The soil is a living web of diversity. Billions of microorganisms thrive in the soil alongwith earthworms, insects,mites and so on. The nature and number of organisms in the soil depends on the food source and supply, temperature and moisture of the soil.
The region of the soil in immediate vicinity of the root systems of plants is called the rhizosphere. The rhizosphere is a zone of intense biological activity between micro-organisms and organic substances exuded by the roots and root caps being sloughed off.
Bacteria benefit most from the food supplied in the rhizosphere and may form a continuous film around the roots. Bacteria reciprocate by releasing essential nutrients from organic matter and soil minerals and making them available to the roots. Some bacteria have been found to release plant hormones which stimulate root growth. Bacteria also cycle nitrogen, by either fixing nitrogen on plant roots or independent of plant association. Bacteria have also been found useful in increasing the solubility of nutrients, improving soil structure, fighting root diseases, and detoxifying soil (by bio-accumulation).
Bacteria are the most numerous among all microorganims in the soil.
The rhizobium bacteria inhabits small lumps on the roots of leguminous plants like beans, peas etc., feeding on carbohydrates from the plant and providing the plant with soluble nitrogen compounds (ammonia and nitrates) by fixing atmospheric nitrogen gas.
Similar to the symbiosis between plant roots and bacteria is the relationship between the plant roots and fungi. Fungi grow as many different species, sizes, and shapes in soil. One of these is the mycorrhizae fungi which colonise the roots of the plants during periods of active growth.
Mycorrhiza = mikès (fungus) + rhiza (roots)
An estimated 95% of all plant species form symbiotic associations with mycorrhizae (with the exception of the Brassicaceae- mustard, cabbage, radish etc. and Chenopodiaceae- beetroot, spinach etc.).
The plant shares the food it synthesises with the mycorrhizae fungi and the fungi shares some of the nutrients it gathers with the plant. The fungi also also increase uptake of water and nutrients by forming an extensive hyphal network on the roots, increasing their effective absorptive surface area. A plant with mycorhiza association is often more competitive for nutrients and water and is able to tolerate environmental stresses.
Effective mycorrhizal colonisation can also increase the nodulations and symbiotic nitrogen fixation in mycorrhizal legumes.
Other fungi aid plants by breaking down organic matter, producing plant hormones, and by producing antibiotics which keep the plant free of diseases.
Arbuscular Mycorrhizas (AM) is the most common type of mycorrhiza, occurring in about 80% of plant species. AM also contributes to maintaining a healthy bacterial population in the rhizosphere.
Actinomycetes are the bacteria that give a biologically active soil, the sweet smell. Their population in the soil is much smaller than other bacteria but they have been found to be important in decomposition and prevention of root diseases.
Algae, like plants, produce their food by photosynthesis, living in/on the surface layer of the soil. Algae produce slimy substances that glue soil into aggregates, lending a good structure to the soil. Some species of algae can also fix nitrogen and later release it to the plants.
Protozoa are free living organisms floating in the water between the soil particles. Most protozoa feed on micro-organisms, cycling the nutrients from the micro-organisms to the plants. e.g. amoeba feeding on bacteria.
Nematodes feed on decaying plant matter, bacteria, fungi, algae, protozoa and other nematodes and speed up the nutrient cycling in the soil.
earthworms and arthropods
The earthworms, the millipedes, centipedes, slugs, snails and others perform the role of primary decomposers by chewing and breaking down the organic matter into smaller bits with larger surface area, on which feed bacteria and fungi and release water soluble nutrients to plants.
Worms and insects prey on smaller soil organisms releasing microbial remains which help bind soil particles together. So do the sticky substances on the skin of earthworms and those produced by fungi and bacteria.
Thus, the living part of soil is responsible for giving a good structure to the soil, keeping air and water and nutrients available to the plants and reducing soil toxicity.
The organic matter in the soil consists of living and dead/decomposing plant and animal matter. The decomposing dead material is transformed by soil organisms into humus (final stages of decomposition), in which process nutrients are released and made available to the plants. Both organic matter and humus serve as reservoirs of plant nutrients; they also help to build soil structure.
The rate at which organic matter is broken down depends largely on the climate. In warm, damp conditions the organic matter is broken down faster than in cold or dry conditions.
Humus is the material that has been broken down so far that the original fresh material is no longer distinguishable. It gives the soil a dark colour. Because of the complex structure of humic substances, humus cannot be used by many microorganisms as an energy source and remains in soil for a relatively long time until its further broken down into simple nutrients. This provides a buffer against fluctuations in soil acidity and nutrient availability.
The humus content also influences the physical, chemical and biological properties of the soil by enhancing the water holding capacity, aeration through good structure and CEC by helping to dissolve, mobilise and transport metals and organics in soil and water.
About 35-55 per cent of the non-living part of organic matter is humus.
Humus consists of different humic substances such as fulvic acids (soluble in water at all pH), humic acids (soluble in water only above pH 2) and humin (not soluble in water at any pH). Fulvic acids are produced in the earlier stages of humus formation. The relative amounts of humic and fulvic acids in soils vary with soil type and management practices. The humus of forest soils is characterised by a high content of fulvic acids, while the humus of agricultural and grassland areas contains more humic acids.
Soil, because of its thermal mass, acts as an insulator for the soil life. A healthy soil will maintain a consistent chemical and biological activity within, which in turn keeps the soil healthy.