Development of a new method for testing the nitrogen needs of your soil enables the Iowa State College Soil Testing Laboratory to make more accurate recommendations for you. Here are the details about the new test.
Hanway, J. J. and Gray, R. C.
"Test Your Soil for Nitrogen Needs,"
Iowa Farm Science: Vol. 7
, Article 4.
Available at: https://lib.dr.iastate.edu/farmscience/vol7/iss12/4