Contributed By: Diels Alder
To figure out the ppm of your fertilizer (or fertilizer mix), you need to be able to measure grams and liters. Look at the 3 numbers on the side of a fert bag. These are the percent content of the nutrients. For every one gram of said fertilizer in one liter of water, it contributes 10 ppm of the given nutrient per percentage point. A 20-20-20 gives 200 ppm (10 ppm X 20) of each nutrient for each gram in a liter of water.
The formula is this:
grams of fert per liter = A/B
A=your desired ppm
B=10 ppm X the % of nutrient in mix
or
your ppm = C X B
B=10 ppm X the % of nutrient in mix
C= grams of fert per liter
So to make a 200 ppm-100 ppm-200 ppm NPK mix using a 13-0-44 (potassium nitrate), a 12-62-0 (monoamonium phosphate), and a 33-0-0 (ammonium nitrate) you would work backwards from your sole P and K sources (it makes it easiest in this case), and make up the N at the end. I have rounded numbers to the nearest 0.1 g for the following. You would use 0.5 g of potassium nitrate (200 ppm/(10 ppm X 44 K)) and 0.2 g of monoammonium phosphate (100 ppm/(10 ppm X 62 P)) in one liter. This would give you 89 ppm N (10 ppm X 13 N X 0.5 g + 10 ppm X 12 N X 0.2 g), 124 ppm P (10 ppm X 62 P X 0.2), and 220 ppm K (10 ppm X 44 K X 0.5 g). 111 ppm are needed to raise the N to the 200 ppm level, so we can use 0.3 g of the ammonium nitrate (111 ppm/(10 ppm X 33 N)) to bring us up to finish.
The actual mix would yield a 188 ppm N, 124 ppm P, 220 ppm K mixture in one liter of water. To get more precision, you need to mix larger batches or get a better scale (you would need to make a 10 liter batch of the above with a scale that is only accurate to the gram).
If you mix your own fertilizer, you can adjust your N source to meet your pH needs, rather than being dependent on adding acid or base, which is nice.
This works for formulating hydro mixes, as well as for us dirt farmers