Biological Alternatives for Sustainable Agriculture
By - SUNITA GAIND

Chemical fertilizers are not only expensive but their persistent application is adversely affecting the soil health and causing environment pollution besides influencing the microbial diversity. Excessive application of nitrogen and phosphorus fertilizer due to low fertilizer use efficiency results in leaching of nitrate and eutrophication of underground water respectively. For rejuvenating the soil, there is need for transition to sustainable agriculture that involves

• Diversification of crops and cultural practices to enhance the biological and economic stability of the farm
• Management of the soil to enhance and protect soil quality and
• Efficient and humane use of inputs

Agriculture ecosystem functioning is largely governed by soil microbial dynamics. Crop growth and development are closely related to the nature of microbial flora inhabiting the rhizo-sphere. Most of the biological activities are mediated by these microscopic organisms. Thus to improve the productivity of soil and to meet the nutritional needs of growing population, there is need to build up high population of soil microbial flora. Sustainable agriculture depends upon the healthy community of soil microbes that contribute to the biological recycling of chemical nutrients responsible for crop productivity. A shift from chemical to natural resources seems essential to harvest prolonged productivity benefits from our soils. To minimize the use of chemical fertilizers and building better soil health, new concepts such as alternate agriculture, organic agriculture, use of bio-fertilizers and bio-pesticides are being explored.

Microorganisms are the important component of biological alternates with diverse application in the field of agriculture. Their role as nitrogen fixer in symbiotic association with legumes and their ability to solubilise inorganic phosphate and improve the growth of plants is now well recognised. Microorganisms have also been identified as plant disease suppressor and improve the crop yield and quality. Some of the commonly used beneficial microorganisms in agriculture include Rhizobia, Azospirillum, Azotobacter, Bacillus, Pseudomonas, Trichoderma and Streptomyces spp.

Nitrogen fixation is one of the important biological processes for environment sustainability. Symbiotic association between leguminous plants and root nodulating bacteria of genera Rhizobium, Bradyrhizobium, and Mesorhizobium result in conversion of atmospheric nitrogen to ammonia in the presence of enzyme nitrogenase. Since nitrogen is commonly the most limiting plant nutrient in arable farming in the tropics and also one of the most expensive elements as a mineral fertilizer, biological nitrogen fixation (BNF) holds great promise for marginal farmers. Microbial inoculants containing nitrogen fixing bacteria can reduce the input of chemical nitrogen fertilizer by 20-25 kg N/ha. Free living nitrogen fixing bacteria including Azotobacter, Azospirillum and Acetobacter diazotrphicus, Herbaspirillum, Azoarcus and Bacillus spp. can be used for cereal crops. The non rhizobial nitrogen fixing bacteria can grow as endophytes in number of grasses.

Plant growth promoting rhizobacteria (PGPR) promote the growth of plant due to production of growth stimulating hormone as indole acetic acid, gibberlic acid, ethylene and abscisic acid. The siderophore production and phosphate solubilisation traits are also exploited for crop benefits. Siderophores are low molecular weight compounds that have high affinity for iron. They chelate with iron and making it unavailable to pathogenic microorganisms that die due to iron starvation. PGPR also synthesise anti fungal metabolites such as antibiotics and fungal cell wall lysing enzymes, or produce hydrogen cyanide that suppress the growth of fungal pathogens causing root and plant disease. These bio-control agents can protect the plant from different diseases in an environment friendly manner. However, the success of bio-control inoculants depends upon their ability to establish in soil and maintain an adequate population to control the disease effectively and lengthen the period during which a threshold population density is sustained in the rhizosphere.
In annual cropping systems, crop rotation can be used to suppress weeds, pathogens and insect pests. Cover crops can have stabilizing effects on the agro-ecosystem. They hold soil and nutrients in place, conserve soil moisture with mowed or standing dead mulches, and increase the water infiltration rate and soil water holding capacity.

Phosphate solubilisation and mobilization is mediated by phosphate solubilising microorganisms and mycorrhizal fungi respectively. The latter benefit the plants by transporting the nutrient especially the phosphorus. The extra radical mycelium present in mycorrhizal plants are adapted to explore the soil to a distance much longer than the non mycorrhizal plants and bring phosphorus in soil solution, Arbuscular mycorrhizal fungal symbiosis is one of the key beneficial biological processes for the rehabilitation of contaminated soils. A diverse group of bacteria and fungi of genus Pseudomonas, Bacillus and Aspergillus and Penicillium are capable of solubilising and mineralizing unavailable form of phosphorus and make it available to plants. The application of microphos inoculants developed by including efficient phosphate solubilising microorgansisms can save the input of chemical P' fertilizers.

Composting of undesired crop residues or biodegradable waste offers several advantages as an effective waste disposal method. The high temperature attained during the process of decomposition kills the pathogens and destroys the weed seeds present in animal manures. The organic fertilizer produced in the form of mature compost contains increased availability of plant nutrients and its application to soil improves the physico-chemical and biological properties of soil. It improves the organic matter content of soil, an important component of soil fertility. Farm yard manure, biogas slurry, decomposed cattle manure, poultry manure, green manure are some of the very good substitutes for inorganic fertilizers.

The use of efficient microorganisms as alternative to chemical fertilizer opens up a new horizon for better crop productivity besides sustaining the soil health. However, the performance of inoculated microorganism largely depends on the physico-chemical environment of soil where the inoculated strain has to establish itself. Thus, there is a need for extensive and consistent research efforts to identify and characterize more microorganisms with greater efficiency for their ultimate application under field conditions.

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