Hello, I keep hearing conflicting answers on whether Epsom salts is good to...

Asked April 15, 2016, 11:26 PM EDT

Hello, I keep hearing conflicting answers on whether Epsom salts is good to use as fertilizer for indoor plants. I'm curious about outdoor plants as well. Can you help me? Thank you,

Hamilton County Ohio

1 Response

Epsom salts is one of those supposed miracle ingredients that will make your tomatoes and roses grow strong, increase cold hardiness in certain plants and generally keep things growing strong. It has been touted as a more "natural" amendment than synthetic fertilizers and so helps a grower garden sustainably. This garden myth has been around so long even some longtime, knowledgeable gardeners accept is as gospel. However, the real story is much more mundane.

Epsom salts are hydrated magnesium sulfate. Use of this material in the garden should be to correct a magnesium imbalance as revealed in a soil test. Because it is a micronutrient, meaning plants need only a tiny amount, it is the rare backyard garden that has a magnesium deficiency. Most magnesium deficiencies are caused by intensive farming and irrigation. Most soils have more than enough magnesium. Adding more when it is not necessary may do more harm than good. In addition, the highly soluble Epsom salts could leaches very quickly from the soil, not allowing it enough time to correct actual deficiencies and potentially increasing mineral contamination in nearby water sources. When a legitimate deficiency exists, magnesium found in slower dissolving sources such as Dolomitic lime might be more appropriate.

In short, there is no valid scientific basis for widespread use of Epsom salts in the home landscape. Its use is only recommended when a soil test reveals an actual deficiency in this mineral.

Here are some resources if you care to do more research.

Chalker-Scott, Linda, The Informed Gardener Blooms Again, University of Washington Press, 2010, pp. 111-116.
Chalker-Scott, Linda, "How Plants Work," Timber Press, 2015, p. 70