What actually constitutes soil organic matter?

Written by

picture of Prof. Samuel Fitzgerald, Ph.D.

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Published: November 28, 2025

Updated: November 28, 2025

Soil organic matter is the living base afoot. This complex substance forms when natural processes decompose living tissues. Unlike the vegetable coating, it becomes incorporated into the soil's structure. Soil fertility and general health depend on its composition.

Real organic matter has many different parts that work together. Microbial biomass is the sum of the active bacteria and fungi colonies. These organisms process and supply nutrients, and they contribute to the soil's structure. Their presence is indicative of a healthy soil ecosystem conducive to plant growth.

Stable humus is produced through many years of biological change. This dark material is derived from organic polymers that have been completely decomposed. Unlike fresh residues, humus is not subject to rapid decomposition. It provides long-term storage of nutrients and appreciably improves the soil texture.

Microbial Constituents

Bacteria colonies processing simple sugars rapidly
Fungal networks decomposing complex cellulose compounds
Actinomycetes creating earthy-smelling geosmin compounds
Protozoa regulating bacterial populations naturally

Plant-Derived Elements

Root exudates released by living plants
Fine root hairs decomposing within weeks
Charcoal fragments from incomplete combustion
Lignin polymers breaking down over years

Organic Component Decomposition Timelines
Component	Decomposition Process	Persistence
Microbial Biomass	Rapid cycling through predation	Days to weeks
Root Exudates	Immediate microbial consumption	Hours to days
Humus	Slow mineralization	Years to decades
Charcoal	Oxidative surface breakdown	Centuries

Persistence depends on soil conditions and management

Organic Component Decomposition Timelines

ComponentMicrobial BiomassDecomposition ProcessRapid cycling through predationPersistence

Days to weeks

ComponentRoot ExudatesDecomposition ProcessImmediate microbial consumptionPersistence

Hours to days

ComponentHumusDecomposition ProcessSlow mineralizationPersistence

Years to decades

ComponentCharcoalDecomposition ProcessOxidative surface breakdownPersistence

Centuries

Persistence depends on soil conditions and management

These elements work together to promote optimal conditions for growing. Humus contributes to water-holding capacity, and microbes will mineralize nutrients. Charcoal, also known as biochar, will serve as a long-term carbon sink. As a result, balanced organic matter will be vital for sustainable agriculture, eliminating the need for chemical inputs.

Farmers are capable of handling organic processes adequately. Frequent compost applications increase the populations of microbes. Cutting back on tillage protects delicate humus structures. Plants cultivated as cover crops contribute fresh root exudates that can increase seasonally. These practices foster adaptable soil ecosystems.

Read the full article: Soil Organic Matter: The Essential Guide

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