Introduction

Rhizobium: Rhizobium is the symbiotic bacteria that fix atmospheric N2 gas in plant through root nodules and have a mutually helpful relationship with their host plants. The plant roots supply essential minerals and newly synthesized substances to the bacteria. Rhizobium is considered the most efficient biofertilizer because of a higher amount of nitrogen fixation. Rhizobium is characterized in seven genera which are highly specific in forming root nodules in legume plants and crops. It was revealed that rhizobium can fix 50-300 kg N/ha. A strain of Rhizobia that nodules and fixes a large amount of nitrogen in association with one legume species may also do the same in association with certain other legume species. 

Azotobacter: Azotobacter are free living and non-symbiotic nitrogen fixing organism that also produces certain substances which are considered good for the growth of plants. The population of Azotobacter is generally low in the rhizosphere of the crop plants and in uncultivated soils. The Azotobacter mostly colonize crop plants such as rice, maize, sugarcane, bajra, vegetables and plantation crops. They are considered to be dominantly inhabite the arable soils and have capacity to fix 2-15 mg N2 fixed /g of carbon source) in culture media. 

Azospirillum: Azospirillium is a nitrogen-fixing microorganism beneficial for non-leguminous plants. Azospirillum transcend nitrogen enrichment through production of growth promoting substances. Two species of Azospirillum i.e. Azospirillum lipoferum and A. brasilense are primary inhabitants in soil horizons. Bacteria of Genus Azospirillum are capable of N2 fixation and can be isolated from the above-ground parts and root of various crop and plant species. They have a Vibrio or Spirillum shape and are Gram-negative bacteria with an abundant accumulation of polybetahydroxybutyrate (70 %) in the cytoplasm of their cells.Along with nitrogen fixation, they performvarious beneficial functions that are growth-promoting substance production (IAA), inducing disease resistance, and drought tolerance. 

Azolla: Azolla is a free-floating water fern/pteridophyte which passively floats in water and fixes atmospheric nitrogen with nitrogen-fixing blue-green alga Anabaena azollae association. The rice-growing areas in South East Asia observed the increase in the application of Azolla fern either as an alternate nitrogen source or as a supplement to commercial chemical-based nitrogen fertilizers. Azolla is known to contribute 40-60 kg N/ha per rice crop and it is used as biofertilizer mainly in wetland rice production. 

Phosphate Solubilizing Microorganisms (PSM): Many soil microorganisms secrete organic acids and lower the pH in their vicinity which leads to the dissolution of bound phosphates in soil horizons, notably species of Pseudomonas, Bacillus, Penicillium, Aspergillus, etc. helps to enhance this process. PSB’s increase the surface area of the plant roots by increasing the availability of the nutrients in the soil to the plants, assisting the N2 fixation and enhancing the other beneficial effects of symbiotically. PSB solubilise phosphate by production of organic acids. These acids can either dissolve the phosphorus directly by lowering the pH of soil which can help in ion exchange of PO42- by acid ions or they can chelate heavy metal ions such as calcium, aluminium, iron and release associated phosphorus with them. They also increase the soil fertility and productivity of the crops. 

Arbuscular Mycorrhizal Fungi (AM Fungi): Mycorrhizae are mutually beneficial (symbiotic) relationships between fungi and plant roots. VAM fungi infect and spread inside the root. These fungi helps in the transfer of various nutrients from the soil to the cells of the root cortex like phosphorus, zinc, and sulphur.This function is mediated by intracellular obligate fungal endosymbionts of the genera Glomus, Gigaspora, Acaulospora, Sclerocysts,and Endogone.These fungi possess vesicles for the storage of nutrients and arbuscles for funneling and transport of these nutrients into the root system of plants (Rana et al. 2013). They are beneficial to crop and are capable of enhancing crop growth by colonizing the roots of the plants. Plant growth promoting rhizobacteria or nodule promoting bacteria are associated with rhizosphere of the soil and enhance the yield by providing the nutrients to the crop plats. 

Liquid Biofertilizers: Liquid biofertilizers which contains living or dormant microbes (bacteria, fungi, algae, actinomycetes etc.) alone or in combination which help in fixing atmospheric nitrogen or solubilizers of different soil nutrients as well as the secretion of growth promoting substances for enhancing crop growth and yield. 

Need of bio-fertilizers: Indiscriminate use of synthetic fertilizers has led to the pollution and contamination of the soil, has polluted water basins, destroyed micro-organisms and friendly insects, making the crop more prone to diseases and reduced soil fertility. Demand is much higher than the availability. Depleting feedstock/fossil fuels (energy crisis) and increasing cost of fertilizers. Depleting soil fertility is due to widening gap between nutrient removal and supplies. Besides above facts, the long term use of bio-fertilizers is economical, eco-friendly, more efficient, productive and accessible to marginal and small farmers over chemical fertilizers.