Fields are variable in nutrients and pests.
Under-fertilization may also be a concern because it can result in a reduction of crop residues returned to the soil which may lead to increased erosion and other undesirable soil properties. If plant nutrient levels are identified site-specifically, it is possible to reduce fertilizer application in areas that are high and increase levels in areas that are low. However, at most sites and most years, the total N used was redistributed throughout other areas of the field, resulting in similar total N used. The result was that at most sites, the level of residual N gradually decreased and levels of N were less variable following site-specific application of N fertilizer. Sometimes traditional soil testing alone, even if conducted in zones or grids, is not enough to identify fertility patterns.