How to convert phosphorus to oxide

Phosphorus in the results of soil analysis

One of the questions that our clients periodically ask us is: “Why in the results of soil analysis the amount of available phosphorus is given as P (as phosphorus expressed in ppm or mg / kg), and in the recommendations for fertilizers as P₂O₅ (as phosphorus oxide expressed in kg / ha) ”?

Phosphorus and potassium oxide

It was previously believed that all elements exist in the form of oxides. After it was found that phosphorus and potassium do not always exist in the form of oxides, one of the phosphorus fertilizer companies replaced the phosphorus DR on the label of its products in ammophos: instead of 52 (% P2O5), 23 (% P) was indicated.
However, farmers did not buy such fertilizer because it seemed to them that it contained 2.3 times less phosphorus than ordinary ammophos (although in fact this fertilizer contained the same amount of phosphorus). After this case, fertilizer manufacturers decided to indicate the concentration of phosphorus exclusively in P₂O₅.

Conversion of P to P2O5?

For example, you are going to apply 20 kg of phosphorus (P) in the form of a fertilizer with the formula 11-52-0. Use the following formula to convert:

1 kg/ha P₂О₅ = 2,3 kg P/ha
For 11-52-0:
2,3 ∙ 20 kg = 46 kg P₂О₅/ha
46 kg P₂О₅ / 0,52 (percent P₂О₅ in 11-52-0) = 88,5 kg/ha.

Therefore, to the question: “Why in the results of soil analysis the amount of available phosphorus is given as P, and in the recommendations for fertilizers, as P₂O₅?” The answer is obvious: “Because that’s the way it is!”