I know part of the problem with weighing powders is just how much moisture
they absorb, as that affects both the apparent weight and how the powders
pack, which both affect density, and that changes over time.

By the way, I put the summary of my "diggings" in a page on my website so I
won't lose the info:

http://www.firstrays.com/measuring_fertilizers.htm

--

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


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

When the computer said I was to use .49 teaspoons per gallon, who was I to
question.

I have weighed some of the stuff around here and here are the results.

.78 g/ml Peters 20-20-20 powder
.84 g/ml Excel 15-5-15 cal mag powder
1.05 g/ml 15.5-0-0 calcium nitrate powder
1.12 g/ml 13.75-0-46 potassium nitrate prill
1.18 g/ml Dyna-Grow 7-7-7 liquid
1.21 g/ml Jacks 10-30-20 powder
1.52 g/ml 0-54-0 phosphoric acid liquid

Pat


"Ray" wrote in message
...
Pat,

You got me thinking about this one...

I was considering taking an example fertilizer and back-calculating the
makeup, throwing in the bulk densities of the components, and seeing
where that got us, but then I realized:

The bulk density of powders varies by not only what the material is, but
how finely it is ground, what shape those particles are, etc. As an
example, the bulk density of ordinary silica sand, with it's fairly
uniform, quite rounded particles, runs about 1.5 g/cc. If you have the
fine particle size silica used as a thickener in everything from paints
to cosmetics to ketchup, its bulk density is about 0.05 g/cc.

But now I'm on a quest.

I figure that most water soluble, powdered fertilizers contain
more-or-less the same chemicals, and they are probably similar in their
ground properties (there are a few exceptions), so the range of bulk
densities is probably reasonably narrow, but I'm going to do some digging
and see what I can find out...

FWIW, the bulk densities of most of the major components is about 1 g/cc,
according to some large-scale mineral suppliers I have already contacted,
suggesting that our volumetric approach might not be all that bad.

--

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


"Ray" wrote in message
...
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