Nutrient Recommendations

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.

If a significant acreage of a particular crop is to be grown, fertilizers should generally be tailored to the specific needs of that crop, based on the amounts of nutrients that the crop is expected to remove during the growing season (see Table 4). If the soil tests indicate that a nutrient is optimum/high it is likely that the soil will supply enough to meet the crop's needs. However, many growers will apply enough of the nutrient to replace what is removed by the crop. If the test level is above optimum/very high, additional applications should normally be avoided unless the crop has an unusually high demand for a specific nutrient. Occasionally, nutrient applications may exceed the soil test recommendation or the expected average removed by the crop (Table 4) because a particular cultivar is considered a heavy feeder, such as long season Russet potatoes. Or, for example, a large crop of tomatoes can be expected to remove a large amount of potassium and it may be justified to apply some of this nutrient even if the soil test indicates a level somewhat above optimum. The nutrient recommendation tables for each crop have been developed on this basis of expected crop removal. This can also be a practical way to determine nutrient needs of high value crops, even when they are grown on a small scale. It is important to keep in mind that many factors can limit crop yield potential, and that simply adding more nutrients will not address any issues beyond nutrient deficiency.