To get an accurate reading of any deficiency, the soil can be tested, but in the case of magnesium, the results are quite variable. Magnesium content can deteriorate quickly, especially during rain or watering. Magnesium is quite water-soluble and gets leached to the lower layers of the soil easily. It is brought back up by tree roots. It is therefore important to return the falling leaves to the topsoil.
Unless you live in an area where the dolomitic rocks is dissolve in the water, you can be pretty sure that magnesium would benefit your garden, especially if you are not adding tree leaves. Magnesium shortage is a real problem in most parts of the world. It is caused when we water or irrigate instead of growing what is natural for the climatic conditions. It causes a serious calcium metabolism problem in people and animals, because calcium cannot be used without magnesium.
Extreme magnesium deficiency is recognized by pale green leaves and by blossom and fruit rot, but don't wait for that. Sprinkle dolomite or epsom salt on the soil from time to time, or add a little epsom salt to the water. Using a little frequently is better than using a lot once, because the excess just gets leached.
Epsom salt recipe: Dissolve 2 tablespoons of Epsom salt in 1 gal. of water. For healthy nightshade plants (tomatoes, peppers, eggplants) water just as flowering starts. Or use this mixture as a foliage spray in the garden and on house plants.
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