Are there any other symptoms that you have not mentioned or may not have noticed so you didn’t mention them, things like reddish stems?
Your first pictures were the most informative of them all. In them you can see that the new growth, the upper leaves appear to be healthy with no signs of what is seen on the older lower leaves.
The most common causes of yellowing, if of course your pH levels are in range, which they seem to be are:
Nitrogen deficiency: Pale plants, red stems, smaller growth. Rapid yellowing of lower leaves progressing up the plant. Add any chemical fertilizer containing N. Treated plants recover in about a week.
Having used MG soil with a slow release fertilizer I would say that is highly unlikely to be your problem plus not mentioning red stems it doesn’t seem likely.
Potassium deficiency: Affected plants are usually tallest and appear to be most vigorous. Necrotic spots form on lower leaves. Red stems. Leaves appear pale or yellow. Add chemical fertilizer containing K.
Again without mention of red stems and with having the slow release ferts I don’t think that is the case but it is at least somewhat more likely since it does show up on lower leaves but then your leaves are not pale and yellow and more brownish so I still do not think it to be all that likely.
Molybdenum deficiency: Yellowing of middle leaves. Foliar feed with chemical fertilizer containing Mb.
Again not a perfect fit with your pictures because again you do not seem to have yellowing but more brownish leaf coloring.
When you have yellowing or color change between the veins of leaves it can be several things.
Magnesium: Lower leaves yellow and may even turn white while veins remain dark green. Blades die and curl upward.
That one is a tough call because in one picture you have leaves that are curling up and also leaves that are curling down so the pictures do not help pinpoint what is going on thanks to what appears to be conflicting ‘evidence.’
Manganese: Necrotic and yellow spots form on top leaves. Mn deficiency occurs when large amounts of Mg are present in the soil. Foliar feed with any chemical fertilizer containing Mn.
Not likely because that normally hits upper/top leaves first and your top/upper leaves seem fine.
You do not seem to have 'cooked' tips and outer edges of leaves so it does not appear to be good old fashioned nute burn, still it is possible that it is in the early stages but then without it showing up on the more tender new growth near or at the top of the plant is seems unlikely to me.
I wish there were some consistency in the pictures when it came to leaf curl. Like I said in one picture one leaf is curling up and another appears to be curling down. If you just had leaves curling up you would likely have a magnesium deficiency but I don’t think that is the case and since I just took another look at the picture with the upturned leaf after looking a little longer it seems like it might be cooked or burnt, though what I guess what was the light flash from taking the picture washed out any color for the tip so you really can’t see if it is cooked but it still seems it might be and if so that is not actual upward leaf curling so I would say anything that causes upward leaf curling is very unlikely now that I took a second closer look at your pictures.
Potassium deficiency: Will cause leaves to turn under. Affected plants are usually tallest and appear to be most vigorous. Necrotic spots form on lower leaves. Red stems. Leaves appear pale or yellow. Add chemical fertilizer containing K.
The necrotic spots somewhat fits with this one but you did not mention red stems and your leaves do not look pale or yellow so again it does not seem to fit. Only a bottom leaf or a few seem to be curling under and not anywhere else on the plant so it doesn't seem to fit either, or at least not very well at this point.
Copper deficiency: It will cause leaves to curl under but again it does not seem to be the culprit. If it was the fix is a fungicide, sulfate [Cocus] Apply foliar.
Then there is what I said I did not think to be the case but through the process of elimination might have to be considered to be at least somewhat likely and that is over fertilization: Your pics do not show the most typical reaction to it but then you do have some symptoms, or at least appear you might like the leaf tip that appears to be burnt and also how one pic seems to show some leaf under curling. I wouldn’t bet the farm on it from what little I could see in the pictures but you did say you used a slow release fert soil, MG, and they have a habit of being to ‘hot’ for many plants because they do not always release as slow as they should and can at times have higher amounts of ferts than they should.
In one picture a few leaves seemed to be very dark green and that would be a sign of getting the maximum amount of nitrogen and that would fit in this case but then it may have been the lighting in the picture too. You will have to look at them in person and see if some leaves are very dark green or if it was lighting in the picture that made them look that way. If it is over feeding it will cause leaf tips to appear yellow or burnt. To correct soil should be flushed with three gallons of water per one gallon of soil.
I wish I could be specific about one or even just two possibilities but what I see in your pictures is somewhat contradictory. It gives some good clues like what level of your plants are being affected but then other things seem to make the answer for what affects what is see on the lower portion of your plants to be incorrect.
I think what I might do is to give them another good flush and then wait and watch and see if anything spreads or if it stops and if anything else pops up and also look close to see if there are any symptoms that you missed or if they pop up soon and then maybe you will have enough clues to be totally sure of what it up because based on what I could see and what you said I definitely am not totally sure what is up.
