RELATIVE EFFICIENCY OF FERTILIZER N AND SOIL NITRATE AT VARIOUS DEPTHS FOR THE PRODUCTION OF SOFT WHITE WHEAT

Abstract

Soft white wheat was grown on a clay loam soil for 2 yr with a wide range of soil nitrate (SN) (70–280 kg ha−1) and fertilizer nitrogen (FN) (0–400 kg ha−1) treatments The field experiment was designed to determine the slopes of the yield response curves to FN (δy/δFN) and to SN (δy/δSN), to determine how the ratio of (δy/δSN)/(δy/δFN), or marginal rate of substitution, is affected by FN, SN, and depth of SN and to determine if refinements to the current FN recommendations for irrigated soft white wheat are required. The δy/δFN values in both years were high at low levels of FN and SN but declined as FN and SN increased. The δy/δSN in 1985 when most SN was situated in the 0- to 30-cm soil layer was initially high (26.0) at low levels of SN but rapidly declined as SN increased. The δy/δSN in 1986, when most SN was situated in the 30- to 120-cm soil layer, was intially low (6.0) but increased as SN increased. Since the δy/δFN and δy/δSN values were sensitive to changes in FN and SN as well as the depth of SN the MRS values were variable in both years. The δy/δSN increased as SN increased when SN was situated in the 30- to 120-cm soil layer so there was only a small effect of depth of SN on FN recommendations. The FN at maximum profit occurred at a greater combined amount of SN and FN for the higher SN levels in both years. This preliminary field study suggested that fertilizer N recommendations could be improved by using an equation for predicting FN that considers a variable yield response to FN and SN, an interactive effect of FN and SN on yield, and an effect of depth of SN on yield. Key words: Soil nitrate, fertilizer nitrogen, soft white wheat, soil test N