Is this a magnesium deficiency?

Would really appreciate some help with this. its day 30 of my girl scout cookie auto grow. Older fan leaves have rapidly started to yellow over the last few days. I fed them 1/4 does of plant magic magne-cal with 1.5ml/L a+b advanced nutrients sensi bloom ph perfect. soil is plagron light mix. The PH was around 6.7. Maybe 1/4 does is not enough? symptoms include stunted growth, yellowing lower and middle fan leaves with dead spots and on the bottom fan leaves. the stems on lower fan leaves are actually black :/ there are also small rust spots on upper leaves of one girl. I will post some pics. measured the run off a while back and it was around the 6.5-7 ph range. Temp avg 26c and humidity around 40-50%

"A while back" don't cut it. Soil can go acidic using chemical salt fertilizer. I'd bet your current run off measures in the 5.0 range.

It's one of those things you gotta stay on top.

RO or tap water? Lights??

I recently had a similar looking problem: https://www.rollitup.org/t/possible-rootbound-what-do.946437/

My diagnosis from forum members was overwatering. I feel the same applies to yours given the curling of your leaves.

Not enough cal/mag and too much nutes.

SouthCross said:

"A while back" don't cut it. Soil can go acidic using chemical salt fertilizer. I'd bet your current run off measures in the 5.0 range.

It's one of those things you gotta stay on top.

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I measured it today. Using tap water. Didn't adjust it. I added .5ml/l plant magic magne-cal (half suggested does), figured it wouldn't do any harm. PH just below 8.5 After watering and measuring run off PH 7.5 give or take.

Just watered one girl with .5ml/L magne-cal with a adjusted ph of 6. Run off came out between 6- 6.5.

Also fed one ph 7 adjusted and the run off was between 7- 7.5. So I'm guessing my PH is too high and what it seems is the symptoms I'm seeing are likely boron and iron deficiency due to lockout. Is that reasonable? I'm ruling out overwatering as the pics were taken a day from being bone dry. Cheers for all the comments folks, appreciate the help

blakjak said:

Also fed one ph 7 adjusted and the run off was between 7- 7.5. So I'm guessing my PH is too high and what it seems is the symptoms I'm seeing are likely boron and iron deficiency due to lockout. Is that reasonable? I'm ruling out overwatering as the pics were taken a day from being bone dry. Cheers for all the comments folks, appreciate the help

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The biggy, calcium. Has a sweet spot of 6.5.

7.0 ph is ok. 7.5 is too high. It all comes together with a slight ph swing between 6.0 to 7.0. Within that window the nutrients can be used. Outside that range and things get locked out.

SouthCross said:

The biggy, calcium. Has a sweet spot of 6.5.

7.0 ph is ok. 7.5 is too high. It all comes together with a slight ph swing between 6.0 to 7.0. Within that window the nutrients can be used. Outside that range and things get locked out.

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Hmm.. so what you mean is 6.5 is optimal but within 6-7 will suffice for the uptake of calcium? I've been referring to a lockout chart for guidance but Calcium seems available at what my ph is (I'm guessing around 7.5 based on tests). The only nutrients that seem unavailable are boron, iron and manganese. Am I misinterpreting the charts?? Or missing something.

I think I will adjust tap water to ph6 and run that through the girls, hopefully with the run off around 6- 6.5 the actual soil ph should be in the right place.

After feeding with plain tap water adjusted to PH6 the girls are looking a Lot better
Cheers for the help peeps