The Nitrogen Cycle

1
The Nitrogen Cycle
Component
Input to soil
Loss from soil
Atmospheric nitrogen
Industrial fixation (commercial fertilizers)
Atmospheric fixation and deposition
Crop harvest
Animal manures and biosolids
Volatilization
Plant residues
Runoff and erosion
Biological fixation by legume plants
Plant uptake
Denitrification
Organic nitrogen
Nitrate (NO3)
Ammonium (NH4)
-
Immobilization

Leaching
Mineralization
2
The Phosphorus Cycle
Component
Input to soil
Loss from soil
Atmospheric deposition
Crop harvest
Animal manures and biosolids
Mineral fertilizers
Plant residues
Runoff and erosion
Primary minerals (apatite)

• Organic phosphorus
• Microbial
• Plant residue
• Humus

Mineral surfaces (clays, Fe and Al oxides,
carbonates)
Plant uptake
Weathering
Adsorption
Immobilization
Mineralization
Desorption

• Soil solution
• phosphorus
• HPO4-2
• H2PO4-1

Secondary compounds (CaP, FeP, MnP, AlP)
Dissolution
Leaching (usually minor)
Precipitation
3
The Potassium Cycle
Component
Input to soil
Loss from soil
Animal manures and biosolids
Crop harvest
Plant residues
Mineral fertilizers
Runoff and erosion
Exchangeable potassium
Plant uptake
Soil solution potassium (K)
Fixed potassium
Mineral potassium
Leaching
4
The Sulfur Cycle
Component
Input to soil
Loss from soil
Atmospheric sulfur
Volatilization
Atmospheric deposition
Crop harvest
SO2 gas
Mineral fertilizers
Plant residues
Animal manures and biosolids
Elemental sulfur
Runoff and erosion
Absorbed or mineral sulfur
Plant uptake
Oxidation
Organic sulfur
Reduced sulfur
Bacterial oxidation
Immobilization
Sulfate Sulfur (SO4)
Bacterial reduction
Mineralization
-
Leaching