Yield response of wheat to N and P fertilization on volcanic ash soils in South-Central Chile
Yield response of wheat (Triticum aestivum L.) to N and P fertilization was evaluated on several andisols and alfisols of Region IX, Chile. Seventy-six field experiments were conducted during a ten-year period to evaluate the yield response to N and P under different soil and climate conditions;The mean treatment yields at each experimental site were fitted to a quadratic function and to a square root function. The quadratic form was selected to continue the development of the production function;Soil chemical analyses were correlated to the response to N and P at each site to explain variations in yield and yield response to fertilizer;By use of multiple regression analysis two models were developed for each of two groups of soils; one by regressing the coefficients of each site's response function on soil variables and the other by direct calculation. For the andepts (Trumao) the models included the variables of soil nitrogen (N[subscript] s), available soil phosphorus (P[subscript] s), soil pH, and soil organic matter (OM). The models for the alfisols (Red clay soils) included the same soil variables plus exchangeable soil potassium (K[subscript] s);An economic analysis of response of wheat to N and P fertilizer was performed using the proposed models and actual costs and returns for different soil fertility levels measured by soil chemical analysis. The response to fertilizer N and P at mean soil test values was illustrated for each one of the four generalized yield functions;Maximum yields were in the order of 5000 kg ha[superscript]-1 for the andepts and 4000 kg ha[superscript]-1 for the alfisols. Maximum and economic optimum yields for the andepts show a large effect due to soil pH at low and medium levels of soils N[subscript] s and P[subscript] s. The effect of P[subscript] s on yields is less, with an increase of 1 to 6%;For the alfisols the effect of soil pH is less than for the andepts and not always positive. The increase due to P[subscript] s for maximum and economic optimum yields is between 30 and 40%;The functions presented give a good estimate of general fertilizer requirements, but they can be improved by the addition of other uncontrolled variable parameters.