Extensive use of fertilizers and chemical pesticides had resulted in soil and water pollution. Fossil fuels such as petrol and coal are used in the manufacture of fertilizers and pesticides. To reduce pollution and over usage of our non-renewable resources like coal, petroleum, etc., an alternative method has been successfully developed to safeguard natural resources. To maintain soil fertility and soil improvement, fertilizers of biological origin called biofertilizers have been developed.
Artificial inoculation of rice and other crop fields with cyanobacteria (Anabaena, Calothrix, Gleocapsa, Lyngbya, Nostoc, Oscillatoria, Scytonema) has attracted much attention to increase fertility in several countries. The term 'biofertilizer' denotes all the nutrient inputs of biological origin for plant growth. Biological origin refers to microbes producing nitrogen compounds. Bacteria and cyanobacteria are known to fix atmospheric nitrogen and are known as biofertilizers. Nitrogen fixing bacteria like Azotobacter, Bacillus and Rhizobium increased the crop yield to 20%. Pseudomonas striata are used as seed inoculants as biofertilizer coats for cereals.
Various leguminous plants like Crotalaria juncea, Cassia mimusoides, Glycine max, Indigofera linifolia, Sesbania rostrata, Acacia nilotica,
Leucena, Lathyrus and Mucuna are used as green manures. They accumulate more than 80 Kg of nitrogen per hectare in the soil when grown as green manures.
Azolla is an aquatic fern, which contains an endophytic cyanobacterium Anabaena azollae in its leaves. It is used as a biofertilizer in rice field. Out of six species of Azolla, A. pinnata is widely employed as a successful biofertilizer in Indian rice fields. It adds 30 Kg of nitrogen per hectare where the yield is equivalent to that of urea or ammonium phosphate.
Mycorrhiza is a root inhabiting fungus found around or inside the roots of many plants. It increases growth and yield and also provides protection to the roots against edaphic (soil) stresses, pathogen and pests. It helps in the increased uptake of soil and mineral water solution by the plant root system. It provides many uses for the host plants eg. VAM (Vesicular Arbuscular Mycorrhiza) fungi. Mycorrhiza is of two types.
a. Ectotrophic mycorrhiza, which are found only outside the surface of roots of plants. eg. Basidiomycetous fungi.
b. Endotrophic mycorrhiza, which are found inside the roots, in the intercellular spaces and even inside the cell (intra and intercellular) eg. VAM fungi.
1. Biofertilizers are easy to produce in abundance and are available at low cost to the marginal farmers.
2. It increases soil fertility without causing any damage to the soil.
3. Application of biofertilizers increases yield upto 45 per cent and the left over biofertilizers in the soil increases yield as long as the biofertilizer remains in the soil up to 3 to 4 years.
4. Azolla, which is a biofertilizer amends the soil with organic matter. Cyanobacteria in particular secrete growth promoting hormones like indole 3-acetic acid, indole butyric acid, naphthalene acetic acid, aminoacids, protein and vitamins to soil.
5. Cyanobacteria grow well both in acidic as well as in alkaline soils.
Since, cyanobacteria are potent neutralizers, they help in the neutralization of soil. The process of converting untenable, fallow land to cultivable soil is termed as soil reclamation. Blue green algae play a vital role in this conversion.
6. Symbiotic nitrogen fixing Rhizobium is a biofertilizer. It adds 50 to 150 Kg of nitrogen to soil per hectare. Azatobacter and Azospirillum secrete antibiotics which act as biopesticides.
Ectotrophic mycorrhiza, which acts as a biofertilizer, increases the surface area of the roots of host plants, so that more absorption of nutrients by the roots is made possible.