For those interested in understanding how to calculate ppm mathematically, here’s an example that might be helpful. First of all, ppm is not a true unit, it’s kind of like percentage.
PPM is a ratio between two numbers that have the same units. So, if you’re using a powdered fertilizer, technically you would need to go by weight. That is, your calculations would only be perfectly accurate if the fertilizer in your teaspoon weighed the same as a teaspoon of water. However, this is not rocket science. Using a teaspoon of your powdered fertilizer, without worrying about it’s weight, will be accurate enough. Your plants won’t know the difference :-)
That said, here’s one way to calculate ppm mathematically. Let’s say your water soluble fertilizer analysis is 20-10-10 and you want to add the proper amount to one gallon of water to come up with a 250 ppm nitrogen solution.
1) There are 768 teaspoons in one gallon.
2) One millionth of that (1ppm) would be .000768 teaspoons.
3) 250 ppm would be 250 times that or .192 teaspoons.
4) Now, if your fertilizer was full strength nitrogen, that would be your answer. However, your fertilizer contains only 20 percent nitrogen. That means you need to add five times that amount.
5) .192 times 5 = .96 teaspoons.
So to get a 250 ppm solution, we need to add about one teaspoon of your fertilizer to one gallon of water.
