Prof (Dr) R D Gupta
After nitrogen phosphorus is a vital nutrient for the growth and development of plants and micro-organisms. In soils, it exists both in inorganic and organic forms. The inorganic forms of phosphorus are compounds of calcium, iron, aluminium and fluorine, while the organic forms are compounds of phytin, phospholipids and nucleic acids which mainly come from decaying vegetation. The soils rich in organic matter are, therefore, rich in organic forms of phosphorus also. Rock phosphate or tri-calcium phosphate is one of the basic raw material for production of phosphatic fertilizer viz., single or triple superphosphate and diammonium phosphate. Tri-calcium phosphate having insoluble form of phosphorus cannot be directly applied to all soils except acidic ones. However there are certain micro-organisms which can bring out dissolution of insoluble phosphatic compounds in soils. For example use of cultures like Psudomonas striata, Bacillus polymyxa and Aspergillus awamori have been found efficient in solubilization of low grade Missouri rock phosphate and soil fixed P. PSM contain phosphate solubilising microbes which solubilize phosphates from the soil and make it available to the crops The efficiency of utilization of phosphatic fertilizers is very low due to their chemical fixation in soils. Besides, native soil P is mostly unavailable to crops because of its low availability. Most of the Indian soils are characterized by poor to medium status with respect to available P. Further, there is building of insoluble phosphates in soils where phosphatic fertilizers have been applied over long periods. In view of the aforesaid facts, it becomes imperative to introduce efficient P-solubilizers in the rhizosphere of crops and soils to increase the availability of P from insoluble phosphate compounds and, thereafter, its utilization for growing crops efficiently. Most important phosphate solubilizing organisms are bacteria and fungi. Mechanism of phosphate solubilization: A number of soil bacteria especially belonging to the genera of Bacillus and Pseudomonas and fungi belonging to the genera of Penicillium and Aspergilus possess the ability to bring insoluble phosphates to soluble forms by producing a number of organic acids. Such acids include formic acid, acetic acid, propionic acid, lactic acid, glycolic acid, fumaric acid, succinic acid, etc. These acids lower the soil pH and bring about the dissolution of bound forms of phosphate. Some of the hydroxy acids may chelate with calcium and iron resulting in-effective solubilization and utilization of phosphate by crops. Among the bacteria, Pseudomonas striata, Bacillus megatherium, Bacillus mycoides, Bacillus circulan, Bacillus polymyxa etc., are the most important species which bring about the dissolution of the insoluble phosphates. Penicillium digitatum and Aspergillus awamori, are the main fungi which also help in dissolving insoluble phosphate groups in soils. These micro organisms also produce amino acids, vitamins, growth promoting substances like Indole acetic acid (IAA) and Gibberellic acid (GA). Various species of fungi associated with legume root nodules have brought about the solubilization of Ca3 (PO4)2 to the extent of 9.10 to 39.02 per cent as compared to control. The extent of solubilization brought about by four species of bacteria in rock phosphate has shown that soil Bacillus megatherium caused gradual increase in available P upto 6 weeks and similar was noticed with B. cereus till the 8th week. However, B mycoides and B circulans showed altogether different trend, marked solubilization was taking place only upto 15 days of incubation. Subsequently, it increased again at 4th, 6th and 8th week.
Agronomic aspects: A commercial biofertilizer under the name “Phosphobacterin” was first prepared from B.megatherium var phosphaticum and is widely used in Russia and East European countries with yield increase in the order of 5-10 per cent over control. Field experiments conducted under various agroclimatic zones of India have indicated significant increase in yield over control. In general 40 percent of the superphosphate could be saved by the application of rock phosphate and phospho-micro-organisms. Without phosphate solubilizing bacteria their yields were 25.0 and 34.0 q ha-1 while with culture of these bacteria the yields were 38.6 and 36.5 q ha-1. There are a number of factors which are responsible for enhancing efficiency in phosphate dissolving micro-organisms. Some of them, are detailed here as under:
(a) Organic carbon and nitrogen contents: As most of the phosphate dissolving microorganisms are heterotrophic in nature, so these micro-organisms will definitely solubilize more amount of rock phosphate in presence of organic carbon and nitrogen content.
(b) The use of phosphorus solubilizing micro-organisms along with FYM was effective in dissolution of insoluble forms of phosphates. It has also been reported that response of phosphorus dissolving micro-organisms was positive in soils with high organic matter content and low available P. It was found that performance of Aspergillus awamori with or without FYM was slightly better than Bacillus polymyxa in increasing the yield of soybean as compared to control in moderately deep soil. However, in other two soils, Bacillus polymyxa did well than Aspergillus awamori, particularly more in soil II. Addition of FYM and low available P in this soil might be indirectly responsible for enhanced activities of Bacillus polymyxa. In calcareous (vertisol) soil application of Bacillus polymyxa + FYM and vermiculture produced the highest yield of soybean than other treatments. Introduction of Pseudomonas striata, Bacillus polymyxa etc., in the rhizosphere of rice and wheat increased availability of phosphorus from insoluble phosphate (rock phosphate) and increased its utilization efficiency.
Vermicompost and phosphobacteria in combination with two inorganic P sources viz., single superphosphate (SSP) and Tunis rock phosphate (TRP) were tested in a soil calcareous black soil for their effect on the yield parameters of black gram and cotton mix. SSP and TRP were applied in soil at 100 per cent with and without Vermicompost and phosphobacteria. The application of TRP (100 per cent) with Vermicompost and phosphobacteria in black gram recorded the highest grain yield. PS Mare well known to play a significant role in solubilizing the unavailable phosphates from soils into available forms for the uptake of growing plants, so these must be added to increase the agricultural production.
(The author is ex-Associate Dean
cum Chief Scientist, KVK, SKAUST-J).
