Disclaimer: Probably not everything I say here will be true. I’m just a guy who reads and experiments, and does my best to continually improve.
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Part 1 - Individual nutrients and the grow cycle
Macronutrients: Nitrogen (N), Phosphorous ( P), Potassium (K)
Secondary nutrients: Calcium (Ca), Magnesium (Mg), Sulfur (S)
Micronutrients: Iron (Fe), Zinc (Zn), Boron (B), Manganese (Mn), Copper (Cu), Chlorine (Cl), Molybdenum (Mo), Nickel (Ni)
I won’t get too in depth into nutrient basics. There are already plenty of resources online that can do a much better job detailing the roles of various nutrients in plants. However, something that is less commonly discussed is when cannabis requires them. In some amounts, all nutrients are needed at all parts of the grow cycle.
When you feed synthetic nutrients, the plant consumes what is fed. Unlike organics, the plant does not have much (if any) ability to direct which nutrients it is consuming. So when we use synthetic nutrients, we are on the hook to figure it out.
Nitrogen is probably the easiest macronutrient to provide. For the most part, it can be provided at roughly constant amounts throughout the grow. We tend to think of using nitrogen more in veg, but in fact, it is only higher, in relation to potassium – which is consumed much more heavily in flowering.
Phosphorous is perhaps the most difficult nutrient to get right, and for reasons I’ll discuss later, needs special consideration when mixed. Phosphorous is consumed very heavily for a short period of time. The most important time to feed P is in the first half of flower. It is most critical somewhere around weeks 2-4; however, it can be beneficial to begin increasing P even in mid to late veg.
Here you can see the sharp rise in P accumulation in the month of July (outdoor growing). I’m not sure at what latitude this refers to, but in most Northern latitudes (30-50N), this is going to be late veg and early flower.
To make this situation even worse, phosphorous deficiency can be caused by overwatering. Often people will flower shortly after a transplant in containers, which compounds the problem - wetter media since roots have not filled out, and sharply increasing demands for the nutrient.
Phosphorous deficiency can be difficult to diagnose. Often people will incorrectly mistake phosphorous deficiency with nitrogen toxicity. You will usually see phosphorous deficiency in early to mid flower, and it will look like dark green top leaves (also with clawing), but the lower leaves will be yellowing.
Potassium demands are highest at the end of flower. Potassium deficiency is easily identified by necrosis at the leaf serrations. It is not typical to see potassium deficiency prior to mid flower. Potassium (1+) interacts with other cations, especially Calcium (2+) and Magnesium (2+). For this reason, people often discuss K, Ca and Mg as ratios, or in relation to each other.
The potassium/calcium/magnesium recipe is especially important in coco coir. Coco has a propensity to bind cations, and it will prefer calcium to potassium. Often “unbuffered” coco contains high levels of potassium. It can soak up calcium and release potassium, and throw this ratio off balance (usually resulting in calcium deficiency).
Calcium is practically a macronutrient in cannabis, and it has very few options for synthetic sources. Because calcium sulfate does not dissociate in water, it cannot be used hydroponically (unlike magnesium sulfate, which is also called epsom salt). There are some forms of chelates, but their density of calcium is too low to be used as a primary source. Most commercial formulas will provide 100% of calcium requirements from calcium nitrate.
, thanks for sharing … 
