Expressing the phosphorus (P) and potassium (K) content of fertilizer in the elemental form will place these two nutrients on the same basis as nitrogen (N) and other nutrients already expressed in elemental form.
Stritzel, J. A.
"Better Names for "Phosphate" and "Potash","
Iowa Farm Science: Vol. 18
, Article 7.
Available at: https://lib.dr.iastate.edu/farmscience/vol18/iss2/7