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

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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.