Extractable soil phosphorus in Blackland Prairie soils

Abstract

The Texas Agricultural Extension Service (TAEX) Soil Testing Laboratory currently utilizes a single phosphorus (P) extractant consisting of 1.43 M NH4OAc, 1. 0 M HCl, and 0.025 M EDTA-PH 4.2 to estimate plant available P for all soils in Texas. Research is needed to determine the suitability of this extractant for predicting crop P response for soils within the Blackland Prairie region of Texas. The objectives of this study were to determine the suitability of the TAEX P extractant for predicting crop P response, evaluate crop response to P fertilizer placement and rate, and compare extractable P levels from TAEX, Bray 1, Olsen, TAEX 1, TAEX 2, and TAEX 3 in selected Blackland Prairie soils. Five field locations were established with replicated P fertilizer treatments of 0, 20, 40, and 60 kg P ha-1 applied as both band and broadcast applications to each site for two years. Crop yields were evaluated to determine the effects of P fertilization. Representative soil samples (O to 10 cm) were obtained and extracted with the six P extractants. The current TAEX P extractant failed to predict three significant crop responses and all small responses to P addition. The Bray I and Olsen extractants recognized the need for P fertilizer, but recommended P fertilizer rates below the 40 to 60 kg P ha-' needed to obtain maximum yield. The TAEX 3 extractant appeared to be better suited for Blackland soils since it predicted the need for higher P rates. Band and broadcast P fertilizer applications were equally effective at all sites. TAEX and TAEX I extractants removed a larger proportion of total soil P than Bray 1, Olsen, TAEX 2, and TAEX 3 extractants. Olsen and Bray I extractable P were most significantly influenced by the addition of broadcast P fertilizer. Estimated critical P limits for the TAEX, TAEX 1, TAEX 2, TAEX 3, Bray 1, and Olsen extractants were approximately 152, 76, 26, 24, 27, and 14 mg P kg respectively. The TAEX 3 extractant better separated responding and nonresponding sites when compared to TAEX, Bray I, and Olsen extractants and may better crop P response on Blackland Prairie soils.