What's the Deal on PPM?

[BajaMitch]

Been doing some research and preparation for next year's set of container experiments focusing on fertilization variances. Ran into a problem researching the recommended PPM recipe for the macro and micro nutrients. Discovered that much information out there is inconsistent on the weight of the fert used to calculate PPM, and therefore, the recommended PPMs for the nutrients.

The most common version of PPM reflects milligrams per liter as equivalent to Parts Per Million in a solution. The problem is whether or not the PPM uses the gross weight of the fertilizer, or the compound weight or strictly the "elemental" weight of the N-P-K-Ca-Mg-S elemental nutrients.

For example, Hydor-Garden's ChemGro formulation uses the gross weight of the three fertilizers that are in their recipe for 100 gallons of solution: 1/2 lb of 4-18-38, plus 1/4 Lb of Calcium Nitrate and 1/4 Lb of Magnesium Sulfate for a total of 1 Lb. of fertilizer to make 100 gallons of solution. Their recipe says that this should be equivalent to 1200 PPM...it is, if you use the entire 1 pound as your weight for calculation of the PPM. Obviously, there are other things in the 4-18-38 fert (which come to a total gross % of 60 % (4+18+38=60). Well, what about the other 40% of that fert? Hydro-Gardens has included the weight of that 40% in calculation of the PPM = 1200.

If you use only the weight of the N-P-K-Ca-Mg-S where P= the compound P2O5 and K = the compound K2O and eliminate the 40% mentioned above, then your "compound" PPM is 533 for the Hydro-Gardens solution. If you replace the P2O5 and K2O with elemental P and elemental K, respectively, they your "elemental" PPM is 434. Big difference. Huge difference!

Deep breath. Now then, how does one know which representation of PPM is being used when you look at PPM recipes for N-P-K-Ca-Mg-S wherever you encounter it?

What my research is showing me is that the PPM recipes are all over the place. I have come across all three different PPMs and the authors never tell you which weight basis they have used in the PPM recipe. If there is enough info in the article (rarely), then I can calculate which weight basis they have used. Other than that, as far as I can tell, it's a crap shoot.

I think that most of the PPM recipes use "elemental" weight of the fertilizer, but not that sure.

If anybody has any comments on this, please post.