N, P, and K Cycles · 2013-03-04 · DEPARTMENT OF ENVIRONMENTAL SCIENCE & TECHNOLOGY...

DEPARTMENT OF ENVIRONMENTAL SCIENCE & TECHNOLOGY

Laboratory for Agriculture and Environmental Studies

N, P, and KCycles

Joshua M. McGrathAssistant Professor

Soil Fertility and Nutrient Management Specialist

Macronutrients (Primary)

Element SymbolForm Absorbed

by PlantsCarbon C CO2

Hydrogen H H+, OH-, H2OOxygen O O2

Nitrogen N NH4, NO3

Phosphorus P HPO42-,H2PO4

-

Potassium K K+

Nitrogen Cycle

• the most complicated of the nutrient cycles

• N undergoes oxidation and reduction

• many reactions mediated by soil organisms

• has gaseous forms

Nitrogen Use Efficiency Concept

• Worldwide NUE in cereals 33% • Developed countries NUE’s approximately 42%• Current NUE for corn in US 30 – 60%

NUE =N removed in grain – (soil N + atmospheric N)

Fertilizer N

Raun and Johnson, 1999

Split Application of N is starting point

• If the N is not yet applied, it can not be lost

• Apply the N when it is required by the crop

• Here’s why it works

Bandel et al. UMD FS-559

PS

U A

gron

omy

Gui

de 1

2

How does yield relate to N rate?

Iowa State, 2006

Sites not responsive to N

If N was related to yield 1:1

Soil Nitrate & Corn N Uptake

0

20

40

60

80

100

120

140

0 1 2 3 4 5 6 7 8

Soil NitrateCorn N Uptake

Soil

Nitr

ate

Con

cent

ratio

n

Cor

n N

Acc

umul

atio

n

Planting 12” tall Tasseling Harvest

Four Basic Mega-processes in Soils

• addition of materials• loss of materials• translocation of materials• transformation of materials

Materials, such as nitrogen, can get added, lost, moved or changed in soils.

Soil Nitrogen Forms• Inorganic forms of soil N:

– ammonium (NH4+) – nitrite (NO2-) – nitrate (NO3-) – nitrous oxide (N2O gas) – nitric oxide (NO gas) – elemental N (N2 gas)

• Organic soil N:– amino acids, amino sugars, and other complex N compounds

NH4+, NO2-, and NO3- are the most important plant nutrient forms of N

and usually comprise 2 to 5% of total soil N.

Modified from the Potash & Phosphate Institute web site at www.ppi-ppic.org

Atmosphericnitrogen

OrganicNitrogen

AmmoniumNH4

+

NitrateNO3

-

Input to soilComponent Loss from soil

Cropharvest

Volatilization

Denitrification

Runoff anderosion

Leaching

The Soil Nitrogen Cycle

Atmospheric fixation and deposition

Animal manure and biosolids

Industrial fixation(commercial fertilizer)

Plant residues

Biological fixation by legumes Plant

uptake

Days/wks

Weeks/months

YearsDecades

Nitrate leaching

Note: N Cycle Losses areDriven by Hydrology

Why is N so difficult?

(J.J. Meisinger)

Denitrification losses:

Small – moderate: 10 – 20%

Ammonia losses:

Small – moderate 5 – 20%

Eroded N:

Small: 1 – 5%

Leaching losses:

Large: 10 – 40%

N fixation

• Conversion of atmospheric nitrogen (N2) to inorganic N

• Ultimate source of all soil N

Oklahoma Farm Bureau

Biological N fixationSymbiotic N fixation:

• Mediated by bacteria with the ability to convert atmospheric N2 to plant-available N while growing in association with a host plant.

• Example: Rhizobium bacteria fix N2 in nodules present on the roots of legumes such as soybeans.

Non-symbiotic N fixation: • N2 fixation process that is performed by free-living

bacteria and blue-green algae in the soil. • Amount of N fixed by these organisms is much lower

than that fixed by symbiotic N fixation.

Industrial N fixationAtmospheric additions:

• Small amounts of N (5-15 lbs/acre/year) from rain or snowfall, or N fixed by the electrical discharge of lightning in the atmosphere and industrial pollution.

Synthetic or industrial processes of N fixation: • Industrial fixation of N is the most important source of N as a plant

nutrient. • Based on the Haber-Bosch process where hydrogen (H2) and N2

gases react to form NH3:• N2 + 3H2 � 2 NH3• NH3 produced can be used directly as a fertilizer (anhydrous NH3)

or as the raw material for other N fertilizer products, including ammonium phosphates, urea, and ammonium nitrate.

Nitrification• Biological oxidation of ammonium (NH4

+) to nitrate (NO3-)

in the soil. • Two-step process where NH4

+ is converted first to NO2-

and then to NO3- by two autotrophic bacteria in the soil

(Nitrosomonas and Nitrobacter).• Aerobic conditions and moderate pH: suppressed below

pH 5.5Nitrosomonas

2NH4+ + 3O2 → 2NO2

- + 2H2O + 4H+

Nitrobacter2NO2

- + O2 → NO3-

Significance of Nitrification• Nitrate (NO3

-) is readily available for uptake and use by crops and microbes.

• NO3- leaching is generally a major N loss mechanism (10 – 40%)

– Minimized through proper rate and timing of N fertilizer applications

• During nitrification, 2 H+ ions are produced for every NH4+ ion that is

oxidized. These H+ cations will accumulate and significantly reduce soil pH; thus, any ammonium-containing fertilizer will ultimately decrease soil pH due to nitrification.

Mineralization and Immobilization• N mineralization is the conversion of organic N to NH4

+

– Important process in the N cycle since it results in the liberation of plant-available inorganic N forms.

• N immobilization is the conversion of inorganic plant available N (NH4

+ or NO3-) by soil microorganisms to organic N forms (amino

acids and proteins). – Reverse of mineralization– Immobilized forms of N are not readily available for plant uptake.

conversion of organic N to inorganic N

• Estimates of mineralization potential are used in nutrient management plan development

Mineralization

ImmobilizationOrganic N

FormsInorganic N

Forms

C:N Ratio• Whether N is mineralized or immobilized depends on the

C:N ratio of the organic matter being decomposed by soil microorganisms:

– Wide C:N ratios (> 30:1): Immobilization of soil N will be favored.

• Residues with wide C:N ratios include hay, straw pine needles, cornstalks, dry leaves, and sawdust.

– C:N ratios of 20:1 to 30:1: Immobilization and mineralization will be nearly equal.

– Narrow C:N ratios (< 20:1): Favor rapid mineralization of N.

• Residues with narrow C:N ratios include alfalfa, clover, manures, biosolids, and immature grasses.

Mineralization, Immobilization, and C:N Ratio

High C:N ratio material added to soil

Available soil N is immobilizedC02 evolution

increases Available N increases through N

mineralization

Time

Nitrogenavailability

N Loss Pathways

♦ denitrification

♦ volatilization

♦ leaching

♦ erosion / runoff

♦ crop removal

NO3- NO2

- NO N2O N2

Denitrification

• Biological reduction of nitrate to gaseous nitrous oxide (N2O) or elemental N (N2) and lost to the atmosphere.

• Favored in saturated (anaerobic) soil

VolatilizationAnother Gaseous

Loss♦ loss of ammonia-N to the atmosphere♦ ammonium in the presence of hydroxyl (OH-)

can produce ammonia gasNH4

+ + OH- H2O + NH3

♦ affects all surface-applied N sources* urea, ammonium nitrate, manure

♦ enhanced by warm, dry atmospheric conditions

Ammonia Volatilization

• Loss of ammonia-N to the atmosphere• Ammonium in the presence of hydroxyl (OH-)

can produce ammonia gas• Affects all surface-applied N sources

– urea, ammonium nitrate, manure• Enhanced by warm, dry atmospheric conditions• Dominated by hydrolysis of urea catalyzed by

the urease enzyme

NH4+ + OH- H2O + NH3

Nitrate Leaching• N moves freely downward

transported by drainage water

• Can lead to pollution of groundwater

• Economic loss with environmental consequences

• Minimal under natural vegetation (forests)

• Greater under modern row-crop productions

• Excessive if N management is sub-optimal– inefficient N management– heavy one-time

applications– improper timing– over-application of

manure/sludge• Enhanced by periods of

heavy rainfall

DEPARTMENT OF ENVIRONMENTAL SCIENCE & TECHNOLOGY

Laboratory for Agriculture and Environmental Studies

The Phosphorus Cycle

Plantuptake

Soil solutionphosphorus•HPO4

-2

•H2PO4-1

Primaryminerals(apatite)

Secondarycompounds

(CaP, FeP, MnP, AlP)

Mineralsurfaces

(clays, Fe and Al oxides)

Organic phosphorus•Microbial•Plant residue•Humus

Crop harvest

Runoff anderosion

Leaching(usually minor)

Loss

Animalmanures

and biosolids Mineralfertilizers

Plant residues

Input

(www.ppi-ppic.org)

P in soil solution

Organic amendments

Crop residue

Organic matter

Soil biota

Crop removal

Runoff, erosion, and

leaching

Phosphate minerals

Commercial Fertilizer

P in soil solution

Traditional Ag P Cycle

Crops

Local

Manure

AnimalsSoil

¼

¾

Ag P Cycle Has Become Fragmented

AnimalsSoil

Crops

Manure? ? ?

Feed mill

Global

¼

¾

Why P management?

• P is essential to all forms of life on earth –no known toxic effects

• Adequate P levels in soils are essential for production of agronomic crops

• In most fresh surface water bodies growth of algae or aquatic plants is limited by P availability

• Vigorous crop (Shoot/Root) growth

• Improved resource utilization

• water, nutrients• positive environmental implications

• Better resistance to stress

• disease, pest, moisture, temperature

• Earlier maturity

• good grain & fruit development• better crop quality, yield

P impact on crops

Agronomic characteristics of P deficiency

… purpling of leaves / stems

… darkened leaves

• stunted growth

P Deficiency

• reduced leaf number, expansion & surface area

Factors influencing P availability and mobility

• Soil pH:– In acid soils P precipitates as insoluble Fe

and Al minerals– In neutral and calcareous soils P precipitates

as insoluble Ca phosphates– Soil P is most available in the pH range of 5.5

to 6.8

Phosphorus Fixation

0

20

40

60

80

100

3 4 5 6 7 8

Soil pH

Perc

ent

of t

otal s

oil P

(%)

Fe & Al chemical fixFe & Al oxidesSilicate minerals"Available" PCa phosphates

pH and P availability

P precipitates as Fe and Al phosphates

P precipitates as Ca

phosphates

P is most available at pH

5.5 to 6.8

Phosphorus availability

3 4 5 6 7 8

Soil pH

Factors influencing P availability and mobility

• P moves from soil solids to plant roots through diffusion– Occurs over short distances < 0.25”

• Plant roots can only obtain P located in close proximity

– Dry soils reduce diffusion

Factors influencing P availability and mobility

• Crops use only 10 – 30% of P fertilizer in the first year– The remainder goes into reserve and may be

used by later crops– Many growers have built up large reserves of

soil P

Factors influencing P availability and mobility

• Timing and placement• Most agricultural soils are naturally low in

available P– Many years of P fertilization have resulted in

many soils that test high in available P• A small amount of starter fertilizer placed

close to the seed may prove beneficial in these soils under cold conditions

P transport to surface waters• Occurs primarily via surface

flow– Dissolved P – 100%

biologically available– Particulate P – carried on

eroded particles, not immediately bio-available

• Leaching and lateral subsurface flow in limited situations

• If the soil becomes saturated with P the potential for P loss increases significantly

• Repeated applications of P in excess of crop needs can saturate a soil with P

• Very high levels of soil test P can result from over-application of manure, biosolids, or commercial phosphate fertilizer. Soils with these high soil test P levels will require several years of continuous cropping without P additions to effectively reduce these high P levels.

Modified from the Potash & Phosphate Institute web site at www.ppi-ppic.org

Animalmanures

and biosolids

Mineralfertilizers

Crop harvest

Runoff anderosion

Leaching

Soil solution potassium (K+)

Plant residues

Plantuptake

Mineralpotassium

Fixedpotassium

Exchangeable potassium

Input to soilComponent Loss from soilThe Potassium Cycle

Potassium Availability• Plant-available K:

– Mineral K accounts for 90 to 98% of the total soil K, – Readily and slowly available K represent only 1 to 10% of the total soil

K. Available K (can be readily absorbed by plant roots) includes:• the portion of the soil K that is soluble in the soil solution.

• Exchangeable K held on the exchange complex and is in equilibrium with K in the soil solution:

Exchangeable K ↔ Solution K• K is continuously made available for plant uptake through the cation

exchange process. There is a slow transfer of K from soil minerals to exchangeable and slowly available forms as K is removed from the soil solution by crop uptake and leaching.

K Forms and Mobility• Fate of K fertilizer in the soil:

– CEC: K+ cations can be attracted to the cation exchange complex where they are held in an exchangeable form and readily available for plant uptake.

– Soil solution: Some of the K+ ions will remain in the soil solution.– Plants: Exchangeable and soluble K may be absorbed by plants.– Fixation: In some soils, some K may be “fixed” by the clay fraction.– Leaching: Applied K may leach from sandy soils during periods of heavy

rainfall.

• Movement of K in the soil:– K moves more readily in soil than P, but less readily than N. – Since K is held by cation exchange, it is less mobile – in fine-textured soils and most readily leached from sandy soils. – Most plant uptake of soil K occurs by diffusion.

Placement of K Fertilizer• Placing K fertilizer:

– K fertilizers are completely water-soluble and have a high salt index, so can decrease seed germination and plant survival when placed too close to seeds or transplants.

– One method of applying K fertilizers is by broadcasting and mixing with the soil before planting:

• Fertilizer injury is minimized by this method but, on sandy soils, some K may be lost by leaching.

– Row placement of K fertilizer is generally more efficient than broadcast applications when the rate of application is low or soil levels of K are low.

Timing K Fertilization

• Timing of K fertilizer applications: – Luxury consumption is a term used to

describe the tendency of plants to take up K far in excess of their needs if sufficiently large quantities of available K are present in the soil.

– Split application of K can minimize luxury consumption and provide adequate available K during the latter part of the growing season.

U.S. Fertilizer Prices at Record Levels

50

100

150

200

250

300

350

400

450

50019

95

July

1996

July

1997

July

1998

July

1999

July

2000

July

2001

Jul y

2002

Jul y

2003

Jul y

2004

Jul y

2005

Jul y

2006

Jul y

2007

Jul y

2008

July

1990

-199

2 =

100

Index of Fertilizer Prices Paid by Farmers, Jan. 1995 - Oct. 2008

Source: National Agricultural Statistics Service, USDA.

Increase of 347 percentJanuary 2000 - October 2008

http://www.enst.umd.edu/laes

Lab for Ag & Environmental Studies

mcgrathj@umd.edu