Magnesium Deficiency starts showing at the bottom leaves of the plant just like with nitrogen deficiency. This is because the new more important growth up top begins pulling the Magnesium out of those lower leaves so they can use it. If you don't take care of the issue right away, the problem will begin to spread very rapidly and your plant will begin to lose lots of leaves! Early stage of this issue will cause the leaves to yellow and lighten in color, and as it develops the leaves will begin to wilt and get crispy.
How to Cure:
Just like with a Calcium Deficiency, this issue requires a dose of Cal-Mag in your watering mix. Once you have given this nutrient to your plants, the issue will stop spreading after just a couple of days, and the plants should return to normal growth around that time as well. Remember that leaves and foliage that have been damaged by a deficiency will not return to their normal color after the problem is corrected. So be sure to look at new growth when trying to decide if the issue has been cured or not.