A phosphorus soil test interpretation for corn grown on acid soils varying in crystalline clay content

Abstract Although there is considerable knowledge about P sorption in relation to soil properties, little of this information has been applied to the refinement of P soil test interpretations. Perhaps the most Important soil property, and one that could be readily Incorporated, is clay content. Corn was grown in residual P studies on two Ultisols that differed in clay content. Yield was first expressed as an exponential function of soil test P concentration, then the P concentration replaced with the rate of P required to maintain a given soil test P level for one year. Net income was computed from the yield, price of corn, rate of fertilizer, and fertilizer cost. When net Income was maximized, the optimum P rate for each soil was calculated in relation to the Initial soil P concentration. These relationships were quite linear and differed markedly between soils due to clay content. A continuous function was then developed to express the optimum P rate for soils of any clay content and P concentration. Key words: Soil testingPhosphorus Zea mays Notes Paper No. 9337 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, NC 27695. Professor and Graduate Research Assistant, N.C. State University, Raleigh, NC 27695.