The Economics of Soil Testing

Matt Ruark, Department of Soil Science

Soil testing is the only tool we have to evaluate pH, phosphorus (P), and potassium (K) status of the soil.  Knowing this information allows growers to make economically optimum applications of lime and phosphorus and potash fertilizer.  In these times of economic uncertainty, managing input costs is an important component of farm sustainability.  Many growers have opted to cut costs by eliminating soil testing as part of their management program.  This is not a recommended strategy.  There are two main misconceptions about soil testing:

MISCONCEPTION #1: Soil testing is expensive.

FACT: Routine soil testing costs about $0.35 per acre.  University of Wisconsin soil testing recommendations are to collect one composite sample per five acres and to soil test at least once every four years.  Using a standard rate of $7.00 for routine soil analysis (which includes pH, P, K, and organic matter), this averages out to $0.35 per acre per year.  All Wisconsin Department of Agriculture, Trade and Consumer Protection Certified Laboratories are required to provide fertilizer recommendations based on University of Wisconsin guidelines along with the soil test values.  (Note: actual cost of soil test may be slightly higher depending on the lab and if shipping costs are incurred)

MISCONCEPTION #2: Maintenance applications of P and K are appropriate in all situations.

FACT: Soils testing in the "Very Low" and "Low" range for P and K require additional inputs beyond removal rates to optimize yield.

FACT: Soils testing in the "High" and "Excessive" range for P and K require less than removal rates to optimize yield.

In either case, money is lost from either reduced yields or over-application of P and K.  When soil tests indicate the soil is in the very low to low category, this suggests that there is a high likelihood that yields will increase due to application of fertilizer.  However, it also indicates that the subsequent crops would benefit from building the "fertility" of the soil through additional P and K inputs over time.

For more information on soil testing, please visit our website www.soils.wisc.edu/extension/soilsampling.php