Mineralization of soluble P fertilizers and insoluble rock phosphate in response to phosphate-solubilizing bacteria and poultry manure and their effect on the growth and P utilization efficiency of chilli (Capsicum annuum L.)

The ability of soil microorganisms and organic manure to convert insoluble phosphorus (P) to an accessible form offers a biological rescue system for improving P utilization efficiency in soil-plant systems. Our objective was to examine the P mineralization potential of two soluble P fertilizers (SPF), i.e., single superphosphate (SSP) and diammonium phosphate (DAP), and of insoluble rock phosphate (RP) with and without phosphate-solubilizing bacteria (PSB) and poultry manure (PM) and their subsequent effect on the growth, yield and P utilization efficiency (PUE) of chilli (Capsicum annuum L.). An incubation study was carried out on a loam (slightly alkaline) soil with 12 treatments: T-0 - control; T-1 - RP; T-2 - SSP; T-3 - DAP; T-4 - PM; T-5 - 1/2 RP+1/2 SSP; T-6 - 1/2 RP+1/2 DAP; T-7 - 1/2 RP+1/2 PM; T-8 - RP+PSB; T-9 - 1/2 RP+1/2 SSP+PSB; T-10 - 1/2 RP+1/2 DAP+PSB; and T-11 - 1/2 RP+1/2 PM+PSB. Phosphorus mineralization was measured by analyzing extractable P from the amended soil incubated under controlled conditions at 25 degrees C for periods of 0, 5, 15, 25, 35 and 60 days. A complementary greenhouse experiment was conducted in pots with chilli (Capsicum annuum L.) as a test crop. Growth, yield, P uptake and PUE of the chilli was determined during the study. Results indicated that P mineralization in soil amended with RP was 6.0-11.5 mg kg(-1), while both soluble P fertilizers resulted in 68-73 mg P kg(-1) at day 0, which decreased by 79-82% at the end of incubation. The integrated use of PSB and PM with RP in T-11 stimulated P mineralization by releasing a maximum of 25 mg P kg(-1) that was maintained at high levels without any loss. Use of PSB decreased soil pH. In the greenhouse experiment, RP alone or RP+PSB did not have a significant impact on plant growth. However, the combined use of RP, PM and PSB in T-11 resulted in similar growth, yield and P uptake of chilli as DAP. The PUE of applied P varied from 4 to 29% and was higher in the treatments that included PSB. We conclude that the use of PSB and PM with insoluble RP or with soluble P fertilizers could be a promising approach to enhance P availability from both low-grade RP and SPF for crop production in intensive cropping systems.