Epsom salts are often recommended as ways to improve your garden. A quick Google search turned up claims that they will improve seed germination, increase the size and number of flowers, reduce fruit drop, increase nutrient absorption, counter transplant shock, green up your lawn, prevent leaf curling, deter slugs, kill weeds, grow sweeter fruit, produce sweeter tomatoes with fewer problems such as blossom rot, increase pepper yields, and result in more and bigger roses on healthier plants. Wow. With benefits like these, we should all be putting Epsom salts in our gardens!

As with anything that’s hyped to this extent, my skeptical nature rears its head and I feel compelled to investigate. What is this miracle substance? Why are there so many claims about its benefits? Does it really work? Will it improve my garden?

Epsom salts are composed of magnesium and sulfur, two elements which are indispensable for plant growth. Obviously, if our soils are lacking one or both of these elements, our plants will suffer. So, are our soils lacking?

The only way to know for sure what nutrients are in our soil is with a soil test. However, sulfur is rarely lacking in US soils. Areas with acid rain have high sulfur levels, and animal manures add more. Ammonium sulfate, a common fertilizer, is another source of sulfur. However, as sulfur eventually leaves the soil through plant uptake, leaching, and volatilization, a deficiency could occur.

Sulfur is an essential micronutrient. According to Cornell University, “Sulfur deficient plants will grow slower and have a delayed maturity. The plants tend to develop thin stems and petioles, and become spindly.” While that’s a bit vague, if your plants are not doing well, a soil test may be in order. If you need to add sulfur, using Epsom salts is one way to replenish soil levels. Using finely ground elemental sulfur is another.

(Note that while sulfur applications are often recommended for lowering your soil’s pH, CSU’s fact sheet on soil pH points out that this “is not effective in many Colorado soils due to high levels of ‘free lime’ (calcium carbonate)….” To test for free lime, moisten a soil sample with vinegar. If it bubbles, you have free lime.)

How about magnesium? Again, magnesium is an essential micronutrient. However, it’s fairly common, making up 2% of the earth’s crust. As magnesium-containing minerals break down into soil, that magnesium is freed up to be used by plants. It’s particularly abundant in alkaline to slightly acidic clay soils, which is what we often have here in Colorado. However, it too can be depleted through leaching, especially in low pH soils with a lot of sand. Too much potassium (caused by over-fertilization or too much compost) can also be a problem, displacing magnesium inside plant stems and roots.

The primary symptom of magnesium deficiency is chlorosis. Older leaves turn yellow while the veins stay green (similar to chlorosis caused by a lack of available iron). If the problem persists, eventually all the leaves turn all yellow. Additional symptoms include small, woody fruit, premature aging, leaf drop, and plant death.

What do you do if you think your plants need more magnesium? Because magnesium deficiency is very similar to problems caused by zinc or chloride deficiencies, viruses, or natural aging, it’s best to measure the magnesium content in your soil is with a soil test. If the test shows that you need to apply more magnesium, both Epsom salts and lime are two good sources. However, if your soil is already highly alkaline, adding lime is a bad idea. In fact, most Colorado soils are composed largely of decomposed limestone! In this instance, Epsom salts can be helpful.

So should you add some Epsom salts “just in case”? While it probably won’t hurt, if you garden in Colorado it likely won’t help, either. It’s highly likely that our soil already has plenty of sulfur and magnesium, and adding more won’t make our plants grow better. If you don’t mind spending the money, and you do decide to apply Epsom salts, research shows that foliar sprays are more effective than adding them to the soil.

The final word? Once again, get a soil test. It’s much better to know what is in your soil than to always be guessing. Besides, too much sulfur can cause smaller, yellowed or scorched-looking leaves and stunted growth. (Too much magnesium is rare, as calcium buffers the effects of high magnesium levels.) And if you garden in Colorado, save the Epson salts for soothing aching muscles in a hot bath. After all that digging, it will feel amazing.