Many times, organic matter is considered as the plant and animal residues incorporated into the soil. This is actually organic material, not organic matter. Organic material is anything that was alive and is now in or on the soil.
For it to become organic matter, it must be decomposed. Organic material is unstable in the soil, changing form and mass readily as it decomposes.
As much as 90 % of it disappears quickly because of decomposition. Organic matter is stable in the soil. It has been decomposed until it is resistant to further decomposition.
Organic matter in soil has been described as “the most complex and least understood component of soils” (Magdoff and Weil, 2004).
According to SSSA (1987), organic matter in the soil is defined as the organic fraction of the soil exclusive of undecayed plant and animal residue.
However, organizations and individual researchers differ in their opinion about whether undecayed plant and animal tissues (e.g. Stover, dead bugs, earthworms, etc.) should be included in the definition of soil organic matter.
A broader definition of soil organic matter is adopted as proposed by Magdoff (1992), which considers soil organic matter to be the diverse organic materials, such as living organisms, slightly altered plant and animal organic residues, and well-decomposed plant and animal tissues that vary considerably in their stability and susceptibility to further degradation.
Simply put, soil organic matter is any soil material that comes from the tissues of organisms (plants, animals, or microorganisms) that are currently or were once living.
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. Most soil organic matter originates from plant tissue. Plant residues contain 60–90% moisture.
The remaining dry matter consists of carbon, oxygen, hydrogen, and small amounts of sulphur, nitrogen, phosphorus, potassium, calcium, and magnesium.
Read Also: Factors Affecting the Decomposition of Organic Matter in Soil
Organic Matter Decomposition in Soil
Decomposition is a biological process that includes the physical breakdown and biochemical transformation of complex organic molecules of dead material into simpler organic and inorganic molecules (Juma, 1998).
Its speed is determined by three major factors: soil organisms, the physical environment, and the quality of the organic matter (Brussaard, 1994). In the decomposition process, different products are released: carbon dioxide, energy, water, plant nutrients, and re-synthesized organic carbon compounds.
Successive decomposition of dead material and modified organic matter results in the formation of a more complex organic matter called humus (Juma, 1998). This process is called humification.
Decomposition of organic matter occurs when an organism dies (or parts of it die) and is on or in the soil. The invertebrates, fungi, and bacteria living in and on the soil consume it. By so doing, they assimilate many of its nutrients into themselves.
This process is called “nutrient immobilization.” As long as these nutrients are in the bodies of the decomposers, they are not available for plant uptake, hence the term “immobilization;” they are temporarily unavailable.
When the nutrients are immobilized, they are in organic form. Decomposers can also immobilize nutrients that are applied inorganically and tie them up temporarily in organic form. This is particularly common with nitrogen, and particularly when the carbon: nitrogen ratio of the litter is very high, such that nitrogen is in great demand.
The decomposers all tend to be short-lived. They die and are consumed by others, often through a complex succession of organisms. However, gradually, the nutrients that they contained are converted to inorganic forms either through biological action or by leaching by water.
This process is called mineralization; the conversion of organically bound forms of nutrients to inorganic forms as the result of inorganic or biological chemical reactions. At this point, the nutrients are available for plant uptake.
Thus, in soils, death, immobilization, and mineralization, coupled with plant uptake, are constantly occurring. Nutrients gradually become available from the decay of organic inputs, which act essentially like timed-release fertilizers.
This is nutrient cycling. By breaking down carbon structures and rebuilding new ones or storing the C into their own biomass, soil biota plays the most important role in nutrient cycling processes and, thus, in the ability of a soil to provide the crop with sufficient nutrients to harvest a healthy product.
Read Also: General Importance of Soil Organic Matter
