Certified Crop Advisor Practice Exam

If a fertilizer is labeled as 13-13-13 on an oxide basis, what is the analysis on an elemental basis?

1. 13-5.7-10.8

2. 13-10-10

3. 13-15-15

4. 13-17-7

The correct answer is: 13-5.7-10.8

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