Your Reading List

Make sure you mind your fertilizer Ps and Ks

The fertilizer story — why a pound of phosphorus may not really be a pound of phosphorus

When it comes to phosphorus and potassium, farmers and researchers speak two different languages.

In a recent article I mentioned a classic old Alberta document that talks about managing Gray Wooded soils — low in fertility and organic matter. I pointed out that when the old boys had data about rates of application of phosphorus and potassium, they meant just that, P and K, not P2O5 and K2O — which is the way fertilizers are labeled and sold. I then lamented that there were too many cases in modern farm literature where the terminology used is just plain WRONG.

It bothers me greatly to see that we have fallen back to where we have to go right back to the basics that we taught at farm short courses in the 60s and 70s when fertilizer use first took off for real.

That stirred up our fearless editor who hit the nail on the head with her comments. She said if everyone is doing it wrong maybe it is OK, but when we do not know who’s doing it wrong and who’s doing it right, then we are setting ourselves up for a fall down the road.

The kicker

When farmers talk about how much fertilizer they’re applying, they usually talk about the amounts they’re actually applying — the amounts in P2O5 and K2O. This is OK, as long as fertilizer recommendations are in P2O5 and K2O. The problem comes when people switch terminology.

The kicker for me is when I fill out my Crop Insurance Yield Report and it asks for inputs. Fertilizer is listed as N, P and K — actual. I report on paper and correct the P and K on the form to P2O5 and K2O and make a note that the form should be corrected, as I am sure most are doing it that way, rather than doing the math to convert their applied P2O5 to actual P. But the form has not been changed.

How useful is the data down the road when we do not know how folks are reporting?

Please, do not get me wrong . This is not a criticism of our Crop Insurance program. I am a huge fan of Crop Insurance and think it is a very well run program. For years I have thought some other farm programs should be scrapped and the resources put to Crop Insurance.

What are the facts?

Henry’s Handbook of Soil and Water devotes an entire page (32 for those that have the book) to explaining the concept.

The facts are this:

The fertilizer industry expresses the nutrient content of fertilizers as follows:

  • N is reported as per cent N.
  • P is reported as per cent P2O5, referred to as phosphate to differentiate from phosphorus (P).
  • K is reported as per cent K2O, referred to as potash to differentiate from potassium (P).

The conversions are quite simple:

  • To convert P2O5 to P divide by 2.29. To convert P to P2O5 multiply by 2.29.
  • To convert K2O to K divide by 1.21. To convert K to K2O. multiply by 1.21.

For K, the difference is not huge but for P it is out by more than 100 per cent. That’s a big error.

Page 32 of Henry’s Handbook explains the chemistry of it all but the factors are what is important.

Why are N and P done that way?

The plain and simple answer is that there is no good reason and it makes absolutely no sense. It is a throwback to the days when lab folks analyzed soils and rocks and expressed the results as the oxide form so it all added up neatly to 100 per cent. It was a check on the lab.

In the early days of superphosphate fertilizer manufacture in the U.K. the oxide form of reporting P and K snuck in to the fertilizer business and that method was transported to North America. And, it has stuck.

There is no P2O5 in P fertilizer. Our most common P fertilizer is monoammonium phosphate (chemical formula NH4H2PO4). The P form is PO4.

There is no K2O in K fertilizer. Almost all of our potash fertilizer is potassium chloride (chemical formula KCl).

With the new ways of looking at P fertility of crops with a longer time window there is renewed interest in the 1970-80s experiments which porked on gobs of P and followed the results (for the keeners, sources are listed below).

In those experiments the reported rate of P fertilizer was 400 kg P/hectare. To convert that to the actual amount of product used, some math is necessary:

  • 400 kg P/ha x 0.0892 = 357 lbs. P/acre;
  • then multiply by the factor of 2.29 = 818 lb P2O5/acre.

Using 11-52-0 fertilizer, this is 1,573 lbs./acre of product.

In the 1970s, when our Government of Canada was stuffing metric down our throats whether we liked it or not, we tried mightily to get P and K changed to the elemental form of reporting — get it all over once and for all. But, the North American fertilizer market is very closely intertwined so it was impossible for Canada to go it alone.

So, there you have it. It makes no sense but we are stuck with it. And we must make sure that each generation that comes along is schooled in what it is and how to use it so there is no ambiguity.

About the author


Les Henry

J.L.(Les) Henry is a former professor and extension specialist at the University of Saskatchewan. He farms at Dundurn, Sask. He recently finished a second printing of “Henry’s Handbook of Soil and Water,” a book that mixes the basics and practical aspects of soil, fertilizer and farming. Les will cover the shipping and GST for “Grainews” readers. Simply send a cheque for $50 to Henry Perspectives, 143 Tucker Cres., Saskatoon, Sask., S7H 3H7, and he will dispatch a signed book.



Stories from our other publications