摘要：The marginal product of fertilizer applications varies across crop years and both between and within crop fields. This variability is caused by variations in weather and a nonuniform distribution of soil and production factors such as organic material, nutrient availability, soil moisture, landscape position, pest pressure, soil compaction, drainage, and rooting depth [Sawyer, 1994]. Soil sampling has long been advocated as a means of improving the efficiency of fertility management decisions by better matching fertilizer applications with crop nutrient requirements and nutrient availability. Advances in mapping and sensing technologies have renewed interest in soil sampling as a means of moving to variable rate technologies (VRT) whereby a farmer varies fertilizer applications across space and/or time. Significant research efforts are under way to develop the knowledge and equipment needed to allow farmers to move to variable rate technologies (VRT) [National Research Council, 1997].
