Agriculture Reference
In-Depth Information
Phosphorus in phosphate. This is why the Ideal Soil chart says
phosphate
should be 2X potash
.
•
Different soil testing laboratories use different conventions when reporting
results. Some labs report P as actual Phosphorus, some report it as
phosphate. It is the same with Potassium and potash. Some lab soil reports
list Potassium in ppm K, but show Phosphorus as P
2
O
5
phosphate.
Whatever lab one is using, it is necessary to know what form of P and K
they are reporting.
If you are buying a commercial NPK labeled fertilizer in the USA, and you want to
maintain the Ideal Soil ratio of P=K by weight, you would not want one labeled
10-10-10, but rather one labeled 10-20-10, or 5-10-5. Let's look at that a little
closer:
Say you bought an NPK fertilizer that was labeled 10-10-10, and knew that meant
10% Nitrogen, 10% phosphate, and 10% potash. 10% is elemental Nitrogen N, but
the phosphate is only 44% Phosphorus and the potash is only 83% Potassium.
The actual ratio and amount of elemental nutrients in that 10-10-10 is 10% N,
4.4%P, and 8.3%K.Alabel that said 10-20-10 would be 10%N, 8.8%P, and 8.3%K,
much closer to the P=K ratio by weight that we have called for in the Ideal Soil.
This writer has read different stories as to why the P and K on fertilizer labels are
listed as they are, one theory being that early analytical chemists only purified the
samples to the oxide form and then weighed that. The best guess as to why they
are
still
listed that way is that it makes it appear there is more fertilizer in the bag
than there actually is.
Here's why phosphate is 44% actual Phosphorus:
The chemical formula of
phosphate is P
2
O
5
,
2 parts Phosphorus and 5 parts
Oxygen. The periodic table of the elements tells us that the atomic weight of
Phosphorus is 31, and that of Oxygen is 16. So we have
2 x 31 = 62 (the weight of P)
5 x 16 = 80 (the weight of O)
added together, the weight is 142. Divide the weight of P by the total weight
62 / 142 = 0.437, or 44% P
For potash, the formula is K
2
O, 2 parts Potassium and one part Oxygen.
Potassium's atomic weight is 39, Oxygen is still 16.
2 x 39 = 78 (the weight of K)
1 x 16 = 16 (the weight of O)
