Fertilizer Banding Influence on Spatial and Temporal Distribution of Soil Inorganic Nitrogen in a Corn Field

Fertilizer N is commonly applied as a sidedress band application in corn (Zea mays L.) fields. The objectives of this study were to determine the implications of fertilizer band application on spatial and temporal variations in soil inorganic N and to evaluate different sampling strategies for their suitability in estimating the average soil inorganic N concentration of a field. Treatments were 0, 60, and 120 kg N ha⁻¹ in 1994, and 0, 120, and 240 kg N ha⁻¹ in 1995, replicated three times at each of two sites. Fertilizer was applied as a band of NH₄NO₃ 15 cm on each side of each corn row at the corn six-leaf stage. Soil samples taken at eight interrow locations 0, 5, 10, 15, 20, 25, 30, and 35 cm from the corn row and at depth increments of 0 to 15, 15 to 30, and 30 to 60 cm and on five sampling dates were analyzed for soil NO₃ and NH₄ concentrations. Random sampling and five systematic sampling strategies were evaluated with respect to bias in estimation of field soil NO₃ and NH₄ concentration for three depth increments, and with respect to the number of samples required to achieve a given precision and probability level combination for soil NO₃ concentration for the 0- to 15-cm depth. All systematic sampling strategies provided adequate estimates of the true soil inorganic N concentration as estimated based on uniform sampling across all interrow locations. There was no consistent benefit to using any one systematic strategy with respect to the number of cores required to obtain a given level of precision in soil NO₃ concentration at a given level of probability. Systematic sampling strategies were at least as good as, or superior to, random sampling, particularly after fertilizer band application and at high N rates. The apparent insensitivity of estimated field soil NO₃ concentration to sampling strategy was attributed to the relatively high mobility of NO₃ in soil and variation in placement of the fertilizer band relative to the corn row during sidedressing.