Doubts with phosphorus

[osmar amaral]

Sorry my English is not very good. I live in Brazil and the only phosphorus-based fertilizer have found so far is called Foskalium. Its composition is 423 g/l P2O5 and 282 g/l of K2O. Can I use it as a source of phosphorus in the tank?

[Biollante]

Hi,

Welcome!

Your English is better than my Portuguese is.

Foskalium is an anti-fungal, used to control apple scab.

1-ml per 100-liters will add about 5.6-ppm PO₄ (about 1.83-ppm P) and 2.3-ppm K⁺.

There is a little copper but not enough to be of concern.

Biollante

[osmar amaral]

Thanks for the help. Here in Brazil it sold as fertilizer. I was in doubt whether this P2O5 it could be used without problems.

[Tom Barr]

They sell a similar product in the USA as an anti fungus turf grass product, it is called KEEP and is the same stuff I think.

KH2PO4.

[Biollante]

Hi,

I apologize for the thread hijack but Tom Barr makes a good point.

While Foskalium Nutriplant is almost straight Monopotassium phosphite, KH₂PO₃ (not Monopotassium phosphate KH₂PO₄), it is reported in terms of potassium oxide, K₂O and phosphorus pentoxide P₂O₅.

Since Monopotassium phosphite reports as 38.4% K₂O and 57.9% P₂O₅, we can be reasonable sure that osmar amaral’s bag of “fertilizer” 423 g/l P₂O₅ and 282 g/l of K₂O, is Monopotassium phosphite and is a bit more than 98% pure.

Monopotassium phosphite, KH₂PO₃ is often preferred over Monopotassium phosphate KH₂PO₄ since it provides the same “P-K” for 12% less weight.

In the United States of America N-P-K (and I believe all of North America) is always reported as N, total Nitrogen, P as Phosphorus pentoxide, P₄O₁₀, (empirically that is P₂O₅) and K as Potassium oxide, K₂O.

That means the N-P-K of 15-9-12 on my Osmocote Plus label is actually 15%-4%-10%.

Why, you may well ask.

Simply in the olden days they weighed a sample of fertilizer, ignited it then weighed it again, so by convention we express the total elemental Phosphorous, P and Potassium, K as there oxides. Now days there are much more efficient ways to measure, but “they” would have to admit people are not getting as much for their overpriced fertilizers as the people believed they had paid for.

Just remember it is actually N-44%P-83%K.

Usually when you see P₂O₅ and K₂O on fertilizer labels, it is not actually Phosphorus pentoxide or Potassium oxide but the equivalent amount and with a little arithmetic, you can generally figure out what “they” are using, not that it really matters, because unlike Nitrogen the source has little or no effect on the plants or critters.

Biollante

[Tom Barr]

KH2PO4 is cheaper.........and that's why it's more commonly used.