Determining critical value of soil Olsen P for dry direct seeded rice (Oryza sativa) in a greenhouse study in northwestern India

In view of the looming water crisis and shortage of labour during transplanting, dry-seeded rice (DSR) is an attractive option for rice cultivation in Asia. Presently limited information is available on optimum nutrient management in DSR. Compared to puddled transplanted rice, DSR is likely to encounter less availability offertilizer phosphorus. The main objective of the study was to determine critical value of soil Olsen Pfor DSR. A greenhouse experiment was conducted to study the effect of three levels of phosphorus (0, 6.5, 13 mg P kg-1 soil) application on dry matter accumulation and phosphorus uptake of DSR at maximum tillering (MT), panicle initiation (PI) and flowering stages (FL) in ten different soils varying in Olsen P and texture. Bulk soil samples (0-15 cm) were collected from different locations in Punjab state of India and analyzed for initial physical and chemical properties (pH, electrical conductivity, cation exchange capacity, organic C, NaHCO3-extractable P, NH4OAc-extractable K, DTPA-extractable Zn, Cu, Mn, and Fe and particle size distribution). Soil samples collected at flowering stage were analyzed for Olsen P. Plant samples were analyzed for total phosphorus content. Cate and Nelson (1971) method was used to determine the critical limit of soil Olsen P. Soils low in Olsen P (≤5.3 mg kg-1 soil) showed significant increase in shoot biomass and phosphorus uptake of DSR with the application of 6.5mg P kg-1 soil compared to control. Critical values of Olsen P for DSR, below which the crop would suffer yields determined by Cate-Nelson model, was 9.0 mg kg-1 soil. Recovery of phosphorus fertilizer at 6.5 mg P kg-1 in different soils ranged from 23.3-42.8%, with lower values for high Olsen P soils compared to control. The critical value of Olsen P was low at initial (MT and PI) growth stages than at flowering stage, suggesting that early crop growth in DSR is more sensitive to phosphorus deficiency. Response of DSR to phosphorus application depends on the availability of phosphorus in soil. A critical level of Olsen P at 9 mg kg-1 soil can be used for applying fertilizer phosphorus to DSR.