Meaning & Definition

Soil organic matter is any material produced originally by living organisms (plant or animal) that is returned to the soil and goes through the decomposition process.

  • It consists of a range of materials from the intact original tissues of plants and animals to the substantially decomposed mixture of materials known as humus.
  • Crop residues added to the soil are decomposed by soil macro fauna and micro-organisms, increasing the organic matter content of the soil.
  • Most soil organic matter originates from plant tissue. Plant residues contain 60-90 percent moisture. The remaining dry matter consists of carbon (C), oxygen, hydrogen (H) and small amounts of sulphur (S), nitrogen (N), phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg). Although present in small amounts, these nutrients are very important from the viewpoint of soil fertility management.

Components of SOM

Soil organic matter (SOM) is the organic component of soil, which consists of three primary parts-

  • small (fresh) plant residues and small living soil organisms,
  • decomposing (active) organic matter, and
  • Stable organic matter (humus).

An active organic fraction including microorganisms (10-40 percent), and resistant or stable organic matter (40-60 percent), also referred to as humus. Soil organic matter serves as a reservoir of nutrients for crops, provides soil aggregation, increases nutrient exchange, retains moisture, reduces compaction, reduces surface crusting, and increases water infiltration into soil. Plant residues on the soil surface such as leaves, manure, or crop residue are not considered SOM.

The analysis of SOM content provides estimates for Nitrogen, Phosphorus and Sulfur mineralized available for crop production and adjust fertilizer recommendations.

Inherent factors affecting soil organic matter

  • Factors such as climate and soil texture cannot be Climatic conditions, such as rainfall, temperature, moisture, and soil air affect the rate of organic matter decomposition.
  • Organic matter decomposes faster in warm and humid climates and slower in cool and dry climates.
  • Organic matter also decomposes faster when soil is well aerated (higher oxygen levels) and much slower on saturated wet
  • Soils formed under grass vegetation usually have high organic matter as those formed under forests because organic material is added to topsoil from both top growth and roots that die back every year. Soils formed under forests usually have comparably low organic-matter levels because of Trees produce a much smaller root mass per acre than grass plants, and do not die back annually and decompose every Instead, much of the organic material in a forest is tied up in the tree’s wood rather than being returned to the soil annually.

Soil organic matter Management

  • SOM generally increases where biomass production is higher and where organic material additions occur.
  • Plant residue with a low C: N ratio decomposes more quickly than those with a high C: N ratio and do not increase soil organic matter levels as
  • Excessive tillage destroys soil aggregates increasing the rate of soil organic matter
  • Measures that increase soil moisture, soil temperature, and optimal aeration accelerate SOM decomposition.

Some management measures that can increase SOM are as below.

  • Use of cropping systems.
  • Reducing or eliminating tillage.
  • Reduce erosion especially top soil using appropriate.
  • Soil-test and fertilize properly.
  • Use of perennial forages provides for annual die back and re-growth of perennial grasses and their extensive root systems and aftermath contributing organic matter to soil.

Soil OM relationship to soil function

  • Increase Nutrient Supply
  • Water-Holding Capacity
  • Soil Aggregation- Organic matter improves soil aggregation, which improves soil With better soil structure, water infiltration through the soil improves, which improves soil’s ability to take up and hold water.
  • Erosion Prevention- Because of increased water infiltration and stable soil aggregates erosion is reduced with increased organic

Frequently Asked Question (FAQ)

What is the difference between soil organic carbon (SOC) and soil organic matter (SOM)?

Soil organic carbon is a component of soil organic matter. Organic matter is primarily made up of carbon (58%), with the remaining mass consisting of water and other nutrients such as nitrogen and potassium.

What is the difference between soil organic carbon and total carbon in soil?

SOC is the carbon component of organic matter that is measured in soil that passes through a 2 mm sieve. Organic matter that does not pass through a 2 mm sieve is not sufficiently degraded to be considered as part of the SOM; they may be surface or buried organic residues.

What is the content of Soil Organic Carbon (SOC) in dryland agricultural soils globally?

SOC is generally between 0.5–4.0 percent in dryland agricultural soils.

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