RELATING SOIL TEST P TO P UPTAKE BY PADDY RICE

Abstract

In Arkansas, the Mehlich-3 extraetant is used in the determination of soil test P for both upland and lowland crops. However, this technique does not consistently predict a response by paddy rice (Oryza sativa L.) to P fertilization. The objective of this research was to relate soil test P determined by four extractants to P uptake by paddy rice grown on two rice-producing soils. The soils in this study were a Crowley silt loam and two Hillemann silt loam samples collected from fields where P deficiencies in rice were observed the previous summer. Rice was grown on these soils in the greenhouse at fertilizer rates of 0, 5, 10, 20 and 40 mg P kg-1. Plant uptake and tissue concentrations of P were measured and related to soil test P extracted by Mehlich-3, Bray-1, Olsen, or ion exchange resin at 0, 24, 31, and 124 days after flooding (DAF). The Crowley had the highest soil test P obtained with all extractants at all flooding dates compared with the two Hillemann soil samples. Mehlich-3 and Bray-1 extractants consistently gave higher soil test P values on the Crowley soil than did Olsen and resin extractants. Only the resin showed a consistent correlation between soil test P measured at 0 DAF and % P in shoot and total P uptake measured at 24, 31, and 124 DAF. This correlation was improved when P uptake was related to soil test P measured on anaerobic soils. Although Mehlich-3 is convenient for soil test laboratories, the resin appears to be a more appropriate extractant to estimate plant available P for paddy rice.