Mixing Bulk Fertilizer – Phosphate or Phosphorus & Potash or Potassium?

Q.  You say we need 125 kg nitrogen, 68.125 kg phosphorus, and 125 kg potassium to feed a hectare. I understand how to figure the nitrogen, but phosphorus is only shown as P2O5 and potassium is K2O. How do I get the right amounts of those two – or do I not need those amounts of pure phosphorus and potassium?

A.  It is not the pure phosphorus that is 46% in an 18-46-0 fertilizer, but P2O5. To get the % pure phosphorus we would have to divide by 2.3. We do in fact want 125 kg of phosphorus as P2O5 – not 125 kg of pure phosphorus, and 125 kg of potassium as K2O – not pure potassium. However, it is so rare for the ordinary family gardener to know anything about this, we figured it would only be confusing to try and explain it in books meant for the home gardener.Here is how you figure how much to buy of N, P, and K. If your fertilizer sources are 18-46-0 and 21-0-53, you do the following:

  Total Purchased N P K
18-46-0 Figure P first – then N 148.09 kg 26.64 kg 68.125 kg 0 kg
21-0-53 Figure K first – then N 235.84 kg 49.52 kg 0 kg 125 kg
Then find some nitrogen –
34-0-0* or 21-0-0
143.64 kg 48.84 kg 0 kg 0 kg
TOTALS 527.57 kg 125.00 kg 68.125 kg 125 kg

*34-0-0 This is best, use it if possible If you are using pounds and acres, you would substitute 110, 60, and 110 for the pounds per acre needed to feed a crop.