Biological nitrofertilizers play a vital role to solve the problems of soil fertility, soil productivity and environmental quality. Anabaena azollae, is a cyanobacteria lives in symbiotic association with the free floating water fern Azolla. The symbiotic system Azolla- Anabaena complex is known to contribute 40-60 µg N/ha per rice crop. Anabaena azollae can grow photoautotrophically and fixes atmospheric nitrogen. The nitrogen fixing cyanobacteria such as A. azollae and A. variabilis when immobilized in polyurethane foam and sugarcane waste have significantly increased the nitrogen fixing activity and ammonia secretion. The inoculation of cyanobacteria in rice crop significantly influenced the growth of rice crop by secretion of ammoniain flood water. The use of neem cake coupled with the inoculation of Azolla greatly increased the nitrogen utilization efficiency in rice crop. Besides Anabaena, other nitrogen fixing cyanobacteria like Aulosira, Calothrix, Hapalosiphon, Scytonema, Tolypothrix, and Westiellopsis have been held responsible for the spontaneous fertility of the tropical rice field.
In addition to contributing N, the cyanobacteria add organic matter, secrete growth promoting substances like auxins and vitamins, mobilize insoluble phosphate and improve physical and chemical nature of the soil. Algalization has been shown to the saline alkali soils, help in the formation of soil aggregates, reduce soil compaction and narrow C:N ratio.These organisms enable the crop to utilize more of the applied nutrients leading to increased fertilizer utilizing efficiency of crop plant. Most of the cyanobacteria act as supplements to fertilizer N contributing up to 30 kg N/ha/season. The increase in the crop yield varies between 5-25 percent.
Mass Production: For outdoor cultivation of cyanobacterial biofertilizers, the regional specific strain should be used. A mixture of 5 or 6 regionally acclimatized strains of cyanobacteria, species- Anabaena, Aulosira, Cylindrospermum, Gloeotrichia, Nostoc, Plectonema etc. are generally used for starter inoculums. The following methods are used for mass cultivation: a. Cemented Tank method b. Shallow metal troughs method, c. Polythene lined pit method and d. Field method. The polythene lined method is most suitable for small and marginal farmers for the preparation of biofertilizer. In this method, small pits are prepared in field and lined with thick polythene sheets. The mass cultivation of cyanobacteria is done by using any of the four methods.
Azolla- For mass cultivation of Azolla, microplots (20m2) are prepared in nurseries in which sufficient water (5-10cm )is added. For profuse growth of Azolla, 4-20 kg P2O5 /ha is also amended. Optimum pH (8.0) and temperature (14-30ºC) should be maintained. Finally, microplots are inoculated with fresh Azolla (0.5 to 0.4 kg/m2). An insecticide (Furadon) is used to check the insect's attack. After 3 weeks, the mat azolla is ready for harvest and the same microplot is inoculated with fresh Azolla to repeat the cultivation. Azolla mat is harvested and dried to use as green manure.
Biofertilizers aiding Phosphorous Nutrition
Tropical soils are deficient in phosphorous. Most of the solubilize P and thus make it available for plant growth. It is estimated that in most tropical soils, 75% super phosphate applied is fixed and only 25% is available for plant growth. There are some fungi such as Aspergillus awamori, Penicillium digitatum etc. and bacteria like Bacillus polymyxa, Pseudomonas striata, etc. which can solubilize unavailable form of P to available form. India has 250 mt of rock phosphate deposits. The cheaper source of rock phosphate like Mussoorie rock phosphate, Udaipur rock phosphate available in our country can be used along with phosphate solubilising microorganisms.
Vesicular-arbuscular mycorrhizal (VAM) fungi colonize roots of several crop plants. They are zygomycetous fungi belonging to genera Glomus, Gigaspora, Acaulospora, Sclercystis, etc. These are symbionts and can not be cultured on synthetic media. The help plant growth through improved phosphorous nutrition and protect the roots against pathogens. Nearly 25-30% of phosphate fertilizer can be saved through inoculation with efficient VAM fungi.
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