I spent some time this morning talking to technical folks at a number of
different fertilizer companies. While having come at the answer from
multiple directions, they all pretty much concluded the same.
Areas of absolute agreement:
1) The bulk density of a fertilizer depends upon the chemical components,
their percentages, their as-ground particle sizes, how much moisture they
have absorbed, et cetera.
2) Because of those issues, the bulk density of a ground product will vary
not only between products, but even lot-to-lot for an individual product, or
how long it traveled to get to you from the manufacturer (packing in
3) Fertilizers should be metered by weight, not by volume.
While that's fine for someone like me who used several pounds to make up 5
gallons of concentrate to be metered by a device into the water stream, it
is unlikely that most hobby growers have the capability of measuring out a
gram or two or a quarter of an ounce of powder to make up a gallon of
There were basically three responses to that:
1) Use "kitchen conversions" gleaned from websites and cookbooks. That
is, a level 1/2 teaspoon is 2.4 g, a level teaspoon is 5 g, and a level
tablespoon is 14 grams. Volumetrically, if we "standardize" on the
teaspoon, they would be 2.5 g, 5 g, and 15 g, but that's pretty close,
either way. As a teaspoon is 4.9289 ml, 5 g gives us a density of 1.014
2) Use an average bulk density of 60 pounds per cubic foot, That's
3) Use a standardized (but admittedly inaccurate) conversion for nitrogen
concentration per teaspoon: one teaspoon of powder to make up a gallon of
fertilizer contributes 12.5 ppm N for each percent in the formula, i.e.:
one teaspoon of a 20-20-20 fertilizer used in a gallon of solution will have
250 ppm N. If I back-calculate that, I get an apparent bulk density of 0.95
Considering the inaccuracy in measuring powders ("is that level or slightly
heaped?"), any other inaccuracies coming from metering devices like hose-end
sprayers or siphons, etc., and the very small concentrations of dissolved
minerals in out fertilizer solutions, it looks like a straight 1 g/ml
density is reasonable for both powders and liquids!
Ray Barkalow - First Rays Orchids - www.firstrays.com
Plants, Supplies, Artwork, Books and Lots of Free Info!
> 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!
>> First, keep in mind those calculators were developed for liquid
>> 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
>> Ray Barkalow - First Rays Orchids - www.firstrays.com
>> Plants, Supplies, Artwork, Books and Lots of Free Info!
>>> 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?