Variability of soil salinity and nitrate within a saline area: consequences for soil testing and fertilizer response

Non-Peer ReviewedSoil test field calibration trials are most often established within apparently uniform, usually small, areas within fields. Results from many such sites over several years are then used to develop criteria for providing fertilizer recommendations based on analysis of samples representing entire fields, typically 40 to 160 acres. Implicit in this approach is an assumption that optimum fertilization for the field (if fertilization is not to be varied within the field) will be similar to optimum fertilization for the average soil condition (as determined by the soil analysis) present in field. The validity of this assumption should always be a concern in soil test interpretation; it becomes a major concern where a field soil test shows a sufficient test level of a nutrient which is expected to be highly variable within the field In those cases potentially large fertilizer response in deficient portions of the field may justify (economically) fertilization of the entire field. Soil salinity is known to be highly variable within fields, and to influence crop growth and soil residual nitrogen (N). This study examined the variation in levels of salinity and N present as nitrate (NO3-N) within a 40-acre saline stubble area, to assess the effectiveness of routine soil testing procedures for this situation. It is recognized that truly economic optimum fertilization cannot be achieved for variable fields unless the fertilizer applications are correspondingly varied within the fields. However, the objective of this study was to evaluate conventional soil testing and fertilization practices, rather than to develop a variable rate fertilization system