How Much Phosphorus and Potassium are Really in Your Fertilizer?

The chemical formulas P₂O₅ and K₂O are the traditional means of expressing the percentage of phosphorus (P) and potassium (K) in fertilizer. However, P₂O₅ and K₂O compounds do not exist in fertilizers. In the event that you have to determine the actual percentage of phosphorus and potassium in your fertilizer, use the following formulas:

% P₂O₅ x 0.44 = % P

% K₂O x 0.83 = % K

Examples

A fertilizer containing 20% P₂O₅ has about 9% P (20% P₂O₅ x 0.44 = 8.8% P)

A fertilizer containing 10% K₂O has about 8% K (10% K₂O x 0.83 = 8.3% K)

Note

Typically, you do not need to perform these calculations because fertilizer recommendations are almost always provided as lbs P₂O₅/1000 sq ft and lbs K₂O/1000 sq ft.