A fertilizer labeled 10-6-6 indicates what composition by weight?

• A. It contains 10% nitrogen and 6% potassium, with no phosphorus.
• B. It contains 6% nitrogen, 10% phosphorus, and 6% potassium.
• C. It contains 10% phosphorus, 6% nitrogen, and 6% potassium.
• D. It contains 10% nitrogen, 6% available phosphorus (P2O5), and 6% potassium (K2O) by weight.

Answer: (D)

Three numbers on a fertilizer label show the amounts of the primary nutrients in weight percent: nitrogen (N), phosphorus as P2O5, and potassium as K2O. For a 10-6-6 label, you have 10% nitrogen, 6% P2O5, and 6% K2O by weight. Phosphorus is typically shown as P2O5 (phosphorus pentoxide) in the guaranteed analysis, sometimes referred to as available phosphorus. The rest of the bag may be inert filler or moisture. This interpretation matches the numbers in order: N, P2O5, K2O. So the statement that it contains 10% nitrogen, 6% available phosphorus (P2O5), and 6% potassium (K2O) by weight is correct.