Soil tests are necessary to establish the proper lawn fertility program for a specific site. The proper ratio of nitrogen, phosphorus, and potassium to apply to a lawn can only be determined by soil testing.
Since nitrogen can move rapidly out of the root zone with infiltrating water, soil tests for available nitrogen are not very meaningful and are generally not performed.
The most important soil test is one that establishes base levels of phosphorous and potassium in the soil. Phosphorus and potassium move little in the soil such that subsequent sampling may not be necessary for three years if phosphorus and potassium levels are adequate.
The label on lawn fertilizer bags lists the percentages of the three primary nutrients as a series of three numbers called the fertilizer analysis. As an example, a common lawn fertilizer analysis is 23-3-6. In this case the fertilizer contains 23 percent nitrogen, phosphorus equivalent to 3 percent P2O5, and potassium equivalent to 6 percent K2O. One fertilizer analysis is not best for all soils, since soil phosphorus and potassium levels determine needs. If a soil is already high in phosphorus and potassium, a fertilizer with an analysis of 34-0-0 or 46-0-0 would be sufficient, while a soil with low phosphorus or potassium would require additions of fertilizer containing phosphorus or potassium.
Information on soil testing and analysis are available through your county extension office or online at University of Minnesota Extension: Soil Test Interpretations and Fertilizer Management for Lawns, Turf, Gardens, and Landscape Plants.
Soil tests can be obtained by submitting a soil sample to a private soil fertility testing laboratory or to the University of Minnesota Soil Testing Laboratory.