Magnesium is essential for nutrient formation in plants and is partially responsible for the dark green color of leafs. Deficiency of magnesium often results in a chlorosis of the lower and older leafs, while the veins remain green.
Chlorosis of the leafs, while veins remain green. With time leafs turn brown and die.
Epsom salt treatments can help with magnesium deficiencies, as does integrating hydrated lime.