Certified Crop Advisor Practice Exam 2026 – Complete Study Resource

When a fertilizer is labeled as 13-13-13 on an oxide basis, it indicates the percentage of three essential nutrients: nitrogen (N), phosphorus (P), and potassium (K) in their oxide forms: N as ammonium nitrate, P as phosphorus pentoxide (P2O5), and K as potassium oxide (K2O).

To convert these oxide values to their elemental forms, certain conversion factors must be applied.

- For phosphorus, to convert from P2O5 to elemental phosphorus (P), you use the factor of 0.436. Thus, 13% of phosphorus pentoxide (P2O5) converts to about 5.7% of elemental phosphorus (13 x 0.436).

- For potassium, to convert from K2O to elemental potassium (K), the factor is 0.830. Therefore, 13% of potassium oxide (K2O) translates to approximately 10.8% of elemental potassium (13 x 0.830).

Since nitrogen is reported directly as an elemental value, it remains at 13%.

Therefore, when you convert the fertilizer analysis labeled as 13-13-13 on an oxide basis to an elemental basis, you