Soil tests can be made in several ways; some are better than others. Fully as important as the methods, however, is how well fertilizer recommendations based on soil-test results are related to actual crop responses in the field.
Stritzel, J. A.
"What Is a Soil Test?,"
Iowa Farm Science: Vol. 17
, Article 3.
Available at: https://lib.dr.iastate.edu/farmscience/vol17/iss12/3