108 Indian Journal of Agricultural Sciences 91 (3): 440–4, March 2021/Article Residual effect of rock-phosphate and PSB on rice yield and soil properties SIDDHARTHA SANKAR BISWAS 1 , DIPAK RANJAN BISWAS 2 *, TAPAN JYOTI PURAKAYASTHA 2 , ABHIJIT SARKAR 2 , RAJESH KUMAR 3 , TAPAS KUMAR DAS 3 , MANDIRA BARMAN 3 , SUNIL PABBI 3 , AVIJIT GHOSH 4 and RAM PAL 1 ICAR-Indian Agricultural Research Institute, New Delhi 110 012, India Received: 25 September 2020; Accepted: 10 November 2020 ABSTRACT Use effciency of direct application of phosphorus (P) hardly exceeds 15-20% because of fxation to soils. The fxed-P could be utilized subsequently if it is brought to plant usable form. An experiment was conducted at Division of Soil Science and Agricultural Chemistry, ICAR-Indian Agricultural Research Institute, New Delhi during 2017–18 to investigate the residual effects of different P sources like di-ammonium phosphate (DAP), rock phosphate (RP), RP+ phosphorus solubilizing bacteria (PSB) and DAP+RP+PSB on rice yield, P uptake, P dynamics and enzyme activities in an Inceptisol. Results indicated that application of DAP 30 +RP 60 +PSB had 20.7, 24.3,19.6, 21.0, 28.1, 33, 15.2, 14.4 and 29.4% higher rice grain and straw yields, grain and straw P uptakes, available and saloid P, dehydrogenase, acid phosphatase and alkaline phosphatase activities respectively, as compared to DAP treatment. Thus, it can be inferred that application of DAP+RP+PSB can reduce fertilizer P consumption by 50%, and support the residual (second) crop better than conventional P-fertilizers, and improve soil available and saloid P status as well as enzymatic activities. Keywords: Phosphorus, Phosphate solubilizing bacteria, Residual effect, Rice, Rock phosphate. Low phosphorus (P) use efficiency (PUE) of conventional fertilizers has become a challenge in the era of intensive agriculture. Inadequate reserve of high-grade rock phosphate (HGRP, >30% P 2 O 5 ) is forcing India to utilize low-grade rock phosphate (LGRP, <20% P 2 O 5 ) as P source to crops. Earlier researches showed that LGRP could be utilized by partial acidulation (Biswas and Narayanasamy1995); organic acid loaded RP formulations (Roy et al. 2018) and developing polymer coated P-fertilizers (Sarkar et al. 2015, 2018, 2020). But, high cost in synthesis of new RP- formulations made us to think in a different way. Zhou et al. (1992) reported that only 0.1% of total P in soil is present in bioavailable form during residual fertility assessment. Thus, there is an opportunity to utilize the fxed-P by subsequent crops after converting them into plant usable forms. Here, application of phosphate solubilizing bacteria (PSB) can play a vital role as they may continue to solubilize P for the subsequent crops after the main crop. Use of PSB like Pseudomonas spp., Bacillus spp., etc. has the potentiality to solubilize fxed-P in soil and signifcantly increase available P in soil by producing organic acids (Roy et al. 2019, Sarkar et al. 2021). However, very scarce information is available on the residual effects of PSB and RP application on soil total P, inorganic P (IP), organic P (OP), microbial biomass P (MBP), and different fractions of inorganic P like saloid P (Sal-P), aluminium P (Al-P), iron P (Fe-P), calcium P (Ca-P) and reductant soluble P (Res-P). For this, we aimed to study the residual effect of LGRP and PSB on rice yield and P uptake; and to assess their infuence on P dynamics among different soil P fractions and enzyme activities. We hypothesized that PSB will continue to solubilize P from RP and provide better residual effect as P source as compared to conventional fertilizer P. MATERIAL AND METHODS Bulk soil sample (0–15 cm) was collected from the experimental farm, ICAR-IARI, New Delhi (2017–18). Part of soil sample was processed and analyzed for physicochemical parameters. The other portion of soil was passed through 5-mm sieve and used for pot-culture experiment. Mechanical analysis was done following the procedure of Bouyoucos (1962). Soil pH and electrical conductivity was determined using soil: water ratio of 1:2.5 (Jackson 1973). Oxidizable organic C was determined by rapid titration method (Walkley and Black 1934). Available P was extracted with 0.5 M NaHCO 3 (pH 8.5) and determined spectrophotometrically using ascorbic acid blue colour method (Watanabe and Olsen 1965). Total P, IP and OP were estimated following the method outlined 1 ICAR-National Research Centre for Orchids, Pakyong, India; 2 ICAR-Indian Institute of Soil Science, Bhopal, India; 3 ICAR- Indian Agricultural Research Agricultural Research Institute, New Delhi; 4 ICAR-Indian Grassland and Fodder Research Institute, Jhansi. *Corresponding author e-mail: drb_ssac@yahoo.com https://doi.org/10.56093/ijas.v91i3.112528