How2Day: Epsom Salts for Plants

Epsom salts is a source of magnesium and sulfur. Lucas Tiedge/Polka Dot/Getty Images
It's right on the calendar: Tomato plants setting fruit -- buy Epsom salts. Sprinkling Epsom salts on the tomato plants at our community garden is an annual ritual the garden team never misses. After years of faithful application, however, we find that we no longer know why. We have become a classic cautionary tale: following old, well-meant gardening advice without examining the reasons behind it. Epsom salts constitute a ready source of magnesium and sulfur for plants that need them. Learn when and where applying Epsom salts makes sense.

A plant suffering from magnesium or sulfur deficiency has basic problems metabolizing the other nutrients it needs for healthy growth. Magnesium deficiency becomes clear in the yellowing of new leaves. Sulfur deficiency may be reflected in a plant's allover failure to thrive. Sources agree that magnesium deficiency can be suspected in light, sandy, acidic soils and note the occurrence of magnesium deficiency in old, weathered soils in the American Southeast and Northwest. Correcting this deficiency has to be credited with improved leaf-greening, leaf- and shoot-number ("bushiness") and increased production of fruit or flowers.

Correcting magnesium deficiency, especially through a simple application of Epsom salts diluted in water, has a definite appeal to gardeners. The process is easy, requiring little equipment (the diluted salts can be poured on plant roots or applied in a foliar sprayer). The relatively loose guidelines for treatment reinforce the sense of a home remedy triumphing over commercial chemicals. The only difficulty with this process is the distinct lack of research to support the use. The National Gardening Association held planting trials in an attempt to back up the use of Epsom salts; two test-gardeners out of six reported that adding salts to their plant-care produced larger peppers, in one case, and more roses in another. It is unclear, however, what the condition of the original test-soils was. Gardeners report the impossibility of making a firm causal link between Epsom-salts application and corresponding plant health.

GH Organics, while supporting the use of Epsom salts to address magnesium deficiency, makes it clear in detail why application does not always produce the desired results. In a chart describing symptoms to observe, GH notes that the symptoms of magnesium deficiency are nearly identical to those of iron deficiency; the remedy for one does not help the other. The chart accompanying the North Carolina Department of Agriculture's advice on addressing magnesium deficiency makes it clear that addressing the use of one soil nutrient means needing to understand the balance among nutrients.

Linda Chalker-Scott at Washington State University applies hard science to old wisdom and emerges with a compassionate view of the old ways. Reviewing a variety of studies and two mythologies (Epsom salts produce better growth and they cause no harm if applied unnecessarily), Chalker-Scott points out that a magnesium deficiency must exist in the first place. Somehow, gardeners who would agree that taking two multivitamins does not make you better-than-healthy, cling to the belief that plants can be stimulated to grow better if given excess nutrients they do not need. The danger in this belief, Chalker-Scott asserts, is that it can lead to harm, as excess nutrients run off into water sources.

As is so often the case in gardening questions, getting a Cooperative Extension soil test, would have saved worry, work, and uncertain results. Exactly how plants use soil nutrients to grow as they do still remains just enough of a mystery to draw us back to the experience over and over. We can make a difference in that growth only when we know what we're doing. A magnesium deficiency may exist in our community garden; a soil test tells us it does. Next year's calendar needs to read: Get soil test. Buy tomatoes.

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