First, keep in mind those calculators were developed for liquid fertilizers.

The values on a fertilizer label are in weight percentages, including the
modified ones for P & K. As water can be generally thought of as one gram
per ml, and as a milliliter is a volume measurement, the calculators work
with factors based upon grams of fertilizer per milliliter of water. As
such, they will vary depending on the minerals used to make up the
fertilizer, but I've not concerned myself with determining the degree of
difference. The mass of nutrients in solution is so small that when
measuring liquids, using the 1g/ml conversion is probably close enough.

As an example, Dyna-Gro "Grow" formula is around 18 weight percent
nutritional elements. For 100 ppm N, the calculation suggests that 1.41
ml/l is needed, so that means that a liter of solution contains:

0.18 x 1.41/1000 = 0.00025 grams of nutritional elements.

If my "concentrate" density is off by 5%, then my final solution will
contain between 0.00024 and 0.00027 grams, and insignificant difference, as
far as I'm concerned.

Obviously, those factors will vary greatly if you're using powders, and they
should be handled by weight, not volume, unless you know the bulk density of
the fertilizer powder you're using.

If you use the grams of fertilizer per liter of water calculation, and
actually weigh your fertilizer, you'll be much better off, in either case.

--

Ray Barkalow - First Rays Orchids - www.firstrays.com
Plants, Supplies, Artwork, Books and Lots of Free Info!


"Pat Brennan" wrote in message
...
Ray,

I was using your PPM calculator yesterday. Thanks, it came in real handy.
I am used to measuring fertilizer by weight, not volume. I'm guessing
that your ppm calculator assumes the density of fertilizer (weight per
volume) is about the same for all fertilizers. Is that a good guess and
if so, do you have any feel how valid the assumption is?


Pat