MSU fertilizer PPM scale

[DavidCampen]

Originally Posted by Ray

, ... but for ANY fertilizer, for 125 ppm N (not 125 ppm TDS):

How and by whom was the value of 125 ppm of nitrogen determined? Why not a value of 100 ppm or 150 ppm?

Edit:
This is intended as a rhetorical question because some people here seem to have become fixated on this value of 125 ppm nitrogen as some inflexible requirement instead of just a quick rule of thumb. And in fact you mention here ( TDS and EC ) that 125 ppm is just a value that you "typically shoot for".

If we take the total mass or powder added - again for 125 ppm N - that's 3.55g in a gallon, or 3550mg/3.785kg = 938 ppm, but depending upon how the particular species in solution dissociate, some of them are not part of the TDS either, so it's probably somewhere in between.

If you dissolve 3.55 grams of material in a gallon of RO water then the TDS _is_ actually (by your calculation) 938 ppm. The only thing approximate (as you mention here: TDS and EC ) is what reading will be obtained from an EC meter that is being used to estimate the TDS.

If I dissolve 3.55 grams of sugar in a gallon of RO water then an EC meter will give a TDS reading of 0 even though the actual TDS is 938 ppm.

And, actually, what is really important is the osmolarity of the nutrient solution which is yet another topic.

[Ray]

Originally Posted by DavidCampen

How and by whom was the value of 125 ppm of nitrogen determined? Why not a value of 100 ppm or 150 ppm?

There is absolutely nothing "magic" about 125 ppm N, at all.

The number is the one that the folks who developed the so-called "MSU Fertilizers" decided to use when evaluated their formulas, and found it worked well for their varied collection of orchid genera. (Coincidentally, I happened to have discussed that with Bill Argo, on of those developers, just last week.) One of them simply suggested it, and they went with it.

It seems to be a reasonable level to apply frequently without fear of "burning".

[orchids3]

Ray,
Thanks for your explanation - but this guy seems to believe that Phosphorus has something to do with blooming. Not unusual but a chance to get on the soap box if you are anything but a fertilizer salesman.