Soil Organisms & Their Importance
Plants and animal residues deposited on the surface of the soil are decomposed by organisms (flora and fauns) present in the soil. These organisms are called soil organisms. This decomposition results in release of carbon dioxide, water, essential elements of humus. Most of the soil organisms belong to the plant life (flora) which includes soil algae, soil fungi, soil actinomycetes, soil bacteria, etc. The animal life (fauna) includes rodents, earthworms, termites, ants, nematodes protozoans, and rotifers. These organisms play an important role in early stages of breakdown of organic residues. Most soil organisms, plants or animals, are so small that they can be seen only with the aid of a microscope. These are called soil micro-organisms (microflora and microfauna) or microbiota. The larger plants and animals of the earth are called soil micro-organisms or macrobiota.
Soil organisms can be classified on the basis of their source of energy as following:
Autotrophic Soil Organisms: The organisms that obtain their energy from sources other than organic materials. for example, from solar energy (photoautotrophs) and from the oxidation of inorganic elements such as nitrogen, Sulphur and iron (chemoautotrophs). The common examples are soil algae (blue-green algae cyanobacteria) and photosynthetic bacteria.
Heterotrophic Soil Organisms: The heterotrophic soil organisms obtain their energy and carbon from the breakdown of organic soil materials. These are more number than the autotrophs and are responsible for decaying processes in the soil. The common examples are soil fauna, most bacteria, fungi, and actinomycetes.
The number of soil organisms in a soil depends upon many factors such as vegetation and physical and chemical characteristics of the soil. The number and species vary from soil to soil, i. e., those found in an arid desert would be quite different from a humid forest or in a cultivated field.
Activities of Soil Organisms
The activities of soil organisms are related. When a plant part, leaf, a stalk, or a piece of a bark, drops to the ground it is attacked by microflora such as fungi, etc., and detritivores (e.g., mites, woodlice and earthworm). The microflora act chemically only, whereas the fauna act physically as well chemically. These plants eating animals (primary consumers or herbivores) chew the plant parts, move them from place to place on and in the soil and incorporate the plant residue into the soil. These primary consumers act as food sources for predators and parasites present in the soil (secondary consumers) such as bacteria, fungi, algae and lichens; and carnivores such as centipedes and moles, etc. The secondary consumers are prey for still other carnivores (tertiary consumers), for example ants, centipedes, spiders, mites, and scorpions. The microflora is intimately associated and involved in decomposition of organic material.
Importance of Soil Organisms
The soil organisms exhibit both beneficial as Well as injurious effects on higher plants.
Benefits of Soil Organisms
The soil fauna and flora have many beneficial effects on higher plants. The most important ones are:
Organic Matter Decomposition
The most significant contribution of soil flora and fauna is their ability to decompose organic matter. Decomposition of organic matter results in release of nutrients such as nitrogen, etc. Slimy intermediate products are also produced during decomposition of organic matter which help in stabilization of soil aggregates. The production of humus, an amorphous colloidal substance improves water holding capacity, ionic adsorption capacity and fertility of the soil.
Soil micro-organisms transform inorganic compounds into plant available forms, i.e., forms in which the inorganic compounds can be absorbed by plants through their roots. For example, inorganic nitrogen, Sulphur and phosphorus are converted by soil microorganisms into nitrates, sulphates, and phosphate ions which are readily absorbed by the plant roots. Similarly, the availability of iron and manganese is determined largely by micro-organisms. These elements are oxidized by autotrophic organisms to low solubility forms, i.e., their higher valence states, so that these remain in the soil in greater proportions. In this way the quantity of iron and manganese in soil solution remain low and does not reach toxic levels.
The plants are unable to use atmospheric nitrogen directly. Soil micro-organisms such as blue-green algae, certain actinomycetes, and bacteria capture gaseous nitrogen, convert it into nitrogenous compounds such as nitrites and nitrates, and add these to the soil from where these are absorbed by plants through their roots. Thus, soil micro-organisms are the main architects for supplying nitrogen to the plants.
Harmful Effects of Soil Organisms
The harmful effects of soil micro- flora and fauna are as under:
Effect of Soil Fauna
Certain soil fauna are injurious to higher plants. For example, some rodents and moles damage the crops. Snails and slugs in some climates act as pests especially of Vegetables. Ants transfer aphids on certain plants. Most plant roots are infested with nematodes affecting the growth of certain crops.
Microflora and Plant Diseases
Microflora produces more injurious effects on higher plants than soil fauna. The cause Plant diseases. The most pathogenic being the fungi which. are responsible for soil-borne diseases of crop plants. These include wilts, damping off, root rots, and clubroot of cabbage and similar crops.
Competition for Nutrients
Soil organisms are able to absorb nutrients quickly so that an amount of nutrient is left for higher pants to absorb. The competition is greatest for nitrogen. The other elements competed are phosphorus, potassium, calcium and micronutrients.
Nutrient Deficiencies & Toxicities
In poorly drained soils, active soil microflora utilizes oxygen supply quickly. The shortage of oxygen affect the growth of higher plants due to lack of energy that results in poor absorption of nutrients. Similarly, microflora reduce oxidized forms of several elements as nitrogen, Sulphur, iron and manganese. Some of these, for example nitrogen and Sulphur exist in gaseous state in reduced states, therefore are lost to the atmosphere. In acidic soils the reduction of these elements produces more soluble forms of these elements. These dissolve in soil solution and produce toxic effects. Thus, nutrients deficiencies and toxicities induced by soil microorganisms effect the growth of higher plants.
Production of Antibiotics in Soils
Certain soil bacteria, fungi and actinomycetes can produce antibiotics, the substances injurious to organisms other than those that produce them. Antibiotics are killers of many soil organisms vital for the growth of higher plants.