Calibration of Soil Test P With Rop Response to Applied Phosphorus

Soil P test-crop response studies were performed in a long term experiment that was continued for seven cropping seasons from Rabi 1983-84 to Rabi 1986-87. The purpose of the study was to determine the comparative value of three soil P tests in assessing soil P status and P fertilizer requirements of wheat , corn and soybean based on their response to applied P. The study involved three wheat-based cropping systems (wheat-corn (WC) , wheat-fallow (WF) and wheat-soybean (WS)) that were assigned to main plots in a split plot design. Sub-plots received six rates of P (0, 30, 60, 90, 120 and 150 kg P20ÿ/ha) that were applied to each crop in the system during each season. The soil was a Sultanpur series (clay loam surface) with pH 8.5 and lime content 8.7%. It was low in organic matter (0.6%) and adequate in exchangeable K (1.0 me/lOOg). Soil P content (Olsen P - 2.79 mg/kg, AB-DTPA P - 1.8 mg/kg and 0.01M P - 0.0067 mg/L) was low by any standard. Phosphate sorption capacity of the soil was also low and it required 83 kg P/ha (190 kg PÿOÿ/ha) to establish standard P concentration of 0.2 mg/L. CaCl2 During the cropping period, 4 crops of wheat, each of maize and soybean were grown under the three cropping systems. Crop yields were recorded for each crop during each season. Soil samples were collected from each treatment after each cropping season and analyzed for available P by Olsen, AB-DTPA and 0 . 0 1 M CaC 1 methods. Phosphate sorption curves were for each sample. Plant samples secured also determine and 3 l from each crop at different stages of growth were analysed for P. These data were utilized to determine the influence of applied P on soil P status and P fertilizer requirements of each crop under different cropping systems. Fertilization and cropping for the seven seasons increased the soil P status (Olsen) from 2.79 mg/kg in 1983-84 to an average value of 12.86 mg/kg at the maximum P rate (P150) after 1986-87. Corresponding values by AB-DTPA method were 1.8 mg/kg and 7.83 mg/kg, and those by 0.01 M CaCÿ were 0.0067 mg/L and 0.04 mg/L respectively. However, AB-DTPA method extracted 61% as much soil P as that by Olsen method. The relationship between any two of the three soil test methods was highly significant (r = 0.80 to 0.96). It was easier to build-up P fertility under WF system which r equ ired 19 units of fertilizer P to raise Olsen soil P by 1 unit at P90. Corresponding values under the other two systems were: 32 units for WS and SO units for WC. cropping All the three crops viz. wheat, corn and soybean responded to applied P and showed progressive increase in grain yield, P content and P uptake with increase in P rate during each of the seven cropping seasons. Nature of response to P depended on the crop, cropping system and the year of cropping. Regression analysis of the pooled wheat grain yield (relative) data showed that wheat yields under WC system decreased significantly over WF or WS system. Phosphorus fertilizer required for 95% yield of wheat was higher under WC system than under WF or WS system. Mean P removal by wheat was 16.55, 17.14 and 17.64 kg P/ha under WC. WF and WS systems respectively. In case of corn and soybean, mean P uptake values were 19.36 and 14.38 kg P/ha respectively. In order to integrate soil P test values with P buffering capacity of soil and to translate P fertilizer additions into the levels of soil P established, phosphate sorption approach was used. Phoshorus in solution values associated with near maximum yield (95%) of the three crops were: wheat - 0.035 mg/L (WF/WS systems), 0.045 mg/L (WC system); corn - 0.036 mg/L and soybean -0.047 mg/L. Accordingly, the fertilizer requirements were 64 , 65 and 80 kg for wheat, corn and soybean respectively. Po°s/havwas grown under WC system, it required 78When kg wheat to attain 95% yield.