The Critical Limit of Available Phosphorus for Predicting Response of Groundnut to Applied Fertilizer Phosphorus

For determining critical level of available P to predict response to applied P, a pot experiment was conducted for two years at the Punjab Agricultural University, Ludhiana. Thirty four soils were collected for the study from Ludhiana district, which varied from low to high in available P. Four levels of P viz, 0,10,15 and 20 ppm P₂O₅ as single superphosphate were applied to the crop along with a basal dose of 7.5 ppm N as ammonium sulphate and 12.5 ppm K₂O as muriate of potash. The pod and straw yield data indicated a linear response to P application up to 20 ppm P₂O₅ on low P soils. In medium and high P soils a significant response was observed up to 15 and 10 ppm P₂O₅, respectively. However, the maximum calculated P dose for low, medium and high P soils was 30.0, 15.1 and 9.9 ppm P₂O₅. Further, Bray's per cent yield gave a significant positive relationship with Olsen's extractable phosphorus of the soils. The pod and straw yield and P uptake were also significantly correlated with Olsen's extractable P in these soils. The critical soil fertility limit of available P for groundnut was found to be 16.6 kg/ha.