Normally it is not necessary to supply all the nutrient needs of a crop from fertilizer alone, because the soil already contains some quantity of nutrient elements available for crop growth. It is necessary to test the soil to determine its nutrient status, because only then can you determine the additional amounts of appropriate fertilizer materials to apply. Frequently, growers apply more fertilizer than the crop can use and the excess, especially nitrogen, is leached from the root zone into groundwater.
The Plant Nutrient Recommendations tables in each crop section can be used to determine nutrient needs based on soil test results and expected removal of nutrients from the soil by crops. See the following section, Removal of Nutrients from the Soil, and the section on Soil Testing to better understand nutrient recommendations.