reduced to sulfide ions by certain bacteria

• Requires anaerobic (saturated} conditions

• Helps prevent iron toxicity as sulfide ions react with iron and

immobilize it

• Useful in rice paddies and marshes

Sulfur in the Soil

� Sulfate ion is form used by most plants

Sulfate compounds can be easily leached from soils

especially in humid regions

• Sulfate ions are attracted to positive charges

associated with acid soils containing Fe and Al oxides

or 1:1 silicate clays

Guides for Educators (May 2014)

Nitrogen (N) is the most abundant element in the atmosphere, and it generally is the most limiting nutrient for crops. Nitrogen cycles in soil through various processes and in various forms (fig. 1). Some of the processes convert N into forms that can be used by plants, and some of the processes, such as leaching and volatilization, can lead to N losses. Nitrogen is added to soil naturally through N fixation by soil bacteria and legumes and through rainfall. Additional N typically is applied to the soil by use of fertilizer, manure, or other organic material. Because N is so dynamic, it can easily be lost to the environment, making it difficult to determine adequate N levels throughout the growth cycle of crops. The soil nitrate N level is a reasonable indicator of N cycling in soils, and it can be used to help determine whether carryover nitrogen was used by the previous crop and whether additional nitrogen is needed. In addition, the nitrate N level is an excellent indicator of soil organic matter mineralization and excessive or inadequate application of N fertilizer for optimum crop or forage production. Excessive application of N fertilizer can result in leaching of nitrates below the root zone and into groundwater at a shallow depth or into drainage tiles.

Figure 1.—Nitrogen cycle (“Soil as a Plant Sees It,” University of Nebraska, 1991).

Soil Health – Nitrogen USDA-NRCS

Page 2 Guides for Educators (May 2014)

Inherent Factors Affecting Soil Nitrogen Inherent factors, such as soil drainage, soil texture, calcium carbonate content, and steepness of slope impact the transport and transformation processes of N and limit availability to crops or increase potential for loss. Inherent factors, such as rainfall and temperature, and site conditions, such as soil moisture and aeration (oxygen level) and salt content (electrical conductivity), affect the rate of N mineralization from organic matter decomposition, affect N cycling, and can result in N loss through leaching, runoff, or denitrification. Organic matter decomposes, releasing N, more quickly in warm, humid climates than in cool, dry climates. Nitrogen is also released quicker in well-aerated soils than in saturated soils. Nitrogen can readily leach out of the root zone in the form of nitrate N. The potential for

leaching is dependent on soil texture (percentage of sand, silt, and clay) and soil water content. Water moves more quickly through the large pore spaces in sandy soils than it does through the small pore spaces in clayey soils, and the available water capacity in sandy soils is much lower. Denitrification occurs in soils that are poorly drained and are ponded or saturated with water, resulting in loss of N as gases. This can result in emission of potent greenhouse gases, a reduction in yields, and a need for additional applications of N fertilizer, which can increase cost. The presence of excessive calcium carbonate in soils can cause ammonium (NH4+) to convert to ammonia (NH3), which can then volatilize. Applying lime does not significantly affect this process.

Nitrogen ManagementManagement factors, such as the rate, timing, and method of application, source of N, irrigation management, residue management, and type of crops grown, and the presence of a tillage pan can affect how efficiently N is used by crops and the amount of N loss. Management of N in sandy soils is important because of the high potential for loss through leaching. Applying N at the proper rate is the primary management consideration; however, selecting the appropriate source of N, properly timing application to coincide with plant uptake, and using the proper method of application, such as injecting to minimize loss, are also important. Management practices that increase organic matter content and minimize compaction are needed to help stabilize the supply of N to crops, increase aeration, and limit N loss as a result of denitrification in saturated soils. Apply nitrogen based on the amount needed to optimize yields with consideration of the

agronomic, economic, and environmental effects. When determining the proper rate of application of N fertilizer or manure, consider the residual nitrate N in the soil, mineralization of organic matter, N supplied by legumes, manure or other organic amendments applied, nitrate N content in irrigation water, decomposition of residue, and natural sources of N. Properly time application to provide adequate amounts of N when plants are actively growing and using N rapidly. Loss of applied N can be minimized by delaying application until the crop has emerged (side dressing). Applying some N prior to crop emergence and some after emergence can also increase the efficiency of use by crops. Select the proper source of N and proper method of application to maximize N recovery by crops and minimize loss from volatilization and leaching. Anhydrous ammonia commonly



is the least expensive source of N, but it must be handled properly and injected or knifed into the soil under ideal soil moisture conditions. Urea should be injected to reduce loss from volatilization of ammonia. Surface-applied urea N fertilizers should not be applied during warm, humid conditions or on wet residue because of the high potential for N loss from volatilization. Other sources of N are manure and other organic amendments. Care must be taken to apply manure uniformly at an appropriate rate, accounting for mineralization. If urea and ammonia fertilizers are used on calcareous soils, they should be incorporated or injected under moist conditions to minimize volatilization.

Placement of N fertilizer can be accomplished by several methods. These typically include side dress application after crop emergence; knife application, which places a band of fertilizer below the soil surface; uniform application by broadcasting; and application through sprinkler irrigation systems. The method should be selected according to the type of fertilizer or manure to be applied.

Timing of irrigation is important. Enough water should be applied to optimize yields while avoiding excessive amounts that can increase cost and leach N below the root zone.

To efficiently manage N: 1. Apply recommended rate based on

expected yields.2. Time N application to align with crop needs.3. Select an ammonium fertilizer that

maximizes N recovery by crops.4. Inject N, if possible, to avoid loss through

volatilization of ammonia.5. Use N inhibitors unless it is applied during

the growing season.6. Consider all sources of N in the soil.7. Irrigate properly.

8. Monitor the N needs of the crop grown.9. Test the soil regularly for nitrate content

(include deep samples) and salt content(EC).

Yellow coloration of corn leaves in a V-shaped pattern is symptomatic of nitrogen deficiency (fig. 2). This pattern progresses from the end of the leaf to the collar and from the lower leaves to the upper ones. Commonly, the lower leaves die when nitrogen deficiency is severe.

Figure 2.—Nitrogen-deficient corn characterized by yellow coloration in V-shaped pattern.

Nitrogen cycle: Nitrogen gas (N2) is the largest constituent of the Earth's atmosphere and is in soil pore spaces. It is relatively unuseable by plants; therefore, it must go through a natural fixation process to become available to plants. Greenhouse gases, including nitrous oxide (N2O) and nitric oxide (NO), can also be in pore spaces and be released into the atmosphere. The main inorganic forms of N in soils are ammonium (NH4) and nitrate (NO3), which are both useable by plants. Organic nitrogen may be present as a living organism, as humus, or as the intermediate products of organic matter decomposition. The processes included in the nitrogen cycle transform nitrogen from one form into another. Nitrogen processes in soils are carried out by microbes to harvest energy, for microbial growth, or for plant use. These processes make up the nitrogen cycle (fig. 1).



Nitrogen fixation, mineralization, nitrification, leaching, plant assimilation, ammonia volatilization, denitrification, and immobilization

can occur in the N cycle in a soil, depending on soil conditions and management.

What management practices being used affect N loss from leaching and volatilization, etc.? ___________________________________________________________________________

___________________________________________________________________________

___________________________________________________________________________

Are soil nitrate N levels expected to be high or low? Why?

______________________________________________________________________________________

______________________________________________________________________________________

______________________________________________________________________________________

Measuring Soil Nitrate/Nitrite Levels Materials needed to measure nitrate/nitrite levels: ____ Plastic containers and probe for

gathering and mixing soil samples ____ Nitrate/nitrite test strips ____ 1/8-cup (29.5 mL) measuring scoop ____ Calibrated 120-mL vial with lid for

shaking ____ Squirt bottle ____ Distilled water or rainwater ____ Pen, field notebook, permanent marker,

and resealable bags

Considerations: Electrical conductivity (EC) should always be measured before measuring nitrate or nitrite content on a soil sample. Soil phosphates and pH can also be measured on the sample.

Quick in-field hand test:

1. Soil nitrate N levels are highly variable,depending on management history, fieldlocation, and time of year. Factors includethe rate of erosion, soil texture, organicmatter content, and applications of manureor fertilizer. Using a soil probe, gather atleast 10 small samples randomly from anarea that represents a particular soil typeand management history. The probe shouldextend to a depth of 8 inches or less for asurface layer sample and to a depth of asmuch as 3 feet for a subsurface layersample. Place samples in a plasticcontainer and mix. Surface layer andsubsurface layer samples should be placedin separate containers. Do not include largestones and plant residue. Repeat step 1 foreach sampling area.



2. Neutralize hands by rubbing moist soilacross palms. Discard soil. Place a scoopof mixed soil in palm of hand and saturatewith “clean” water (distilled water orrainwater).

3. Squeeze hand gently until a soil and waterslurry forms.

4. Touch tip of nitrate test strip to soil andwater slurry. Leave until the slurry isdrawn up at least 1/8 to 3/16 inch on strip(fig. 3).

5. After 1 to 2 minutes, measure nitrate bycomparing the color of the wet test strip tothe color chart on the test strip container(fig. 4). The color that most closely matchesthe test strip indicates the amount of nitratein the saturated soil.

6. Bend or cut off end of strip to expose the tipfor the nitrite test. Repeat steps 2 through 5to determine the amount of nitrite in thesaturated soil.

Figure 3.—Quick nitrate N hand test.

Figure 4.—Nitrate color chart.

1:1 soil to water nitrate/nitrite test for classroom: 1. Gather samples as instructed in step 1

under “Quick in-field hand test.”2. Fill scoop (29.5 ml) with mixed soil, tamping

down during filling by carefully strikingscoop on a hard, level surface. Put soil invial. Add one scoopful (29.5 ml) of water tothe vial, resulting in a 1:1 ratio of soil towater, on a volume basis.

3. Tightly cap the vial and shake 25 times. Letsettle for 1 minute. Remove cap, andcarefully decant 1/16 inch of the soil andwater slurry into the cap.

4. Allow to settle for 2 to 3 minutes. Touch endof nitrate test strip to soil and water slurry.Leave until liquid is drawn up to least 1/8 to3/16 inch beyond the area covered by thesoil (fig. 5).

5. After 1 to 2 minutes, compare color of wettest strip to color chart on the test stripcontainer (fig. 4). The color on the chart thatmost closely matches the color on the teststrip indicates the amount of nitrate in thesaturated soil.

6. Bend or cut off end of strip to expose the tipfor the nitrite test. Repeat steps 2 through 5to determine the amount of nitrite in thesaturated soil.

Figure 5.—1:1 soil to water test for nitrate.



Interpretations Soil nitrate N is an excellent indicator of N cycling in soils. The levels help in determining whether carryover nitrogen was used by the previous crop and whether additional nitrogen is needed. High nitrate N levels indicate excessive application of nitrogen, a high rate of organic N mineralization, and potential for loss from denitrification, leaching, or volatilization.

Typically, an early-season measurement of 20 parts per million (ppm) nitrate N in the surface layer (upper 6 to 8 inches) is sufficient to produce a good yield of corn and most other crops that use a high amount of N. A value of 14 ppm is sufficient in areas where animal manure is applied or a cropping rotation of corn or other crops that use a high amount of N and legumes is used. A nitrate N level of more than 40 ppm in the surface layer restricts the ability of bacteria and legumes to fix nitrogen; therefore, nitrogen applications should be managed to maintain nitrate N levels below this level. Adjust nitrate N levels according to soil bulk density and moisture content, as shown in table 1.

Table 1.—Average bulk density of organic matter and upper part of soils with varying textures (Rawls, 1983; C. Smith, 2014 [red clayey andvolcanic ash soils]).

Organic matter and soil textures

Average bulk density (g/cm3)

Organic matter 0.22 Sand 1.56 Loamy sand 1.54 Sandy loam 1.50 Loam 1.45 Silt loam 1.20 Sandy clay loam 1.63 Silty clay 1.55 Clay loam 1.45 Silty clay loam 1.40 Red clayey soils 1.10 Volcanic ash soils 0.80

Soil nitrate-N calculations (for completing table 2): Example—silty clay loam; 8 inch sample depth; 2 percent organic matter estimate determined from soil color chart; no bulk density (BD) measurement (BD based on estimated values in table 1).

Sampling depth conversion: Sampling depth in centimeters (cm) = sampling depth in inches (in) x (2.54) Example— 8 in x 2.54 = 20 cm sampling depth

Subsample soil water content (gram/gram [g/g]): (weight of moist soil – weight of oven-dry soil) (weight of oven-dry soil)

Example— (41.0 g – 32.5 g) = 0.262 g of water/g of soil

32.5 g



Dry weight of soil: (weight of moist soil in scoop) ÷ (1 + soil water content [decimal]) Example—

41.0 g ÷ (1 + 0.262 g/g) = 32.5 g

Volume (weight) of water: Water added to soil (g) + (dry weight of soil x soil water content [g/g]) (1 milliliter water = 1 g) Example— 29.5 g + (32.5 g x 0.262 g/g) = 38.0 g

Adjusted ppm soil nitrate N: (ppm of NO3 N in 1:1 mixture) x (volume of water in extract and soil) dry weight of extracted soil

Example—(20 ppm x 38.0 g) ÷ 32.5 g = 23.4 ppm (adjusted)

Estimated bulk density (BD) (refer to “Bulk density” guide for educators to calculate actual bulk density or use estimates in table 1): Estimated bulk density (record in table 2) = 100

(percent organic matter) ÷ (organic matter BD) + (100 - percent organic matter) ÷ (average soil BD)

Example— 100 = 1.26 g/cm3

(2.0 ÷ 0.22 g/cm3) + (100 – 2) ÷ (1.40 g/cm3)

Pounds (lbs) of nitrate N/acre-depth sampled: (adjusted ppm NO3 N) x (soil sample depth [cm] ÷ 10) x estimated soil bulk density x 0.89 (conversion factor)Example—(23.4 ppm NO3 N) x (20 cm ÷ 10) x (1.26 g/cm3 [estimated BD]) x (0.89) = 52.5 lbs nitrate N/acre-20 cm sampling depth (8 in)



Table 2.—Soil nitrate nitrogen (Based on sampling depth.)

Site Sample depth (in or cm)*

Nitrate N from

test strip

(ppm)

Adjusted nitrate N (ppm)*

Bulk density (g/cm3)*

Nitrate N/ acre/sample depth (lbs)*

Nitrite N from test strip

(ppm)**

Notes

Ex. 8 in, or 20 cm 20 23.4 1.26 52.5 N/A

N tested in spring prior to planting

* Follow steps for calculating soil nitrate based on sampling depth.** If nitrate N exceeds 40 ppm or EC exceeds 0.6 ds/m, soil nitrite should also be measured.

Nitrate and nitrite levels in water: Nitrate and nitrite levels can be estimated for drinking water, irrigation water, and other water sources. Treatment of drinking water should be considered if nitrate N levels are higher than 10 milligrams per liter, which is considered by the Environmental Protection Agency (EPA) to be the maximum contaminant level. If levels are high, consult your local health officials. Nitrate levels in groundwater may vary seasonally. If the water tests high or borderline high, retest it every 3 to 6 months. Nitrate in irrigation water can be credited as N for use by crops. Use the formula given in the footnote for table 3. To test cloudy water samples*— 1. Collect water sample in small plastic vial.2. Tightly cap the vial and shake 25 times. Let

settle for 1 minute. Remove cap, andcarefully decant 1/16 inch of water into thecap.

3. Allow to settle for 2 to 3 minutes. Touch tipof nitrate test strip to water. Leave untilliquid is drawn up to at least 1/8 to 3/16 inchon the strip.

4. After 1 to 2 minutes, compare the color ofthe wet test strip to the color chart on thetest strip container (fig. 4). The color thatmost closely matches the test stripindicates the amount of nitrate in thesaturated soil. Record appropriate nitratelevel (ppm) in table 3.

5. Bend or cut off end of strip to expose the tipfor the nitrite test. Repeat steps 2 through 4to determine the amount of nitrite in thesample. Record appropriate nitrite level(ppm) in table 3.

*To test clear water for nitrate and nitrite, dipthe entire strip into water for 1 second or passthe strip under a gentle stream of water. Followthe rest of the instructions on test strip bottle.



Table 3.—Water nitrate and nitrite nitrogen.

Site Nitrate N from test

strip (ppm)

Nitrite N from test

strip (ppm)

Irrigation water

applied (in/ac)

Irrigation water N credit

(lbs/acre)

Ex. 20 N/A 12 54

* (Pounds of N/ac credit = (water pumped [in/ac]) xnitrate [ppm] x 2.7) ÷ 12)

Are the nitrate N levels adequate for the needs of the crops grown?

______________________________________________________________________________________

______________________________________________________________________________________

______________________________________________________________________________________

Is the total amount of nitrate N in the soil too high or too low based on the rate, method, and timing of N application?

_____________________________________________________________________________

_____________________________________________________________________________

_____________________________________________________________________________

Will nitrate N likely be lost through leaching, volatilization, denitrification, or other processes? Why or why not?

_____________________________________________________________________________

_____________________________________________________________________________

_____________________________________________________________________________



If the drinking water was tested for nitrate or nitrite levels, do the levels exceed the maximum contaminant level of 10 mg/l nitrate N (ppm)? If so, what management practices should be considered?

______________________________________________________________________________________

______________________________________________________________________________________

______________________________________________________________________________________

Glossary Ammonification.—Production of ammonium (NH4) as a result of soil organic matter decomposition and other processes. Ammonium.—Nitrogen, referred to as NH4, thatis available for plant use and is part of the N cycle in soils. Produced from soil organic matter decomposition and other processes. Bulk density.—Weight of dry soil per unit of volume. More compacted soil has less pore space and higher bulk density. Denitrification.—Conversion and loss of nitrate N to various nitrogen gases when soil is saturated with water (fig. 1). Immobilization.—The temporary “tying up” of inorganic N as a result of soil micro-organisms decomposing plant residue. Immobilized N will eventually become available to plants as the decomposition continues; thus, it is not considered as a loss. Leaching.—Loss of nitrogen in the form of nitrate N, which is water soluble and mobile, as excess water moves below the root zone or into drainage tiles. Mineralization.—Release of nutrients in a form available for plant use through organic matter decomposition (e.g., phosphorus, nitrogen, and sulfur).

Nitrate N.—Form of nitrogen referred to as NO3that is available for plant use and is part of the N cycle in soil. Nitrate N is highly susceptible to loss from leaching. Nitrification.—Part of the N cycle in which soil organisms convert ammonia and ammonium to nitrite and then to nitrate N, which is available for plant use. Nitrite N.—Form of nitrogen referred to as NO2that is not available for plant use and is part of the N cycle in soil. The susceptibility of nitrite N to loss from leaching normally is short-lived because it converts to NO3 rapidly under aerobic conditions. Soil nitrogen fixation.—Conversion of nitrogen in the air into organic forms, which occurs either as a result of soil organisms or in association with legumes. Volatilization.—Ammonia nitrogen loss from nitrogen fertilizers and other sources. Losses can be especially high if nitrogen fertilizer containing urea is applied directly on the surface of moist residue or on a calcareous soil. Volatilization also occurs through denitrification if free O2 is absent (as NO, NO2, and N2).

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Soil Quality Indicators

Indicator

BTest

LFunction

DNS

Potentially Mineralizable Nitrogen (PMN)PMN can be defined as the fraction of organic nitrogen converted to plant available (or mineral) forms under specific conditions of temperature, moisture, aeration, and time. Determining levels of PMN can provide an estimate of available N in the soil.

PMN originates mainly from microbial biomass and plant and animal tissues—the main source of the organic nitrogen pool. It represents the fraction of nitrogen easily decomposable by soil microorganisms and is considered an indirect measure of nitrogen availability during the crop-growing season (if measured during that period).

While anaerobic N mineralization potential may be a good indicator of the potential for soil to deliver N, it does not necessarily reflect microbial biomass N levels. It was suggested that the ratio of N mineralized to total organic nitrogen N could serve as a sensitive indicator of differences in soil organic matter (SOM).

Factors AffectingInherent — Levels of PMN may be greatest in humid climates and lower in drier climates because humid climates usually enhance biomass production. Clay soils have the capacity to physically protect organic matter and organic nitrogen and, thus, associated PMN from degradation by microorganisms. During the soil incubation to measure PMN, soil clay particles can attract and temporarily retain ammonium on cation-exchange complexes. The depth to bedrock affects soil hydrologic properties (e.g., fluctuation of water table and subsequent soil hydromorphy, causing excess or low

moisture amounts), which in turn determine the chemical end products of N mineralization, namely ammonium or nitrate. Low areas (topographic depressions) of a field tend to accumulate more organic matter and total N, and probably available N and PMN, than higher areas.

Dynamic — Soil properties and soil management practices that affect organic matter and organic N dynamics will ultimately affect available N and PMN levels. Continuous cultivation of lands without the replenishment of external organic matter depletes the land’s organic matter and nitrogen reserve and the related available PMN. Conversely, repetitive additions of farm manure or crop residues (as under no-till or cover crops) usually increase the levels of available N and probably PMN (as shown in Figure 1); no-till significantly increases PMN levels compared to the plow and fertilizer addition. Soils with stable aggregates protect soil organic matter and associated available N from microbial degradation compared to soils with unstable aggregates. Small aggregates in soil reportedly contain a larger proportion of readily mineralizable organic N; therefore, a greater amount of PMN may be obtained in soils with small aggregates than in those with larger aggregates. Accumulation and mineralization of N also depend upon the C:N ratio of amendment materials added to the soil.

Figure 1. PMN levels in the 0-7 cm soil layer as influenced by tillage management (+ = NH4NO3 , 0 = no fertilizer, white block = no tillage, dashed block = plow, and * = significant difference at p < 0.05) (Doran, 1987).

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Relationship to Soil FunctionAs a readily available fraction of total N, PMN is an important potential source of N for crop growth and yield, especially in synthetic N-fertilizer-free agricultural operations (e.g., organic farming). PMN can be a source of available N for microorganisms and indirectly enhances microbial growth and activities, including C and N cycling. In well drained soils, PMN is made available to plants and microorganisms, mostly in the form of nitrate, through aerobic mineralization. In poorly drained soils (such as rice fields), PMN is made available, in the form of ammonium, through anaerobic mineralization.

Problems with Poor ActivitySoils naturally low in organic matter or depleted by poor management will have low PMN content. In the absence of live vegetation, a high amount of available N delivered by the PMN pool can build up and become a potential source of nitrate contamination for ground water. An excess of nitrate from the PMN pool can be lost in the atmosphere as gaseous nitrogen products during subsequent very wet seasons or under heavy irrigation (many of those products, like nitrous oxide, are greenhouse gases).

Improving ManagementThe following practices increase available N in soils:

● Application of organic residues and farm manure toincrease microbial biomass and organic nitrogen

● Addition of N fertilizer at recommended rates andoptimal times

● Adoption of conservation practices, such as no-till (seeFigure 1) and cover crops

● Erosion control to reduce the transport of available Nby sediment to streams and other bodies of water

Figure 1 shows that PMN levels were greatest in the surface layer (depth of 0 to 7.5 cm) of soil under no-till management compared with conventional tillage. The results were more dramatic in the humid climate compared with the dry. Biomass and PMN levels were greatest below a depth of 7.5 cm of plowed soil. PMN levels were primarily associated with microbial biomass carbon (MBC) and total N distribution. Figure 2 shows PMN obtained in anaerobic incubation; high levels of PMN

were obtained in soil amended with crop residues (rice residues).

Measuring PMN● Anaerobic method (Waring and Bremner, 1964): PMN

is estimated in the laboratory on soils by measuringammonium N produced in soil incubated underanaerobic conditions for 1 week.

● Aerobic method (Stanford and Smith, 1972): PMNis estimated in the laboratory on soil by measuringammonium N and nitrate N produced in soil incubatedunder aerobic conditions for 30 days. Cumulative N isused in a mathematical model to compute PMN.

● Other methods have been suggested, but they are notdiscussed in this document.

ReferencesDoran, J.W. 1987. Microbial biomass and mineralizable nitrogen distributions in no-tillage and plowed soils. Plant Biology and Fertility of Soils 5:68-75.

Drinkwater, L.E., C.A. Cambardella, J. Reeder, and C. Rice. 1996. Potentially mineralizable nitrogen as an indicator of biologically active soil nitrogen. In J. W. Doran and A. J. Jones (eds.) Methods for Assessing Soil Quality. Soil Science Society of America Special Publication 49:217-229.

Islam, M.M., F. Iyamuremye, and R.P. Dick. 1998. Effect of organic residue amendment on mineralization of nitrogen in flooded rice soils under laboratory conditions. Communications in Soil Science and Plant Analysis 29:971-981.

Stanford, G., and S.J. Smith. 1972. Nitrogen mineralization potentials of soils. Soil Science Society of America Proceedings 36:465-472.

Waring, S.A., and J.M. Bremner. 1964. Ammonium production in soil under waterlogged conditions as an index of nitrogen availability. Nature 201:951-952.

Figure 2. PMN (No) in flooded soils amended with rice straw and pea vine (Islam et al., 1998).

SOIL QUALITY – AGRONOMY TECHNICAL NOTE No. 6

Legumes and Soil Quality Introduction The ability of legumes to fix atmospheric nitrogen is perhaps the most notable aspect that sets them apart from other plants. In addition, legumes can provide a wide range of important soil quality benefits.

Nitrogen Fixation Legume plant and seed tissue is relatively high in protein. This can be directly attributed to a legume’s ability to supply most of its own nitrogen needs with the help of symbiotic Rhizobia bacteria living in their roots.

Inoculated with the proper strain of Rhizobia bacteria, legumes can supply up to 90% of their own nitrogen (N). Shortly after a legume seed germinates in the presence of Rhizobia bacteria in the soil, the bacteria penetrate the root hairs and move into the root itself. The bacteria multiply, causing a swelling of the root to form pale pink nodules.

Nitrogen gas present in the soil air is then bound by the bacteria which feed on carbohydrates manufactured by the above-ground plant during photosynthesis. The bacteria produce ammonia (NH3) from the hydrogen acquired from the plant’s carbohydrates and nitrogen from the air. The ammonia then provides a source of nitrogen for the plant to grow. This symbiotic

Generally speaking, the higher the protein content of a plant the more nitrogen it will return to the soil. Nitrogen is an important element for the formation of soil organic matter.

Nitrogen release from a legume crop occurs as the above-ground plant residues, roots and nodules gradually decompose. Soil microorganisms decompose the relatively nitrogen-rich organic material and release the nitrogen to the soil when they die. Usually about two-thirds of the nitrogen fixed by a legume crop becomes available the next growing season after a legume in a rotation.

In a perennial grass and legume mixture, legumes not only supply their own N, but approximately 36% of the N needs of the grass plants growing alongside them. The total amount of N fixed and released by a legume can be estimated from the amount of seed produced by an annual legume as follows:

Bushels of seed/acre x 1.0 = lbs. of N/acre or,

Pounds of seed/acre x .017 = lbs. of N/acre

For perennial or biennial legumes such as alfalfa or sweetclover, 40 to 70 pounds of N are produced per ton of forage if the crop is left unharvested. If the crop is harvested as forage, the remaining stubble and roots will return 5 to 15 pounds of nitrogen to the soil as a function of each ton of forage removed. Tillage is not necessary to release

United States Department of Agriculture

Natural Resources Conservation Service

Soil Quality Institute 411 S. Donahue Dr. Auburn, AL 36832 334-844-4741X-177

Technical Note No. 6

March 1998

This is the sixth in a series of Soil Quality-Agronomy technical notes on the effects of land management on soil quality. This information is general and covers broad application.

1

relationship between bacteria and legume allows them both to flourish and produce a high-protein seed or forage crop. Even though legumes can fix nitrogen from the atmosphere, they can take up large quantities of soil nitrogen if it is available.

legume-N into the soil. A study involving no-till corn after legumes demonstrated that the N benefits following a legume were the same whether the legume was killed with herbicide or with tillage.

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Soil Quality Benefits of Legumes Soil quality benefits of legumes include: increasing soil organic matter, improving soil porosity, recycling nutrients, improving soil structure, decreasing soil pH, diversifying the microscopic life in the soil, and breaking disease build-up and weed problems of grass-type crops.

Soil Organic Matter As mentioned previously, legumes are high in protein, and therefore, nitrogen-rich. Because most crop residues contain much more carbon than nitrogen, and bacteria in the soil need both, the nitrogen supplied by legumes facilitates the decomposition of crop residues in the soil and their conversion to soil-building organic matter.

Soil Porosity Several legumes have aggressive taproots reaching 6 to 8 feet deep and a half inch in diameter that open pathways deep into the soil. Nitrogen-rich legume residues encourage earthworms and the burrows they create. The root channels and earthworm burrows increase soil porosity, promoting air movement and water percolation deep into the soil.

Recycle Nutrients Because perennial and biennial legumes root deeply in the soil, they have the ability to recycle crop nutrients that are deep in the soil profile. This results in a more efficient use of applied fertilizer and prevents nutrients (particularly nitrate nitrogen) from being lost due to leaching below the root zone of shallower-rooted crops in the rotation.

Improve Soil Structure Research in both the United States and Canada indicate improved soil physical

properties following legumes. The improvements are attributed to increases in more stable soil aggregates. The protein, glomalin, symbiotically along the roots of legumes and other plants, serves as a “glue” that binds soil together into stable aggregates. This aggregate stability increases pore space and tilth, reducing both soil erodibility and crusting.

Lower Soil pH Because inoculated, nodulated legumes acquire their N from the air as diatomic N rather than from the soil as nitrate, their net effect is to lower the pH of the soil. In greenhouse studies, alfalfa and soybeans lowered the pH in a Nicollet clay loam soil by one whole pH unit. Legumes could lower the pH and promote increased plant-soil-microbial activity on soils with a pH above the range for optimum crop growth and development.

Biological Diversity Legumes contribute to an increased diversity of soil flora and fauna lending a greater stability to the total life of the soil. Legumes also foster production of a greater total biomass in the soil by providing additional N. Soil microbes use the increased N to break down carbon-rich residues of crops like wheat or corn.

Break Pest Cycles Legumes provide an excellent break in a crop rotation that reduces the build-up of grassy weed problems, insects, and diseases. A three year interval between the same type (grassy, broadleaf, cool-season, warm season) crop is usually sufficient to greatly reduce weed, insect, and disease pressure.

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Conclusion All totaled, the soil quality benefits of legumes have been shown to be greater than the sum of their parts. Research in Saskatchewan and Minnesota, have shown yield increases of spring small grains of 7 to 14% after alfalfa. This yield increase is in addition to what can be attributed to improvements in moisture and nutrients measured in the soil. The Minnesota research attributes these yield increases to ‘rotation effects’ including improved soil physical

properties, depression of phytotoxic substances, addition of growth promoting substances, and decreased disease pressure.

Legumes can provide a multitude of benefits to both the soil and other crops grown in combination with them or following them in a rotation. Locally adapted legumes can be used in almost any conservation situation to improve soil quality.

References: Bowren, K.E., V.O. Biederbeck, H.A. Bjorge, S.A. Brandt, B.P. Goplen, J.L. Henry, H.

Ukrainetz, T. Wright, and L.A. McLean. 1995. Soil Improvement With Legumes. 24 pp. Saskatchewan Agriculture and Food. (Internet site; www.gov.sk.ca)

Ebelhar, S.A., W.W. Frye, and R.L. Blevins. 1984. Nitrogen from legume cover crops for no-tillage corn. pp. 51-55. Agronomy Journal, vol. 76, January-February 1984.

Graber, L.F., and J.M. Sund. 1975. Laboratory Manual for Students of Agronomy, 10th edition. 246 pp. Kendall/Hunt Publishing Co., Dubuque, IA.

Heichel, G.H., and K.I. Henjum. 1991. Dinitrogen fixation, nitrogen transfer, and productivity of forage legume-grass communities.. Crop Science 31:202-208.

Nyatsanga, Titus, and W.H. Pierre. 1973. Effect of Nitrogen Fixation by Legumes on Soil Acidity. Agronomy Journal 65: 936-940.

O'Leary, Mike, George Rehm, and Michael Schmitt, 1990. Providing Proper N Credit for Legumes. University of Minnesota Extension Service, Clean Water Series. (Internet site; www.mes.umn.edu)

Wright, Sara E., and Abha Upadhyaya. 1997. Extraction of an abundant and unusual protein from soil and comparison with hyphal protein of arbuscular mycorrhizal fungi.. Soil Science 161(9):575-586.

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Helping People Help the Land

USDA Natural Resources Conservation Service

Soil Quality Indicators

Indicator

CTest

FFunction

W

Soil NitrateNitrate (NO3

-) is a form of inorganic nitrogen (N) naturally occurring in soils. Sources of soil NO3

- include decomposing plant residues and animal manure/compost, chemical fertilizers, exudates from living plants, rainfall, and lightning. Eventually, nitrate ions immobilized by microorganisms (nitrate taken up by microorganisms) are converted into organic forms and released back to the soil in plant-available forms when dead soil organisms are fed upon or decompose. In well drained soils, ammonium (NH4

+) and ammonia (NH3) are converted into NO3 by very specific populations of aerobic bacteria. This process is known as nitrification.

Another biological N transformation is denitrification, which is the conversion of NO3

- into nitrous oxide (N2O), nitrogen dioxide (NO2), and nitrogen gas (N2) that often occurs in anaerobic soils, such as waterlogged soils and wetlands. Even when nitrifying bacteria are very active in the outer parts of aggregates in well aerated soils, denitrification may still occur in anaerobic microsites inside the aggregates. Nitrate is very soluble in water and can be easily transported by runoff and other surface and subsurface flows to rivers and lakes or moved downward to ground water.

Factors AffectingInherent — Soil texture influences NO3

- mobility: thecoarser the soil texture (sandy), the faster NO3

- leaches through the soil profile because NO3

- ions do not bind to sandy particles and water infiltration rates are typically very high in sandy soils. Under heavy rain, NO3

- in those soils can eventually leach and contaminate the ground water. Less weathered types of clay minerals (e.g., montmorillonite, illite, vermiculite) have lower NO3

- retention than very weathered clay minerals (e.g., kaolinite) found in tropical and subtropical soils. Soils with a high CEC do not retain NO3

-. These soils, such as those in the Mississippi River basin, have major leaching

issues. Climate (temperature and precipitation) affects the rate of nitrification; the optimum temperature is between 86 and 95 degrees F. Poor soil drainage creates an ideal biochemical environment for denitrification and the release of gaseous NOx into the atmosphere, which is exacerbated in carbon-rich soils (e.g., wetlands or plugged tile drains). Soil NO3

- is sensitive to seasonal fluctuations and crop presence. Figure 1 shows that that the amount of NO3

- increases as the soil warms in temperate regions (May and June) and then, because of NO3

- leaching during the rainy season, sharply decreases in autumn.

Dynamic — Nitrate is usually deficient in acid soils because low soil pH (<5.5) reduces nitrification. Nitrification ceases at pH <4.5, and the optimum pH is between 6 and 8. Because organic matter is an important source of NO3

-, accumulation may correlate with organic matter content patterns across the landscape. Moreover, nitrification depends on microorganisms, which proliferate in the presence of organic matter. Residues with high C:N ratio (>24/1) slow down the release of NO3

- from organic matter; microorganisms initially immobilize all available NO3

--N from soil for their growth. This delays the decomposition of organic matter and the associated nitrification.

Figure 1. Typical seasonal pattern of nitrate concentration in representative surface soil layers with and without growing plants for soils typical in humid temperate regions that have cool winters and rainfall uniformly distributed throughout the year (based on Brady and Weil, 1996).

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Relationship to Soil FunctionThe primary function of NO3

- is to serve as a source of nitrogen for the nutrition and growth of plants and soil microorganisms.

Problems with Poor ActivityDenitrification results in nitrogen loss from soil and produces some forms of intermediate gaseous nitrogen (e.g., N2O) that are harmful to the environment. Problems associated with high NO3

- concentration include the pollution of ground water and surface water and an increased risk of eutrophication that threatens the survival of aquatic life. Nitrification can potentially result in soil acidification by hydrogen ions (H+) released during the process.

Improving ManagementIn a study conducted at the University of Maryland Research Center, soil NO3

- concentrations at any depth (except 0-30 cm) have been found to be consistently lower in no-till plots than in conventional-till plots (see Figure 2) and were related to the amount of N fertilizer applied. The explanations by the authors of the study include: (i) the lack of a winter cover crop on the conventionaltill plots affected the soil N content in the root zone andthe subsequent rates of nitrate leaching; (ii) the no-tillplots had higher rates of denitrification compared tothe conventional-till plots (i.e., higher populations ofdenitrifying organisms in no-till); (iii) crops in no-till plotsused N more efficiently (removal of more N from soil);and (iv) the conventional till plots had an accumulation ofnitrate from the plant residues of previous years.

The following practices add nitrate:

● Crop rotations with legumes

● Addition of organic residues, manure, and compost

● Conservation tillage and field strips or no-till with awinter cover crop

● Split applications of fertilizer that match crop growthstages

The following practices prevent nitrate loss:

● Autumn applications of ammonium-based fertilizer onfrozen soils

● Application of materials that slowly release nitrogen

● Planting cover crop species that use residual NO3-

● Planning the timing and rates of irrigation according tosite water content

● Keeping the soil well drained

● Additions of green manure with a high C/N ratio

Measuring NitrateSoil NO3

- is measured in the field using a test strip as described in the Soil Quality Test Kit Guide. Measurement takes just 15 minutes. The test strip has two pads, one to measure nitrite (NO2

-) and the other to measure NO2-/

NO3- combined. This is not intended to be a substitute for a

fertilizer recommendation but is an indication of potential management remediation needs.

ReferencesAngle, J.S., C.M. Gross, R.L. Hill, and M.S. McIntosh. 1993. Soil nitrate concentrations under corn as affected by tillage, manure, and fertilizer applications. Journal of Environmental Quality 22:141-147.

Brady, N.C., and R.R. Weil. 1996. The nature and properties of soils. Prentice-Hall.

Cabrera, M., and others. 2008. Modeling the nitrogen cycle. In Schepers and Raun (eds.) Nitrogen in Agricultural Systems. American Society of Agronomy Monograph 40.

Dick, R.P., R.A. Christ, J.D. Istok, and F. Iyamuremye. 2000. Nitrogen fractions and transformations of vadose zone sediments under intensive agriculture in Oregon. Soil Science 165:505-515.

Figure 2. The effects of ammonium nitrate and tillage (●=no-till; ∆=conventional-till; 0X=no manure) on nitrate concentrations of soil collected to a depth of 210 cm during the spring of 1986 (after Angle et al., 1993).

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