Biofertilisers for organic farming

Biofertilisers and biodynamic formulations are the buzzwords of organic agriculture. Biofertilisers are the carrier-based inoculums of microorganisms, which are capable of enhancing the availability of nutrients to plants. Biofertiliser is an essential component of organic farming. Important biofertilisers are nitrogen fixers like Rhizobium, Azotobacter, Azospirillum, phosphate solubilisers like Pseudomonas, Bacillus and vesicular arbuscular mycorrhiza (VAM). Seed treatment with biofertiliser has opened up great promise in providing plant nutrients in an economic way.Different biofertilisers are briefly described below.

Rhizobium: Rhizobium provides the host plant with the nitrogen it requires for its growth. In association with legumes, Rhizobium fixes 50-300 kg nitrogen per hectare per year. Recent reports showed that nodulation by Rhizobium are not restricted to leguminous host alone. Rhizobium is also capable of nodulating the non-legume Parasponia of family Ulmaceae.

Azospirillum: Azospirillum are widely distributed nitrogen fixers found in lose association with roots of most of the agriculturally important crop plants. The positive effects of Azospirillum include an overall increase in nitrogen uptake, plant biomass and grain yield.

Azotobacter: Azotobacter is one of the important members of the free–living diazotrophic club due to its broad spectrum utility for crops like cereals (wheat and rice), vegetables (potato and tomato), oilseeds (mustard and rape seed) and fibre crops (cotton and jute).

Azolla: Azolla is an aquatic fern that occurs widely in the tropical and subtropical areas of Asia and Africa. Symbiotic cyanobacteria Anabaena azollae found at the lower part of upper lobes of fern, fix nitrogen by using solar energy. Interest in Azolla stems primarily from its current use and future potential as a biofertiliser. Incorporation of one crop of Azolla grown before or after transplanting is equivalent to the split application of 30 kg fertilizer nitrogen.

Acetobacter: It acts as a nitrogen fixer which is most suitable for sugar producing crops such as sugarcane.

Cyanobacteria: Cyanobacteria constitute the largest, most diverse and most widely distributed groups of photosynthetic prokaryotes. The nitrogen fixing ability of several genera has a vital role in maintaining the soil fertility and sustaining the crop yield even in the absence of any added nitrogenous fertilizer.

Phosphate Solubilizing Microorganisms (PSM): Phosphorus is one of the key elements for the growth of crop plants. Indian soils are generally poor in available phosphorus content. The application of PSM in the soil is known to increase the efficiency of commonly used phosphatic fertilizers as well as the availability of phosphorus from insoluble sources of phosphate. They also solubilise the soil-fixed phosphorus.

Mycorrhizae : It is the relationship between microorganisms (fungi) and roots of higher plants. Mycorrhizal fungi are known to significantly improve plant productivity by enhancing the absorption of soil nutrients especially phosphorus and water and by controlling pathogens and pests. Thus mycorrhizal plants withstand water stress apart from efficiently absorbing plant nutrients.

Actinorrhizae : About 200 plant species covering 19 genera and 8 families, known as actinorhizal plants nodulate with a nitrogen fixing actinomycetes, Frankia. The main tropical species of actinorhizal plants belong to the genera Casuarina, Allocasuarina Gymbostoma and Alnus.