Letters To The Editor: Oct. 1997

Dear Editor:
I very much enjoy reading your publication. It is professionally done and contains some very good information. I felt compelled to contact you after reading the article "As Easy as N-P-K" (May, 1997) because it contained an error.

Under "Learning Your N-P-Ks," it stated that the three numbers on a fertilizer label represent the concentration by weight of N, P, and K. This is correct for N, but not for P and K. The second number on the label represents the concentration of P calculated on the basis of P2O5, while the third number represents K calculated on the basis of K2O. P2O5 is roughly 44% P by weight and K2O is 83% K.

The example you gave was 12-4-8. The actual concentrations of available P and K are:

4 x .44= 1.76% P 8 x .83= 6.64% K

Reporting concentrations as P2O5 and K2O are the way that chemists have traditionally reported metallic substances as their oxide forms for hundreds of years. Reporting on the fertilizer label is a carryover of this practice. Thus, actual concentrations of P and K are considerably different than the numbers on the fertilizer label.

Frank Hons
Professor, Soil Chemistry/Fertility, Texas A&M University
College Station, Texas