ONE of the important characteristics of soil when it comes to supplying nutrients is its "cation exchange capacity," or its ability to supply the nutrients that come in cation form (cation = ion with a positive electrical charge). In the realm of plant nutrition, the cations we usually talk about are potassium, magnesium, and calcium (K, Mg, Ca). Of the three, magnesium is required in the smallest amount in the vine/but that small amount serves some very important functions. Like most of the other essential nutrients, magnesium has a few different roles in the plant. One of the primary ones is as the central atom in the chlorophyll molecule, which explains why leaves in deficient vines have leaves that are chlorotic, or yellowing, between the veinsof older leaves. Magnesium is also important in the formation and functioning of ATP, the plant's cellular energy source, the synthesis of DNA and RNA, and many other enzymatic reactions.
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