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. Read the section on Soil Testing to better understand nutrient recommendations.
In general, the goal should be to maintain nutrient elements within the optimum range as reported on the soil test. When nutrient levels are within this range, the needs of most crops will be met. If levels are below optimum (low or medium), most crops would benefit by adding the appropriate nutrient(s) to increase levels to optimum. However, if levels are at above optimum (very high) levels, there will be no additional benefit and excess levels may reduce crop yield or quality and may cause environmental harm. This happens in fields where soil testing was not used to monitor fertility levels or when nutrients are applied even when soil levels are sufficient. When a nutrient is above optimum levels it should not be included in any amendments until the excess is taken up by crops. In this case, it is wise to temporarily stop applying compost or manures until nutrient levels are in the desired range because the addition of these amendments can add high levels of nutrients, especially phosphorus. This is a practical way to manage nutrient levels if small to moderate amounts of mixed crops are to be grown.