Soil

Microbial Antagonists as soil inoculants and their role in improving the quality of soil Malvika Chaudhary and M. S. Prabhakar Bio-Control Research Laboratories 36/2, Sriramanhalli, Arakere Post Near Rajankunte, Dodballapur Road Bangalore-561203 [email protected] Soil, comprising of trillions of microorganisms like earthworms, nematodes, arthropods, bacteria, fungi and protozoans, is a hot bed of activity which easily outnumbers the actions taking place above the ground in the environment. These microorganisms play an important role in plant health by driving cycle of nutrients through the environment in accordance with decomposition, mineralisation, storage and release of nutrients, and detoxification. Depending on the type and its effect on the environment these microbes attain the status of being beneficial or detrimental. Microbes are termed beneficial, or more recently as effective microorganism, when they can contribute in enhancing the health and quality of soil by fixing atmospheric nitrogen, decomposing organic wastes and residues, detoxifying pesticides, suppressing plant diseases and soil-borne pathogens, enhancing nutrient cycling, and producing bioactive compounds such as vitamins, hormones and enzymes that can stimulate plant growth. The harmful microorganism on the contrary can induce plant diseases, stimulate soil-borne pathogens, immobilize nutrients, and produce toxic and putrescent substances that adversely affect plant growth and health. Though both types are found together in a soil profile but the equilibrium of their coexistence is disturbed due to various factors like weathering, pollution, cropping pattern, excessive use of pesticides and other environmental issues. The shift on either side results into symptoms above ground, which also defines the viguor of the soil for supporting the crop. This results into mosaic of lithosphere, a portion where the pathogenic microbes are able to exert more influence manifests as symptoms of diseases in the plants while increase in population of beneficial microorganism results into disease suppressive soils, where due to natural antagonism between the harmful and beneficial microbes the disease is controlled. To obtain a desirable result from the beneficial microorganisms they should be given suitable and optimum conditions for metabolizing their substrates including available water, oxygen (depending on whether the microorganisms are obligate aerobes or facultative anaerobes), pH and temperature of their environment. Microbial inoculants in form of

biopesticides are flooding the market but their application would be much more meaningful if they are quality products and are applied in an appropriate manner. One of the other reasons of their inconsistent performance is different soil conditions. The actual soil condition that prevail at any point in time may be most unfavorable to the growth and establishment of laboratory-cultured, beneficial microorganisms. To facilitate their establishment, it may require that the farmer make certain changes in his cultural and management practices to induce conditions that will (a) allow the growth and survival of the inoculated microorganisms and (b) suppress the growth and activity of the indigenous plant pathogenic microorganisms. Microbial antagonists yield better result if they are allowed to establish in the rhizosphere proactively. This is preferred because the onset of favourable conditions like high temperature and humidity increases the microbial activity leading to their augmentation. In such case the pathogenic microorganism can grow at alarming rate and produce devastating effect on the crop. Under such circumstances if the soil is pre-inoculated with beneficial fungi or bacteria, conditions for the growth of disease causing microorganism becomes unsuitable. The antagonism is due to production of antimicrobials, mycoparasitism, competition for nutrients to occupy the niche which reduces the pathogens in the soil. Benign organism are compatible to each other, therefore they can be multiplied and can be broadcasted along with fully decomposed manure or other organic amendments. Another application strategy to give a protective coat to the seed before it is sown, which helps the antagonistic microbe to form a zone of inhibition against the pathogen and thus minimising the chances of infection. An endophyte bacterium can also result in bacterisation in the cotyledon itself which can later induce systemic resistance to the plant. A higher cost-benefit is obtained with a re-inoculation of these microbial antagonists, which is preferred to replenish their diminishing population once the environmental conditions become extremely favourable for pathogens as well. It is well established fact that proper use of these microbial inoculants enhances the fertility of soil by reducing deleterious microorganism which although does not pose major risk to the plant but interfere with their nutrient intake, thus affecting viguor. Following the current demand and the emerging trend of use of organic inputs, augmentation of microbial antagonists as soil inoculants in the form of biopesticides forms an important component of Organic Agriculture.