pH - The most important! (kind of)
Now obviously the nutrients are what the plant actually uses to grow and be a plant but, in order for it to be able to absorb the N,P,K,Ca,Mg,B,Zn,Fe, etc., the medium must be at the appropriate pH for these nutrients to actually be available. In soil, the magic range is between approximately 6.0-7.0. (I personally go for about 6.3)
Whenever I begin to see almost any issues (besides pests or other visually obvious problems), the first thing I always do is check my pH. More often than not, it's a bit out out of whack and is much easier to remedy than jumping to a nutrient deficiency conclusion, which can cause the issue to become even more severe by adding more of the nutrient that is not actually deficient. Traditional fertilizers (e.g. Miracle Gro) tend to acidify (lower the pH) the medium over time. However, if there is an issue, a good flush with some distilled water is usually all that is needed.
Make sure you have a decent pH pen, or some other half-ass accurate way (sorry litmus paper) to check your water source! For example, I use my municipal tap water, and it's base pH is around 7.8-8.0 (we are very alkaline around here, lots of calcium carbonate in the water), so I need to add pH Down to it to bring it down, usually to around 6.3, before I water.
Some handy charts adapted from Jorge Cervantes' The Cannabis Encyclopedia.
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