Use of liquid fertilizers and fertilizer solutions has increased rapidly in Iowa since 1950, though such materials have been used in the United States since 1923. The earliest ones used in Iowa contained only nitrogen. Now, however, liquid fertilizers containing nitrogen, phosporous and potassium are being produced.
Pesek, John; Webb, John; Meldrum, H. R.; and Stritzel, J. A.
Iowa Farm Science: Vol. 11
, Article 5.
Available at: https://lib.dr.iastate.edu/farmscience/vol11/iss10/5