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 (<i>Capsicum annuum</i> 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 (<i>Capsicum annuum</i> L.). An incubation study was carried out on a loam (slightly alkaline) soil with 12 treatments: <i>T</i>₀ – control; <i>T</i>₁ – RP; <i>T</i>₂ – SSP; <i>T</i>₃ – DAP; <i>T</i>₄ – PM; <i>T</i>₅ – 1/2 RP+1/2 SSP; <i>T</i>₆ – 1/2 RP+1/2 DAP; <i>T</i>₇ – 1/2 RP+1/2 PM; <i>T</i>₈ – RP+PSB; <i>T</i>₉ – 1/2 RP+1/2 SSP+PSB; <i>T</i>₁₀ – 1/2 RP+1/2 DAP+PSB; and <i>T</i>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 °C for periods of 0, 5, 15, 25, 35 and 60 days. A complementary greenhouse experiment was conducted in pots with chilli (<i>Capsicum annuum</i> 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 <i>T</i>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 <i>T</i>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