Synthetics not from scratch
I’m going to make a more lengthy answer to your question, because it’s more complicated than just NPK ratios. I’m glad you asked it and are interested in experimenting as I am. I am learning a lot of things about nutrient solutions this year as you can see from this pretty lengthy thread, and I think I am at a point where I can explain some things that should give you guidance.
I’m calling this ‘not from scratch’ because if you’re using a premixed formula like 5-12-26, then you have to temper your expectations about what you can or should do with it.
The main thing is that you cannot use it in arbitrary amounts. Jacks specifies like 3.7g per gallon, and you can probably get away with using a little more or a little less, but I wouldn’t stray too far from this number. The reason for this is something people don’t often talk about – micronutrients.
The premix has all the micronutrients included, and if you use different amounts than recommended, you will not have the correct amount of them in your feeds. Micronutrients are essential to plant growth, and also immobile! That means your plant has a limited ability to take them from a different part of the plant. So they must be supplied adequately for pretty much the whole grow. If you use less than recommended amounts of 5-12-26, then you will get less than recommended amounts of micronutrients, and then you are unlikely to have an optimal fertilizer for growth.
So the bottom line is that you should consider this to be a pretty fixed amount per gallon, except for maybe the first and last few weeks of the entire plant cycle, when nutrient demands are fairly low.
What this means is that you have 3.7g of a fairly large number ‘26’ for potassium. You will never get to a 3:1:1 ratio in veg, and probably not a 1:3:2 ratio in early flower. You don’t really need to, and really, those ratios are somewhat meaningless on their own. There are 3 main cations (K, Ca and Mg) and 3 main anions (NO3, PO4 and SO4), and NKP only addresses half of them. There’s actually 4 main cations if you include NH4, which shouldn’t be overlooked either. Also, it doesn’t specify EC/ppm of the solution which is also important.
Given that we already know that we need to use around 3.7g of 5-12-26, we should be asking what else can we do to adjust the nutrient solution at different growth stages.
- We can alter the other salts to change anion ratios – so we can trade calcium nitrate for something else, or epsom for something else.
- We can raise the EC by adding more ‘stuff’
And those are probably the only real viable options, without having a whole cabinet of micronutrients and other things to fix it.
I think option 1 is usually pretty safe, and I would start there. The main things I think you can do is change the anion ratios – ie trading sulfur into phosphorus or nitrogen, or trading nitrogen into sulfur. In general, I feel the formula can use more a little more nitrogen and phosphorus in veg and until the completion of stretch. You can sub magnesium nitrate and magnesium phosphate to accomplish this. In late flower, you may want less nitrogen so you can sub calcium nitrate for calcium sulfate (solution grade) or even calcium chloride.
Magnesium nitrate has a similar % of magnesium to epsom, so you can use it in equal parts as you would epsom. Calcium sulfate has slightly more calcium than calcium nitrate (23% vs 19%), so you could probably use it in equal parts as well or 19 divided by 23 times the amount you used for calcium nitrate if you wanted to be more exact. Calcium chloride is 34% calcium, so if you wanted to remove 0.5g of calcium nitrate you could substitute 19/34 * 0.5 = 0.28g of calcium chloride, etc. Making substitutions is easy without having to use a nutrient calculator.
If you decide to try option 2, it should be done more carefully. Changing the ratios of the cations (K, Ca and Mg) can be trickier and more difficult to understand, since all 3 are important. With the anions, you have sulfur, which plants are tolerant to over a very wide range; however the cations are all very essential in narrower ranges and will inhibit each other. So be careful to make these adjustments modestly. I would probably not recommend adding more than about 0.5g of anything extra without checking hydrobuddy or really fleshing out what all is going on with your other cations.