Interactions affecting response to applied nutrients like nitrogen can be difficult to predict.
Variable rate is most applicable to those nutrients like nitrogen and phosphorus in which a range of application rates can be set out according to differences in such measured parameters as level of available nutrient and moisture availability across the field. In the on-off approach, problem areas where deficiencies are suspected are located in the field through visual inspection or using yield maps produced from the combine yield monitor. Soil and plant testing inside and outside the affected area is used to first determine whether or not a nutrient limitation is responsible for the low yield and then subsequently the nature and extent of the limitation. Furthermore, rather than a range of rates associated with different soil test levels, the recommendations often take the form of a single recommended rate when the test level is below a critical value i.e. apply or don't apply. In order to effectively vary the rate of fertilizer so as to achieve the optimum rate of added nutrient for each specific location or identified management unit in the field, we need to obtain the necessary information that allows us to predict response to added fertilizer in each unit.