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How much phosphorus and potassium are really in your fertilizer?

The chemical formulas P 2 O 5 and K 2 O are the traditional means of expressing the amount of phosphorus (P) and potassium (K) in fertilizer. In fact, P 2 O 5 and K 2 O compounds do not exist in fertilizers. In the event that you have to determine the actual amounts of phosphorus and potassium in your fertilizer, use the following formulas:

% P 2 O 5 × 0.44 = % P

% K 2 O × 0.83 = % K

Examples:

A fertilizer containing 20% P 2 O 5 has about 9% P (20% P 2 O 5 × 0.44 = 8.8% P)

A fertilizer containing 10% K 2 O has about 8% K (10% K 2 O × 0.83 = 8.3% K)

NOTE: Typically, you do not need to perform these calculations because fertilizer recommendations are almost always provided as lbs P 2 O 5 /1000 sq ft and lbs K 2 O/1000 sq ft.