PO 18. Compare and contrast the following approaches for making fertilizer recommendations:

Three main approaches have been developed to determining nutrient status and calculating needs.

Sufficiency level recommendations are most commonly used by Land Grant Universities and their laboratories.  The recommendations utilize the yield response method; i.e. fertilizer rate is based on expected crop response (i.e. increased yield), a function of soil nutrient levels.  The agronomic soil test is an index that can be used, based on a large amount of local field studies, to determine:

1. If a response to extra fertilizer is to be expected
2. How much of that fertilizer needs to be added

Results from soil tests are categorized as being either "high," "medium," or "low" for a given nutrient.  Crops in high-test soils are unlikely to demonstrate any yield response to additional fertilizer; crops in medium-test soils may or may not show a yield response; and crops in low-test soils are likely to respond to additional nutrients.  Fertilizer recommendations are made appropriately.  Overall, this method of recommendation leads to a slower increase in soil test values, and keeps the soil test values lower than result from other methods.  This is primarily due to a decreased fertilizer usage than in the other approaches.