Fertilize with Epsom Salts
By: Charlie Nardozzi
After working with home gardeners for more than 10 years, I know that they love to use home remedies on their plants. From setting out beer traps for slugs to hanging bars of soap to repel deer, if the household product seems to work, gardeners try it. That's why I was intrigued by the often-mentioned idea of using Epsom salts as a fertilizer.
Gardeners apply it to tomatoes, peppers, and roses, hoping to produce more flowers, greener plants, and higher yields. You can use it to improve magnesium content if you know you have a soil that's deficient in that element, but home gardeners are most likely to apply Epsom salts to peppers, tomatoes, and roses.
I wanted to find out if it really works and learn the best ways to apply it for best growth, so last summer I asked some of our test gardeners (home gardeners who tested seeds and products for National Gardening) to test Epsom salts' effects on plant growth and vigor by applying it to pepper plants and roses. Then I talked to researchers about using the salts as fertilizer. Here's what I found out.
The History and Science of Epsom Salts
This natural mineral, discovered in the well water of Epsom, England, has been used for hundreds of years, not only to fertilize plants but to treat a range of human and animal ailments. Who hasn't soaked sore feet in it at least once?
Chemically, Epsom salts is hydrated magnesium sulfate (about 10 percent magnesium and 13 percent sulfur). Magnesium is critical for seed germination and the production of chlorophyll, fruit, and nuts. Magnesium helps strengthen cell walls and improves plants' uptake of nitrogen, phosphorus, and sulfur.
Sulfur, a key element in plant growth, is critical to production of vitamins, amino acids (therefore protein), and enzymes. It's also the compound that gives vegetables such as broccoli and onions their flavors. Sulfur is seldom deficient in garden soils in North America because acid rain and commonly used animal manures contain sulfur, as do chemical fertilizers such as ammonium sulfate.
The causes and effects of magnesium deficiencies vary. Vegetables such as beans, peas, lettuce, and spinach can grow and produce good yields in soils with low magnesium levels, but plants such as tomatoes, peppers, and roses need high levels of magnesium for optimal growth. However, plants may not show the effects of magnesium deficiency until it's severe. Some common deficiency symptoms are yellowing of the leaves between the veins, leaf curling, stunted growth, and lack of sweetness in the fruit.
Magnesium tends to be lacking in old, weathered soils with low pH, notably in the Southeast and Pacific Northwest. Soils with a pH above 7 and soils high in calcium and potassium also generally have low magnesium levels. Calcium and potassium compete with magnesium for uptake by plant roots, and magnesium often loses. Sometimes, a soil test will show adequate magnesium levels in soil, but a plant grown in that soil may still be deficient because of that competition.
Gardeners add magnesium when they apply dolomitic lime to raise the soil's pH. However, this product (46 percent calcium carbonate, 38 percent magnesium carbonate) breaks down slowly, and the calcium can interfere with magnesium uptake. For soils with a pH above 7, many gardeners use Sul-Po-Mag (22 percent sulfur, 22 percent potassium, 11 percent magnesium) to increase magnesium. Although dolomitic lime and Sul-Po-Mag are inexpensive ways to add magnesium, Epsom salts' advantage over them is its high solubility.
When diluted with water, and especially when applied as a foliar spray, Epsom salts can be taken up quickly by plants. Epsom salts' magnesium content, high solubility, and ease of application as a foliar spray are the main reasons for the positive results many gardeners see in their plants.