Hi mate, I'm sorry I'm late but I haven't checked in to RIU for a while.
So the first photos were general underfeeding, as you guessed. The rust spots are calcium whilst the rest of the growth is showing a slight magnesium deficiency (yellowing, slight interveinal chlorosis), zinc (new growth leaf twisting), and possibly boron (slightly red-brown hue from the new growth).
The very tops of the plants have a striped yellowing effect which is light stress. You can tell, because the lower leave don't display this. So you were experiencing a combination of not enough nutrient and too much light – which is common under LED for new users. LEDs tend to be stronger than growers give it credit for and it also contains more blue light, which can more easily lead to leaf temp rises and bleaching. It's more common to see this under so-called "veg" lights than flowering lights that have less blue in them.
I used CX nutrients for years and I can tell you that 1ml per litre is not nearly enough. But you are also using the new CX line, and I used the old line (see below).
CX nutrients are designed to mix with typical tap water which usually has some calcium-carbonate and magnesium-carbonate in it (usually from limestone groundwater sources). Likewise tap water usually has some zinc, copper and iron in it from the metal pipes and plumbing. If your water is soft (low EC) then it will be missing a small but significant source of Cal-Mag and you may need to add more – even though CX coco nutes have quite a bit of calicium and magnesium in them already. The problem may be worse in the beginning with fresh coco that's not charged, or it could get worse during the grow as the coco breaks down and its surface area increases, which causes more cation exchange and greater demand for calcium and magnesium.
Nitrogen also helps with magnesium uptake, so low N levels – especially if you are underfeeding – will exacerbate the problem. Higher pH will help cal-mag uptake but will affect heavy metal uptake (iron, zinc, boron, copper etc). pH5.6-5.7 in veg is a good number to aim for, and around 5.8-5.9 in flower.
I used the old CX formula which had dedicated NPK ratios for veg and bloom – the new formula uses a "base" two-part nutrient that must be supplemented during veg and flower. So the base nutrient will not have enough nitrogen for veg and it won't have enough PK for flower – that's why you have to add the Growth enhancer (NPK 11-0-0) during veg and the head master (1-2-2) during flower and mighty bloom PK (0-15-16) during mid-late flower. Those nutrients are designed to be used in conjunction, not separately.
Finally, you need to be careful reading those CX nutrient charts because they can be quite misleading. For example, have a look at the NPK ratios of the Coco A and B below. The ratio is 4-4-5. But have a look at the nutrient chart – the NPK ratio is 6-1-4. So which one is correct? And does the nutrient chart incude all the additives and if so which ones?
Just be careful about which nutrients you think you are giving your plants and what ratios they are, as you may not be giving yourt plants what you think you are. Excessive K, for examnple, will lock out both calcium and magnesium, so if you were just feeding the base nutrient without the Growth Enhancer, that could be your problem there.
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