Thanks Dobe! I have to say I didn't follow the calculations you made there, but it looks like what you did was back into the math using PPM. Is that correct?
I definitely left some stuff out. Let me try to restate a little more clearly. Hopefully this won’t just add more confusion. But it’s quite long in an attempt to clarify. Also working values are illustrative although I’m pretty sure I have the KB powder’s NPK correct.
All I’m doing is using the PPM of the solution to roughly figure out their impact on my final NPK value.
Let’s say I have a liquid solution and a powder additive. My powder additive added to water in isolation measures out to 2-45-28, 200 PPM. My liquid solution is 1-1-1, 600 PPM.
I know the powder’s NPK because it’s written in the bag. The PPM is too, but I can test that - take plain water, measure PPM before I add it, then measure it after and see the difference. I know the liquid’s NPK because I measured it out (Ase’s thread has a good description of calculating based on liquids in the solution). I know its PPM from measuring it in the same way I could with my powder.
What I did, because I couldn’t find a better way, was to then combine and recalculate based on their proportion of the total.
I figured my total PPM was 800, so my 1-1-1 solution at 600 PPM was 3/4 of the total, and my 2-45-28 at 200 PPM was 1/4 of the total. 3 parts of 1-1-1 ratio gives me raw values of 3-3-3. Add that to my 1 part of 2-45-28, and I get 5-48-31. That’s the equivalent of 1-9.6-6.2.
You could also do the math this way:
0.75 x 1-1-1 = 0.75-0.75-0.75
0.25 x 2-45-28 = 0.5-11.25-7
Added up that’s 1.25-12-7.75. Divide all by 1.25 (to get the formula down to start with a whole number) and you end up back at 1-9.6-6.2.
Let’s say I didn’t want to feed 1-9.6-6.2. The way I’d do that would be to change the solution I’m mixing with. Let’s say I want to hit 1-6-5 instead with 800 PPM.
First my powder, which is 1/4 if my total:
0.25 x 2-45-28 = 0.5-11.25-7
But I don’t know my solution I’m mixing into. I need to add values that get it to 600 PPM, but that will reduce the final mix to 1-6-5.
I can’t just add a solution that gets me to 1-6-5 because the P and K values already exceed 6 and 5. So I need to add enough of each so that when I divide by the N value, the P and K values hit 6 and 5. 2-12-10, for example, is the same as 1-6-5.
I could add 1.5-0.75-3 and I’d hit my target - 2-12-10. 1.5-0.75-3 reduces to 2-1-4. So, I know if I have 600 PPM of a 2-1-4 solution, I’ll hit my mark.
0.75 x 2-1-4 = 1.5-0.75-3
0.25 x 2-45-28 = 0.5-11.25-7
Total is 2-12-10. Which is 1-6-5.
Let’s say instead that I wanted to hit 1-3-3, 600 with the KoolBloom powder. I know my KB powder is at that 2-45-28 with 200 PPM. That means the KB solution is 1/3 of my total (200/600). I would want my solution into which I mix the KB powder to thus be 400 PPM.
Going back to my math:
0.33 x 2-45-28 = 0.67-15-9.33.
Again I can’t just add to hit 1-3-3 since I’ve already exceeded it. I need to hit a value that divides down to 1-3-3.
5-15-15 would do it for example. If I add 4.33-0-5.67 I hit my 10-30-30.
Therefore:
0.67 x N-P-K = 4.33-0-5.67
Dividing all values by 0.67 leaves me with 6.5-0-13.9 as my NPK.
So, if I add 400 PPM of 6.5-0-13.9 and 200 PPM of 2-45-28, my resulting solution should be 1-3-3.
0.67 x 6.5-0-13.9 = 4.33-0-5.67
0.33 x 2-45-28 = 0.67-15-9.33
For a total of 5-15-15, or 1-3-3.
What it all comes back to: I know my KB powder is 2-45-28 and 200 PPM at the amount I want to feed (the GH recommended dose). If that solution is 25% of my total PPM, or 50%, or whatever, I can do some math to figure out the NPK of the solution I add the KB powder to, to get as close as possible on my target NPK while being right in with my target PPM.