Ph or MG? Please Help

Hi Everyone,

Just wondering if its normal for the older leaves to yellow and have crispy brown spots? All the new leaves are healthy and the plants are doing great other than that. I have used epsom salts in my watering a week ago. The plants are 6 weeks in veg, starting to flower today.

Nitrogen would be my guess.

I highly doubt that, My soil contains worm castings and I gave fertilizer for the first time a week ago. Its brown spots that grow and have a yellow lining. When the spots get bigger, they dry out the leaf and it cracks. It crumbles if I touch it.