Christian Prud'homme wrote with an important question:

I attended your seminar you presented on January 28 at the Ontario Superintendent association [handout and slides here] and I really enjoyed. May I ask you a question to make sure I am on the same page?

On page 33 [slide 33, above], you come with a need in fertilizer of 2.1 g/m² of potash [sic]. If 5 g/m² = 1 lb/1000 ft², 2.1 g = 0.42 lb/1000 ft².  Is this 0.42 lb of ‘’elemental’’ K ?  If so, in order to apply this 0.42 lb, everybody understands (I think) they need to apply 0.84 lb of 0-0-50 fertilizer.  Since 0-0-50 is K₂O, should not we multiply by a 1.2 conversion factor and 1 lb of 0-0-50 fertilizer?   Applying the same logic, we should multiply by a 2.3 factor for phosphorus if we amend with a P fertilizer (P₂O₅) =  if we need 1 lb of P, we would need to apply 23 lb of a 5-10-5 fertilizer…?

That is correct. I make calculations of elemental K, and elemental P. When one is making an application of P or K, this should be kept in mind, and adjustments can be made to match the confusing way that fertilizers are labeled.

A little more about this -- when we do a soil test, the P and K data are reported as elemental amounts. When leaf tissue is tested for P and K content, those are reported as elemental amounts. For consistency, I keep everything in elemental units, from soil amounts, leaf amounts, estimated plant use, and also nutrient recommendations.

But fertilizer labeling does not use elemental amounts for P and K. The P is reported on a fertilizer label as P₂O₅ (phosphate, 44% P) and the K is reported as K₂O (potash, 83% K). When making fertilizer applications, one needs to keep this in mind, and make the necessary conversions.

The fertilizer labeling varies from country to country. Here's Wikipedia on Labeling of fertilizer. You can expect that when reading anything I've written, I am referring to the elemental P and K, and not to the phosphate or potash.