Answer: First, a little background. Magnesium is an important component of chlorophyll--in fact, the chlorophyll molecule is identical to the hemoglobin molecule in blood, except that in the chlorophyll molecule contains magnesium where the hemoglobin molecule contains iron. (Fascinating, isn't it?)
The balance of magnesium, calcium and potassium is important, because these nutrients compete for takeup by plant roots. If soil and/or fertilizer is high in calcium and potassium, a magnesium deficiency can occur. There are no characteristic symptoms of mild Mg deficiency, except stunted growth. A more severe deficiency leads to a yellowing of older leaves, especially between leaf veins, while the veins stay green. (These symptoms can vary with the type of plant and other factors, so they aren't always exact.)
Whether the new growth or older leaves are affected first depends on whether the nutrient is mobile within the plant. If a nutrient is mobile, the plant will draw it out of the old growth first, to nourish new, young growth. If the nutrient is not mobile, then the plant can't draw on the reserves in the older growth, and the new shoots and leaves show symptoms first.
Once leaves have begun to yellow--that is, the nutrients are being drawn out of them, they won't turn back to green. If the overall color of the plant is light, it may "green up" with the addition of nutrients. Hopefully, the latter is true for your plants.
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