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Influence of Fertilizer Application Nonuniformity on Crop Response

Levels of nonuniformity in K fertilizer application were established in a field experiment on a K deficient soil. Measures of uniformity of fertilizer distribution (UCF) were calculated from the coefficient of variation (CV) of K application (UCF = 1 − CV/100). The structure of spatial dependence of exchangeable soil K was used to estimate the proportion of a given experimental plot which should be below a specified threshold of exchangeable K. Fertilizer requirements were estimated on the basis of the proportion of the region that was deficient. If deficient areas were identified in the field, then variable rates of fertilization could be applied. The response of chinese cabbage (Brassica oleracea L. Chinensis) to K application was strongly affected by the uniformity of fertilizer distribution. Maximum yield decreased 9.5% with nonuniformity of fertilizer distribution. Amounts of K associated with 95% maximum yield were 97, 113, and 444 kg K ha⁻¹ for UCF values of 1.0, 0.42, and −0.02, respectively. Corresponding tissue K concentrations associated with 95% maximum yield were 3.5, 3.8, and 4.0%. Increased variation in exchangeable K from CV = 44.1% to CV = 96.6% resulted in critical levels of soil K of 0.28 and 0.73 cmolc ha⁻¹, respectively. Joint contribution Hawaii Inst. for Tropical Agric. and Human Resources and the African American Institute.