Roles of Micro-organisms in soil fertility and crop production:
Soil microorganisms are very important as almost every chemical transformation taking place in soil involves active contributions from soil microorganisms.
- Micro-organisms play an active role in soil fertility as a result of their involvement in the cycle of nutrients like carbon and nitrogen, which are required for plant growth. For example,
- Decompose and turn crop residues into manures, humus, carbon dioxide and water and release their nutrients slowly for efficient plant use.
- Soil microorganisms are responsible for the decomposition of the oranic matter entering the soil (e.g. plant litter) and therefore in the recycling of nutrients in soil.
- Certain soil microorganisms such as mycorrhizal fungi can also increase the availability of mineral nutrients (e.g. phosphorus) to plants.
- Other soil microorganisms can increase the amount of nutrients present in the soil. For instance, nitrogen-fixing bacteria can transform nitrogen gas present in the soil atmosphere into soluble nitrogenous compounds that plant roots can utilize for growth.
- Microorganisms, which improve the fertility status of the soil and contribute to plant growth, have been termed 'biofertilizers' and are receiving increased attention for use as microbial inoculants in agriculture.
- Soil microorganisms have been found to produce compounds (such as vitamins and plant hormones) that can improve plant health and contribute to higher crop yield. These microorganisms (called 'phytostimulators') are currently studied for possible use as microbial inoculants to improve crop yield. e.g. colonization of wheat roots by strains of Azospirillium, a bacterial inoculant acts as a phytostimulator.
- In contrast to these beneficial soil microorganisms, other soil microorganisms are pathogenic to plants and may cause considerable damage to crops. Large numbers of pathogenic microorganisms are routinely found in the soil and many can infect the plant through the roots. However, certain native microorganisms present in the soil are antagonistic to these pathogens and can prevent the infection of crop plants.
- Antagonism against plant pathogens usually involves competition for nutrients and/or production of inhibitory compounds such as secondary metabolites (antimicrobial metabolites and antibiotics) and extracellular enzymes.
- Other soil microorganisms produce compounds that stimulate the natural defense mechanisms of the plant and improve its resistance to pathogens. Collectively, these soil microorganisms have been termed 'biopesticides' and represent an emerging and important alternative (i.e. biological control) to the use of chemical pesticides for the protection of crops against certain pathogens and pests.
- Azospirillum induces the proliferation of plant root hairs which can result in improved nutrient uptake.
- Mycorrhizal fungi colonize the root systems of many plants and aid in the uptake of nutrients by the plant, thereby improving plant growth and overall health.
- Improve water absorption, retention, drainage and aeration of the soil creating a better environment for root growth.
- Increase the buffering capacity of the soil to hold nutrients and reduce the toxicity of soil and fertilizer salts.
- Breakdown and eliminate herbicide, pesticide and other chemical residues in the soil.