Whether it comes as a powder or a liquid, by law the values on a fertilizer label are in weight percentages, including the modified ones for P & K.
When I was devising my fertilizer calculators, I was focusing on liquid fertilizers, so I simply used the water standard of 1 milliliter volume weighing 1 gram as the “master” conversion. I recognized that the densities of different formulas will vary depending on the minerals used to make up the fertilizer formula, but considering that 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 1.41ml/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 the “concentrate” density is off by 5% (that is, 0.95 or 1.05), then the final solution will contain between 0.00024 and 0.00027 grams, respectively, which is an insignificant difference.
Obviously, those factors are likely to vary even more when using powders and they truly should be handled by weight, not volume, unless you know the bulk density of the fertilizer powder you’re using.
I was considering taking an example fertilizer and back-calculating the makeup from the formula and the minerals used to make it up, throwing in the bulk densities of the components, and seeing where that got us. But then I remembered that the bulk density of powders varies by not only what the material is, but how finely it is ground and what shape those particles are, etc. As an example, the bulk density of ordinary silica sand, with its 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.
Figuring that most water soluble, powdered fertilizers contain more-or-less the same commodity chemicals, and they are probably similar in their ground properties (there are a few exceptions), the range of bulk densities is probably reasonably narrow.
The bulk densities of most of the major components are 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. Some research into the physical properties of the chemicals states 1 g/cc +/- 20%, but it pretty much averages out when their ground to powders.
I spoke to technical folks at a number of different fertilizer companies. While having come at the answer from multiple directions, they all pretty much came to the same conclusion.
The bulk density of a fertilizer depends upon the chemical components, their percentages, their as-ground particle sizes, how much moisture they have absorbed, or how and how long it traveled to get to you from the manufacturer (packing in transit). 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.
Fertilizers should be metered by weight, not by volume. While that’s fine for someone like me who uses several pounds to make up many 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 fractions of an ounce of powder to make up a gallon of fertilizer.
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, so feel free to use that measuring spoon for either!
Ray Barkalow has been growing orchids for over 45 years, and owns First Rays, which offers horticultural products to the hobby grower. He may be contacted at firstname.lastname@example.org and you can visit his website at FirstRays.com.