By coupling soil-test results with knowledge of crop sequence and plant nutrients carried over, added by manures, and removed or added by crops, you can eliminate more and more of the guesswork in crop fertilization.
Stritzel, J. A.
"Modern Farmers Need to Be -Accountants in the Cornfield,"
Iowa Farm Science: Vol. 18
, Article 6.
Available at: https://lib.dr.iastate.edu/farmscience/vol18/iss2/6