JIMKSI64 said:

It does look like magnesium. There are ways to tell based on the development of the fade but the changes to the vein structure and leaves start on older leaves as it moves mag to new growth to help with photosynthesis.
Unscented Epsom salt is used as a readily available source in some grows but not sure if it is appropriate for Promix.
Led lights at high strength can cause super growth that causes the needs for cal and mag to go up. Might just double check your DLI as my first auto I pushed not taking into consideration the 18 hour schedule for the lights.

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carlosescobar said:

cal /mag deficiency looks more like this, (or rather this is how it manifested on my first grow ) i added calmag and these brown spots stopped straight away and the smaller ones went away..im not saying this isnt magnesium def, but I'm curious as to whats causing it..
''calcium magnesium and potassium compete for space on the surface of the roots, so if there is an overabundance of one element it can stop the uptake of other elements, even if they are available..if the roots are occupied by potassium ions and there isnt space for magnesium ions the older leaves will lose colour, but the veins will remain green.'' soft secrets 2023 issue 6
what light are you using ? Im going to put my money on light stress, it seems to be putting on huge fan leaves to absorb the light which is just a little too much for it . the smaller plant seems to be suffering as well.

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HerbalEdu said:

there is calcium and/or magnesium def., your photo look like calcium def. ; op photo look like magnesium def.

View attachment 2461951View attachment 2461952


a plant could eventually show both deficiencies at the same time but i can't provide any reference photos

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GnomishViking said:

Thanks. I did give them some calmag this water and I am going to do a mag foliar spray tonight before lights out.

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mancorn said:

Ca & Mg are different nutrients. If you’re lacking one, it doesn’t mean you need the other. Pretty sure you’re in peat soil (?) and good chance if you’re using municipal water you‘re getting both. (If using tap you can normally find what elements are in your water online.) That said, a lockout (where one element is prevented from being taken up even if it is being supplied) can develop if you have too much of another element. When you have too much Ca, a Mg deficiency can occur. So better to try and isolate what is more likely lacking, instead of adding both. I don’t think you mentioned what feed you’re using and how much.

Mg is a mobile element, so deficiency will appear on the older leaves or lower foliage. Ca is immobile and affects new growth. Hard (for me) to tell from your pics if this is top growth or (as I suspect) your plant has been topped and these are older leaves. It looks like your new growth is also yellowing, but doesn’t really look like a Ca problem, so perhaps lacking one the micronutrients (which are immobile). I’d guess if your using a standard bottled fert that you’re getting all your plant can eat - but have an excess, or soil is staying too moist, that’s giving you a lockout.

Post up your feed and watering schedule (and if you’re getting a decent dry back).

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carlosescobar said:

BINGO! a light that is too bright makes the plant uptake loads and loads of calcium , so thats locking your magnesium out at the roots. turn the light down or move it up and it should uptake cal, mag and potassium equally.

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