You can use phosphate fertilizer profitably for alfafa and red clover on over two-thirds of Iowa cropland. Potash is profitable on an additional fifth of the land. Alfalfa and red clover hay from some Grade A land is now so low in phosphorus that you can't feed it safely unless you add phosphorus to the ration from some other source!
Nelson, L. B.
"Fertilize for Better Hay,"
Iowa Farm Science: Vol. 1
, Article 2.
Available at: https://lib.dr.iastate.edu/farmscience/vol1/iss8/2