Biofertiliser (jibaja sar) living materials used to increase fertility of soils. Some free-living or symbiotic bacteria and blue-green algae (Cyanobacteria) fix gaseous nitrogen as ammonia and release it increasing the fertility of soil and water. Rhizobium or Bradyrhizobium producing root nodules in legumes and Anabaena azollae living in leaf cavities of Azolla (aquatic fern) are very efficient nitrogen fixers, and contribute about 500 kg N/ha/year.
Another microsymbiont with nitrogen fixing capacity is Frankia having hyphal morphology similar to that of actinomycetes and produces nodules in woody non-legumes, like Alnus, Casuarina, Myrica, etc. It produces vesicles and sporangia in both free-living and symbiotic states. Azotobacter species are free-living (mostly root associated), aerobically nitrogen fixing bacteria. Nostoc, Calothrix, Gloeotrichia, Stigonema, etc are free-living aerobically nitrogen fixing Cyanobacteria. In addition, Vesicular Arbuscular Mycorhizae (VAM fungi) are free-living soil forms that increase nutrient uptake (specially by converting organic phosphorus into inorganic phosphorus), plant growth, nodulation and nitrogen fixation in legumes. In coastal areas of some countries, seaweeds are also used as biofertilisers. However, all these life forms may be grown artificially and inoculated in the soil as biofertiliser. The nitrogen fixers releases nitrogen during their life time and also add other elements after their death and decay, essential for the growth of crops. The biofertiliser also contributes organic matter and maintains a good soil texture. Azolla contains well balanced amino acids and high amount of anthocyanine, a -carotene. These compounds also have positive effects on growth and yield of crops.
Large-scale production (2.5 m tons/year) of Rhizobium or Bradyrhizobium inoculants has been successful at bangladesh institute of nuclear agriculture, Mymemsingh. About 1.5 to 2.0 kg inoculant/ha used with seeds may result increased production of pulses by 20 to 40%. Average Azolla production in a pond could be up to one m ton/ha/day, following the method developed at Botany Department, Dhaka University. It could also be grown along with irrigated rice crop. One cover of Azolla is about 10 m tons/ha and incorporation of two such covers could reduce urea-N fertiliser by about 50%. Azolla could also be made into compost and addition of about 10 m tons could supplement all fertilisers by about 50%.
The benefits of biofertilisers over chemical fertilisers are manyfold. It is environmentally friendly, less expensive, increases soil organic matter, and believed to have growth promoting substances. [Abdul Aziz]