Embed Size (px)
Citation preview
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
