PO 45: Determine crop nutrient needs by using:

Crop yield potential reflects the nutrient requirement of the crop.  For some nutrients, particularly N, an attempt is made to match fertilizer applications with nutrient uptake patters throughout the season to optimize yield.  For other nutrients, the amount of nutrient removed in the harvested portion of the crop is replaced by fertilizer (maintenance).  Soil nutrient supply of P, K, Mg, etc is evaluated using a soil test.  Refer to earlier competency areas for details on testing and evaluating crop nutrient needs.

Crops in a crop rotation impact the nutrient needs in two major ways.  First, fertilization of one crop may also provide sufficient fertility for others in the rotation.  Basically, crop nutrient needs can be provided for the rotation, rather than the individual crops.  Second, the portion of a crop returned to the soil can increase or decrease the need for a nutrient by the succeeding crop.  Nitrogen fertility is one example: if a crop follows a legume (low C:N ratio in the residue), N fertilizer needs are often reduced.  If a crop follows a small grain (high C:N ratio in the residue), N fertilizer needs are often increased.

Soil test information provides an estimate of the productivity of a soil.  Nutrient needs in more productive soils are often greater than those in less productive soils, as more nutrients will be withdrawn from a more productive soil.