Influence of integrated phosphorus management on phosphorus transformation, availability and its critical concentration in green-gram (Vigna radiata L.)

A field experiment was conducted during two consecutive kharif seasons (2004 and 2005) and a pot-culture experiment was conducted during kharif 2005 to study “The influence of integrated phosphorus management on phosphorus transformation, availability and its critical concentration in green gram (Vigna radiata L.)”. Prior to the field experiment, eight phosphate solubilizing bacteria were isolated from rhizosphere of leguminous crops (Pea, French bean and Lentil) and two efficient P-solubilizers isolated from French bean rhizosphere were identified as Bacillus and Pseudomonas sp. In the field experiments, organic manures significantly increased the growth, yield, seed quality, nutrient contents (N,P,K & S) of green gram and microbial population (bacteria, fungi and actinomycetes) in the rhizosphere. Among organic manures, application of FYM (10 t ha-1) proved superior than Dalweed (15 t ha-1). Soil inoculation with P-solubilizers resulted in significant increase in growth, yield, seed protein and nutrient content (N,P & S) of green gram and bacterial/actinomycetes population, whereas fungal population significantly decreased due to inoculation. Increasing levels of phosphorus upto 30 kg P2O5 ha-1 significantly improved the nodule dry weight per plant, leaf area index, yield and nutrient contents (N,P,K & S) of green gram and microbial population in rhizosphere, however further increase in phosphorus level viz., 60 kg P2O5 ha-1 had no beneficial influence on these parameters. The optimum dose for grain yield was 45.93 kg P2O5 ha-1 with a yield potential of 9.19 q ha-1. Application of FYM/ Dalweed significantly increased the various P-fractions (Fe-P, Al-P & Ca-P), available N,P and K of the post-harvest soil. Although inoculation with P-solubilizers also increased the available N and P-content of the post harvest soil, however inoculation with Pseudomonas recorded significantly more available P as compared to Bacillus. The effect of PSB inoculation on available K was statistically non-significant. Increasing levels of phosphorus upto 60 kg P2O5 ha-1 increased the various inorganic P-fractions (Fe-P, Al-P & Ca-P) and available P, whereas available N increased significantly only upto 30 kg P2O5 ha-1 level of the post-harvest soil. The combined application of organic manures along with P-solubilizers/ phosphorus levels significantly increased the number and dry weight of nodules plant-1, yield, seed protein, N and P contents of green gram, bacterial and actinomycetes population in the rhizosphere as well as improved the available N and P status of post-harvest soil as compared to their sole application. Among five extractants, Olsen and Hunter method’s were significantly and positively correlated with yield and P-uptake parameters of green gram. Thus Olsen and Hunter extractable P were equally good indicator of P availability. The scatter diagram reveals that the critical limits of soil P below which an economic response of green gram to applied P can be expected was 14.40 ppm P2O5 for Olsen and 14.80 ppm P2O5 for Hunter method respectively. The critical limit of P concentration in grain and straw of green gram were worked out to be 0.34 and 0.13 per cent respectively