Soil Organic Matter and Nutrient Management

Main organic compounds in plant litter

Proteins, carbohydrates (cellulose, hemicellulose, starches), fats and waxes, lignin.

Decomposition rates

Fastest - proteins, simple sugars; Intermediate - cellulose, hemicellulose; Slowest - lignin, fats, waxes.

Steps of soil organic matter formation

Shredding by soil fauna, enzymatic oxidation, nutrient release or uptake, microbial synthesis, stabilization into humus.

Three ways SOM is protected from decomposition

Aggregate protection, mineral protection (binding to clay), biochemical recalcitrance (complex molecules like lignin).

Why only a portion of added organic matter becomes SOM

Some is mineralized into CO2, some is consumed by microbes, only a fraction stabilizes.

Nutrients released from organic matter decomposition

Nitrogen (N), phosphorus (P), sulfur (S), potassium (K), calcium (Ca), magnesium (Mg), micronutrients.

Effect of C:N, C:S, and C:P ratios

High ratios lead to immobilization (nutrients tied up by microbes), low ratios lead to mineralization (nutrients released).

Labile vs. stable SOM

Labile - fast turnover, high microbial availability; Stable - slow turnover, contributes to long-term soil fertility.

Effects of SOM on soil properties

Improves aggregation, increases water retention, enhances nutrient availability, supports microbial activity.

Steady-state SOM

Inputs (residues, manure) equal outputs (decomposition, erosion).

Effect of carbon input changes on SOC stocks

Increasing inputs raises SOC, decreasing inputs or increasing losses lowers SOC.

Management practices affecting SOC

Conservation tillage, cover crops, manure application, crop rotation.

Environmental conditions with high SOC stocks

Cold/wet climates, high clay content soils, undisturbed soils.

Time scale for SOC changes

Decades to centuries.

Plant use of nitrogen

Essential for amino acids, proteins, nucleic acids, chlorophyll.

Nitrogen deficiency symptoms

Yellowing of older leaves, stunted growth.

Principal forms of nitrogen uptake

Ammonium (NH4+), Nitrate (NO3-).

Largest nitrogen pool in soil

Organic nitrogen in SOM.

Why nitrogen is often limiting despite large atmospheric pool

Plants cannot directly use N2 gas.

Major sources of bioavailable nitrogen

Mineralization, fertilizers, biological fixation.

Estimating nitrogen mineralization from SOM

Use % SOM and mineralization rate (1-4% per year).

Estimating nitrogen mineralization/immobilization from residue

Based on residue C:N ratio.

Key nitrogen transformations

Nitrification (NH4+ to NO3-), Denitrification (NO3- to N2 gas), Ammonia volatilization (NH4+ to NH3 gas).

Conditions promoting nitrogen losses

Nitrification - warm, aerated soils; Nitrate leaching - high rainfall, sandy soils; Denitrification - waterlogged soils; Ammonia volatilization - high pH, surface-applied urea.

Three most common nitrogen fertilizers

Urea (CO(NH2)2), Ammonium nitrate (NH4NO3), Anhydrous ammonia (NH3).

Ammonium-forming fertilizers and volatilization

Urea releases NH3 gas, leading to losses.

Fertilizer nitrogen calculation

Target N rate ÷ (%N in fertilizer × 100).

Plant-available nitrogen from manure

Depends on manure type, C:N ratio, and decomposition rate.

Plant use of sulfur

Amino acids, proteins, enzymes.

Sulfur vs. nitrogen concentrations in soil/plants

Sulfur is lower than nitrogen.

Sulfur deficiency symptoms

Yellowing of young leaves.

Forms taken up

Sulfate (SO4²-), sulfur dioxide (SO2).

Largest sulfur pool

Organic matter.

Soil properties promoting sulfate adsorption

Low pH, high clay, high Fe/Al oxides.

Reasons for increasing sulfur deficiencies

Less atmospheric deposition, high-yield cropping.

Timing of sulfur applications

Elemental S - before planting; Sulfates - shortly before planting.

Plant use of phosphorus

ATP, DNA, membranes.

Phosphorus vs. nitrogen/sulfur concentrations

Phosphorus is lower than both.

Phosphorus deficiency symptoms

Purple leaves, slow growth.

Forms taken up

H2PO4-, HPO4²-.

Soil pH & phosphorus solubility

Low pH - Fe/Al fixation; High pH - Ca precipitation.

Orthophosphate leaching risk

Low, binds to soil particles.

Why pH 6.5 is best for phosphorus availability

Avoids fixation by Fe, Al, or Ca.

Phosphorus fixation causes

Binding to Fe, Al, or Ca.

Soil phosphorus pools

Mostly in organic and mineral forms.

Major phosphorus fertilizers

Monoammonium phosphate (MAP), Diammonium phosphate (DAP), Rock phosphate.