If you’re trying to amend the 321 schedule, I haven’t found much use for potassium nitrate. However, I think there is value in potassium sulfate. Usually I consider adding MKP at the end, when the plant does not need more nitrogen.
Usually what I’m trying to do is just tweak 321 using various other amendments. From experimentation, the “safe” rule of thumb is not to exceed half a gram of “other” amendments. You can also do a full swap in some cases. Some example formulas I have been testing are:
(Veg adjusted)
3g Jacks (Southern Ag 5-11-26)
2g Calcium Nitrate
1g Magnesium Nitrate
(option 0.25 Fertilome 9-58-8)
(Bloom adjusted, first 1-2 weeks)
3g Jacks (Southern Ag 5-11-26)
2g Calcium Nitrate
1g Epsom
0.25-0.5g Fertilome 9-58-8
(Finish adjusted, last 1-2 weeks)
3g Jacks (Southern Ag 5-11-26)
2g Calcium Nitrate
1g Epsom
0.25-0.5g MKP (or possibly split 0.5g 50/50 with potassium sulfate)
A more exotic bloom formulation I’m considering this time is:
3g Jacks (Southern Ag 5-11-26)
2g Fertilome 9-58-8
1g Epsom
0.5g Calcium Nitrate
But it definitely could only be useful for a week or so, due to low Ca. This recipe comes very close to Jacks 10-30-20, but still has some Calcium.
Edit: I should note that these are just the ratios I use, and add water until I hit the specific EC target I’m looking for.