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Living organisms, active dead organic matter (slightly decomposed or undecomposed, labile), and well-decomposed (humified) relatively stable materials are the three main categories of organic matter that may be found in soils.
The organic component can have a significant impact on soil characteristics, ecosystem functioning, and the scale of many necessary ecosystem activities, despite its frequently insignificant proportion to the overall mass of mineral soils.
The three major categories of soil characteristics that organic matter affects are biological, chemical, and physical characteristics. It should be highlighted that these groups have close relationships and depend on one another heavily.
Additionally, changes in soil characteristics seen across various soils may not be exclusively the result of qualitative or quantitative variations in the soil organic component. The impacts of organic matter on soil properties frequently entail interactions with the soil mineral fraction.
Each one of these fractions serves important roles in maintaining and improving soil quality.
These three different parts that together make up soil organic matter will be discussed separately below.
Biological Diversity of Soil Organisms:
Good crop and soil management should prioritize fostering an extremely diversified variety of soil organisms. Similar to variety above ground, biological diversity in the soil plays a significant role in the stability and health of agroecosystems. A diverse array of species forms a system where competition for food supplies and habitats, as well as dynamics between predators and prey, contribute to control populations of pathogenic bacteria, fungus, plant parasitic nematodes, and pest insects.
Microbial populations are influenced by cropping and residue management.
In general, diversity of organisms is decreased, and the amount of microbial biomass is decreased by shifting from natural ecosystems to agroecosystems.
Management of the soil and the crop can affect the population dynamics of soil organisms. Cover crops, decreased tillage, complex crop rotations with a variety of crops, huge quantities of various agricultural residues and manures, and complex crop rotations all encourage a biologically diversified population of soil organisms.
Using mulches or just leaving residues on the soil surface will promote the populations of surface feeding earthworms. Surface residues tend to accentuate the importance of fungi in the decomposition process.
Soil Organisms:
A wide variety of organisms make up the living organic matter in soil. Small to medium sized arthropods, earthworms, tiny viruses, bacteria, fungus, and protozoa are a few examples of these species. In general, population density declines as organism size rises. For instance, according to Smil (1991), there are around 1,014 bacteria, 10 fungus, 10 nematodes, and 102 earthworms per square meter.
It is true that soils contain parasite worms, insects, and germs that can cause illness. The vast majority of soil organisms in all categories, however, do not harm plants and instead feed on crop and other organic leftovers as well as other soil creatures.
The great majority of soil organisms are really crucial to soil quality because of their roles in the cycle of nutrients, the control of insect populations, the production of compounds that encourage the formation of soil aggregates, and the production of humic substances.
Through the process of photosynthesis, green plants obtain their energy from the sun, their carbon, which serves as the "backbone" of all organic molecules, from atmospheric carbon dioxide, the oxygen required for respiration (which recovers and uses the energy stored in organic molecules), and the remaining nutrients (N, P, K, S, Ca, Mg, Fe, B, Mn, Cu, Mo, Cl, Zn, Co) as well as water (H20) from the soil.
The concept of a food web is that the organisms within a particular ecosystem are related to an underlying food source and to each other by virtue of their food source(s).
Primary consumers: Organisms in the soil that are the first to use agricultural and other wastes as fuel. Many fungi are early colonizers of plant detritus and work to soften it so that other species can utilize it more easily. Numerous bacteria, as well as sowbugs, nematodes, fly larvae, and other organisms, are key consumers. Some earthworms are major consumers as well, and their digestive processes work to macerate and combine the leftovers with bacteria so that their castings are easily accessible for other creatures to keep using. Nearly all fertility indicators, including the amount of calcium, potassium, and nitrogen that is readily available, are substantially greater in these earthworm castings than in the nearby soil.
Secondary consumers: Wildlife types that feed on primary consumers. Nematodes and protozoa are two types of organisms that feed on bacteria and fungus. It has been claimed that nematodes may devour bacteria at rates of up to 5,000 cells per minute, and it is believed that secondary feeders consume roughly 50% of the fungus and bacteria produced annually while feeding. More diversified populations of these species in soil may be maintained by the existence of active populations of feeders on bacteria and fungus. Protozoa and nematodes are important contributors to the nitrogen cycle because when they consume bacteria, extra nitrogen is converted to ammonium and discharged into the soil solution. Mites and springtails (Collembola) are some other secondary consumers.
Tertiary consumers: It consists of ants, centipedes, ground beetles. Other soil organisms are the main food source for this fauna. Some centipedes and ants, as well as termites, a key consumer that builds mounds, can aid in blending and loosening the soil due to their huge size and propensity for digging.
Plant roots are also an important aspect of life within the soil. Products derived from photosynthesis above ground are translocated to roots for their own metabolism.
The rhizosphere commonly contains 10 to 50 times the number of organisms as found in soil at some distance from the root.
Well-Decomposed Organic Matter:
The well-decomposed and relatively stable fraction of soil organic matter is usually called humus.
Humus has a significant affinity for the clay and silt fractions and has a typical residence time in soil that is on the order of hundreds or thousands of years. When compared to biological materials linked with clay, silt size mineral particles seem to be more stable.
About 2% to 5% of soil humus decomposes each year, which is a rather moderate rate. The majority of the cation exchange capacity (negative charges that enable the retention of certain nutrients like calcium, magnesium, and potassium) in organic matter is found in humus.
It appears that the extreme circumstances placed on organic molecules by these conditions cause bonds to be broken, either with silt and clay particles or within organic molecules, solubilizing large amounts of organic matter and making it simple for organisms to access the released molecules.
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The Nature of Soil Organic Matter.
By K H Akhil Srinivas.
(KHAS).
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