Knowing how much nitrogen is in a bag of fertilizer is fairly easy. The percentage is listed right on the bag. To determine the actual amount of the element phosphorus (P) that is in phosphate (P2O5) and how much the element potassium (K) is in potash (K2O), use these formulas: 44% of P2O5 is phosphorous, and 83% of K2O is potassium. If you have a 50 lb bag of 20-5-10 fertilizer, how many pounds of nitrogen, phosphorus and potassium are there in the bag?