Estimation of Critical Levels of Available P for Predicting Response of Paddy to Applied P in Submerged Soils

The available P status of intensively rice growing soils of Andhra Pradesh was evaluated by different methods both in air-dry and submerged soils. The redox potential became negative (−37 mV) at 28 days of submergence, attained the lowest value (−165 mV) at 49 days of submergence and remained almost constant thereafter. It was observed that the submergence of the soils increased the available P status due to an increase in iron phosphate, occluded phosphate and calcium bound phosphate. The Fe-P and AI-P significantly contributed to the available P pool in submerged soils and Fe-P was the major contributing P fraction to the P nutrition of rice. The traditional Olsen's method was found to be much superior to the other methods as it showed highest significant relationship with all the plant growth parameters with a critical level of 33.5 kg P₂O₅ ha⁻¹. Since Olsen's method extracted Fe-P and AI-P at peak submergence, which are also the main sources for P uptake by rice, the determination of available P at submergence by this method may be a better index for computing fertilizer P recommendations for rice.