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The chemical formulas P2O5 and K2O are the traditional means of expressing the amount of phosphorus (P) and potassium (K) in fertilizer. In fact, P2O5 and K2O 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:
% P2O5 x 0.44 = % P
% K2O x 0.83 = % K
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)
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.
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