Due to the cost of fertilizer materials, annual soil testing has become the standard recommendation to ensure that application rates are correct.  In addition, since nitrogen is subject to leaching and volatilization, annual soil testing is essential to predict residual soil levels and to determine proper supplemental fertilizer rates. 

The objective in soil sampling is to obtain a composite that represents the entire area to be fertilized or limed. A composite sample is comprised of 12 to 15 cores or slices of soil (approximately 1 inch wide by 6 inches deep) collected from the sampling area and thoroughly mixed.  A sampling area can be any field or pasture or portion thereof that is relatively uniform in soil type and cropping history.  Typically, the sampling area for a composite sample should not be greater than 40 acres.

Typically, soil samples are collected to a depth of 6 inches, measured from the soil surface after non-decomposed plant materials are pushed aside. Deviations from the traditional 6-inch sampling depth may be required if fertilizer has been placed deeper in the soil by tillage or banding.