4.3 Soil organic matter composition and dynamics

Soil organic matter is the lifeblood of healthy soil. It's a complex mix of decomposing plant and animal residues, living organisms, and their byproducts. This vital component plays a crucial role in nutrient cycling, soil structure, and water retention.

Managing soil organic matter is key to sustainable agriculture. Practices like cover cropping, reduced tillage, and adding organic amendments can boost SOM levels. These strategies improve soil health, increase crop yields, and help mitigate climate change by sequestering carbon in the soil.

Soil Organic Matter Composition

Components of soil organic matter

Top images from around the web for Components of soil organic matter

• 

  Frontiers | Crops for Carbon Farming View original

  Is this image relevant?

• 

  The Soil | Boundless Biology View original

  Is this image relevant?

• 

  Soil Organic Matter – Digging into Canadian Soils View original

  Is this image relevant?

• 

  Frontiers | Crops for Carbon Farming View original

  Is this image relevant?

• 

  The Soil | Boundless Biology View original

  Is this image relevant?

1 of 3

Top images from around the web for Components of soil organic matter

• 

  Frontiers | Crops for Carbon Farming View original

  Is this image relevant?

• 

  The Soil | Boundless Biology View original

  Is this image relevant?

• 

  Soil Organic Matter – Digging into Canadian Soils View original

  Is this image relevant?

• 

  Frontiers | Crops for Carbon Farming View original

  Is this image relevant?

• 

  The Soil | Boundless Biology View original

  Is this image relevant?

1 of 3

• Soil organic matter (SOM) forms complex mixture of organic compounds derived from decomposing plant and animal residues including living organisms and their byproducts
• Fresh organic residues encompass recently added plant litter and animal waste undergoing initial decomposition stages
• Particulate organic matter consists of partially decomposed organic materials visible to the naked eye (leaf fragments, root pieces)
• Humus represents stable, complex organic compounds resistant to further decomposition formed through chemical and biological processes
• Dissolved organic matter comprises soluble organic compounds easily transported in soil water (organic acids, simple sugars)
• Carbon content of SOM typically ranges from 50-58% forming the backbone of organic molecules

Decomposition of soil organic matter

• Decomposition breaks down complex organic molecules into simpler forms carried out by diverse soil microorganisms (bacteria, fungi) releasing nutrients for plant uptake
• Initial rapid decomposition targets easily degradable compounds (sugars, proteins) followed by slower breakdown of more resistant materials (lignin, cellulose)
• Temperature influences decomposition rate with higher temperatures generally accelerating microbial activity and organic matter breakdown
• Moisture affects decomposition with optimal levels promoting microbial growth while extremes (too dry or waterlogged) inhibit activity
• Oxygen availability impacts decomposition speed with aerobic conditions favoring faster breakdown compared to anaerobic environments
• Soil pH affects microbial activity with neutral to slightly alkaline conditions (pH 6.5-8) generally promoting optimal decomposition rates
• C:N ratio of organic matter influences decomposition speed with lower ratios (10:1 to 20:1) leading to faster breakdown than higher ratios (>30:1)
• Soil texture and structure impact decomposition by affecting oxygen diffusion clay content and aggregation protecting organic matter from microbial attack

Functions of soil organic matter

• Nutrient cycling enhanced by SOM serving as source of essential plant nutrients (N, P, S) increasing cation exchange capacity (CEC) and improving nutrient retention and availability
• Soil structure improved through promotion of soil aggregation enhancing soil tilth and workability while reducing compaction
• Water retention increased by SOM improving soil porosity and water holding capacity enhancing infiltration and drainage
• pH buffering provided by SOM helping maintain stable soil acidity levels
• Soil biodiversity supported by SOM providing food and habitat for diverse soil organisms
• Carbon sequestration achieved through SOM accumulation mitigating climate change by storing atmospheric CO2 in soil

Management for soil organic matter

• Cover cropping adds organic residues to soil protects from erosion (clover, rye)
• Crop rotation diversifies organic inputs improves soil health (corn-soybean-wheat)
• Reduced tillage or no-till practices minimize SOM oxidation preserve soil structure
• Organic amendments like compost and manure incorporation increase SOM content
• Residue management leaves crop residues on soil surface or uses mulching to protect and add organic matter
• Agroforestry practices integrate trees with crops or pastures enhancing SOM inputs (alley cropping)
• Avoiding bare fallow periods maintains living roots in soil contributing to continuous organic matter input
• Proper irrigation management prevents waterlogging or excessive drying optimizing conditions for SOM preservation
• Balanced fertilization promotes optimal plant growth and residue production contributing to SOM accumulation
• Avoiding excessive nitrogen application prevents accelerated SOM decomposition maintaining long-term soil organic matter levels