Cal-mag deficiency is a term that refers to a combined deficiency of calcium and magnesium in cannabis plants. These two nutrients are often deficient together because they have similar functions and interactions in the plant. The symptoms include: Brown or bronze spots on the leaves, especially on new growth and near the edges Yellowing or curling of the leaves Stunted growth and reduced yields Weak stems and branches Poor bud development and quality Cal-mag deficiency can be caused by low pH, soft or filtered water, high temperatures, overwatering, or overfeeding with nitrogen or potassium.