The efficiency of utilization of phosphatic fertilizers is very low due to their chemical fixation in soil. Besides native soil phosphorus (P) is mostly unavailable to crops because of its low solubility. Poor to medium status with respect to available phosphorus characterize Indian soils. Further, there is building of insoluble phosphates in soil where phosphatic fertilizers have been applied over long periods. The introduction of efficient P solubilizers in the rhizospheres of crops and soils increases the availability of phosphorus from insoluble sources of phosphates and the utilization of efficiency of applied phosphatic fertilizer. Important phosphate solubilising organisms are Pseudomonas striata, Bacillus polymyxa, Aspergillus awamori, Penicillium digitatum. These microorganisms can grow on insoluble phosphatic sources such as tricalcium phosphate, ferric, aluminium and magnesium phosphate, rock phosphate, and bone meal and convert them in to soluble forms. A variety of mechanisms are implicated in the solubilisation and mineralization of insoluble and organic P sources. The nature of organic acids produced has important bearing on amount of P solubilized.

Most of the cultivated tropical soils have good population of rhizobia capable of nodulating legumes.  Presence of nodules should not be taken as an index of establishment of an efficient nitrogen fixing system.  Competitive and efficient strains of Rhizobium used for inoculation will ensure maximum nitrogen fixation.  Absence of effective Rhizobium strains is responsible for poor nodulation encountered in many areas.  Absence of suitable rhizobia, deficiency or toxicity of a nutrient, unfavorable soil conditions, excess water logging, unsuitable pH, predators and pests are other factors which indirectly influence the potentiality of Rhizobium strains.

Drought and temperature stress is known to affect the growth of legume host and nitrogen fixation.  Nodule formation and nitrogen fixation are influenced by the rhizosphere temperature.  The capacity to tolerate elevated temperature varies considerably among different strains of rhizobia and host species.  Therefore, the ability to tolerate higher temperature will be a very desirable trait for  selection of Rhizobium strains in a tropical country like India as this would help in build up and carryover of strains introduced into soil from one season to another.

Azospirillium an associative microaerophilic nitrogen fix is commonly found in association with the roots of cereals and forage grasses.  High nitrogen fixation capacity, low energy requirement and abundant establishment in the roots of cereals and tolerance to high soil temperature (30-40°C) are responsible for its suitability under tropical conditions too.  Azospirillum is also found on root surface, root hairs, epidermal cells and vesicular tissues.

Azospirillum are mesophilic and reports in association of crops grown in acidic to alkaline pH range.  Azospirillum are metabolically versatile and can grow vigorously in the presence of nitrogenous compounds presents in soil but as soon as the external combined nitrogen supply is exhausted the bacteria switch on to diazotrophy.