Authors: Ajit Kumar Dubedi Anal and Manvendra Singh
Biofertilizers are defined as preparations containing living cells or latent cells of efficient strains of microorganisms that help crop plants uptake of nutrients by their interactions in the rhizosphere when applied through seed or soil. They accelerate certain microbial processes in the soil which augment the extent of availability of nutrients in a form easily assimilated by plants. Use of biofertilizers is one of the important components of integrated nutrient management, as they are cost effective and renewable source of plant nutrients to supplement the chemical fertilizers for sustainable agriculture. Several microorganisms and their association with crop plants are being exploited in the production of biofertilizers. They can be grouped in different ways based on their nature and function.
|N2 -fixing Biofertilizers|
|1.||Free-living||Azotobacter, Beijerinkia, Clostridium, Klebsiella, Anabaena, Nostoc|
|2.||Symbiotic||Rhizobium, Frankia, Anabaena azollae|
|Phosphorus solubilizing biofertilizers|
|1.||Bacteria||Bacillus megaterium , Bacillus subtilis|
|2.||Fungi||Penicillium sp. , Aspergillus awamori|
|Phosphorus mobilizing biofertilizers|
|1.||Arbuscular Mycorrhiza||Glomus sp.,Gigaspora sp.,Acaulospora sp. , Scutellospora sp. & Sclerocystis sp.|
|Plant growth promoting rhizobacteria|
Different types of biofertilizers:
Rhizobium inoculant was first made in USA and commercialized by private enterprise in 1930s and the strange situation at that time has been chronicled by Fred (1932). Rhizobium is a soil habitat bacterium, which can able to colonize the legume roots and fixes the atmospheric nitrogen symbiotically. The morphology and physiology of Rhizobium will vary from free-living condition to the bacteroid of nodules. They are the most efficient biofertilizer as per the quantity of nitrogen fixed concerned. The site of symbiosis is within the root nodules. It has been estimated that 40-250 kg N/ha/ year is fixed by different legume crops by the microbial activities of Rhizobium.
In the species of Azotobacter, Azotobacter chroococcum happens to be the dominant in soils capable of fixing N2 (2-15 mg N2 fixed/g of carbon source) in culture media. They produce abundant slime which helps in soil aggregation.
They perform the associative symbiotic relation with the graminaceous plants. Five species of Azospirillum have been found. The organism proliferates under both anaerobic and aerobic conditions but it is preferentially micro-aerophilic in the presence or absence of combined nitrogen in the medium.
Both free-living as well as symbiotic cyanobacteria (blue green algae) have been harnessed in rice cultivation in India. Composite BGA culture contains heterocystous Nostoc, Anabaena, Aulosira etc. The benefits due to algalization could be to the extent of 20-30 kg N/ha under ideal conditions.
Azolla is a free-floating water fern that floats in water and fixes atmospheric nitrogen in association with nitrogen fixing blue green alga Anabaena azollae. Azolla fronds consist of sporophyte with a floating rhizome and small overlapping bi-lobed leaves and roots. Azolla is used as biofertilizer for wetland rice and it is known to contribute 40-60 kg N/ha per rice crop.
Phosphate solubilizing microorganisms (PSM)-
Several soil bacteria and fungi, notably species of Pseudomonas, Bacillus, Penicillium, Aspergillus etc. secrete organic acids and lower the pH in their vicinity to bring about dissolution of bound phosphates in soil.
The transfer of nutrients mainly phosphorus and also zinc and sulphur from the soil milleu to the cells of the root cortex is mediated by intracellular obligate fungal endosymbionts of the genera Glomus, Gigaspora, Acaulospora, Sclerocysts and Endogone which possess vesicles for storage of nutrients and arbuscles for funneling these nutrients into the root system.
Plant Growth Promoting Rhizobacteria (PGPR)-
The group of bacteria that colonize roots or rhizosphere soil and beneficial to crops are referred to as plant growth promoting rhizobacteria (PGPR). Species of Pseudomonas and Bacillus can produce as yet not well characterized phytohormones or growth regulators that cause crops to have greater amounts of fine roots which have the effect of increasing the absorptive surface of plant roots for uptake of water and nutrients. These PGPR are referred to as Biostimulants and the phytohormones they produce include indole-acetic acid, cytokinins, gibberellins and inhibitors of ethylene production.
Constraints in Biofertilizer Technology-
Though the biofertilizer technology is a low cost, ecofriendly technology, several constraints limit the application or implementation of the technology the constraints may be environmental, technological, infrastructural, financial, human resources, unawareness, quality, marketing, etc.
Importance of Bio-fertilizers:
• Biofertilizers are eco-friendly and cost effective.
• They are effective even under semi-arid conditions.
• They increase the yield of plants by 15-35%.
• They improve soil texture and soil health.
• Farmers can prepare the inoculum themselves.
• They produce vitamins and growth promoting bio-chemical.
• They are non-polluting.
About Author / Additional Info:
ICAR-National Research Centre on Litchi