For every ton of commercial fertilizer that Iowa farmers used 6 years ago, they are now using 9 tons—a nine times increase in 6 years. It’s likely that still more fertilizer will be used in the coming years.
Pierre, W. H. and Meldrum, H. R.
"Going to Use Fertilizer?,"
Farm Science Reporter: Vol. 7
, Article 4.
Available at: http://lib.dr.iastate.edu/farmsciencereporter/vol7/iss1/4