We are not doing a very good job of keeping up the available nitrogen content in Iowa soils. W e could step up our yields and we would get a lot more good from the phosphorus and potassium fertilizers which we use if we had more available nitrogen in our soils.
Black, C. A. and Lawton, Kirk
"Iowa Soils Need Nitrogen,"
Farm Science Reporter: Vol. 6
, Article 5.
Available at: http://lib.dr.iastate.edu/farmsciencereporter/vol6/iss1/5