The Many Myths of Epsom Salts

Tom Ingram: Ask a Master Gardener

Monday, July 6, 2020

Q: I keep seeing posts online that recommend adding Epsom salts to the soil when planting tomato plants. Is this something I should be doing? JE

A: The internet seems to be abuzz with many near-miraculous claims attributed to Epsom salts. Before we talk about those, let’s talk about Epsom salts and a bit of soil chemistry.

Epsom salts is magnesium sulfate. Magnesium is one of the secondary nutrients found in healthy soil, and Epsom salts can be utilized as a soil additive if you have a magnesium deficiency. However, magnesium deficiencies are not common in home gardens.

Magnesium deficiencies can be found in soil that has been under intensive production over an extended period or in soil that has experienced a great deal of nutrient leaching due to excessive rainfall or irrigation. In this instance, adding magnesium would be an appropriate strategy to remedy a magnesium deficiency.

Potassium is one of the big three soil nutrients (nitrogen, phosphorus and potassium), and high levels of potassium in soil can interfere with magnesium uptake, but adding additional magnesium to the soil will not overcome the problem created by an abundance of potassium.

On the flip side, adding magnesium to soil that is not deficient in magnesium can interfere with potassium uptake, which may result in a potassium deficiency in plants even if the soil has adequate potassium supply.

Rather than continue to dig deeper into soil chemistry, let’s look at some of the claims we often see associated with adding Epsom salts to your gardens.

1. Epsom salt helps with seed germination.

Seeds have all the nutrients they need to germinate within the seed and can germinate in a wet/damp paper towel. Epsom salts does not aid in this process.

2. Put a scoop of Epson salts into each hole when planting tomatoes to prevent blossom end rot.

Blossom end rot is a common malady in tomatoes that is caused by a calcium deficiency. Adding Epsom salts when planting tomatoes not only does not help prevent blossom end rot but also can contribute to its occurrence because magnesium and calcium compete for uptake into the plant. An abundance of magnesium present in the soil will encourage less calcium uptake, thus increasing the potential for blossom end rot.

3. Use Epsom salts as a foliar spray to help tomato plants grow and enable a larger harvest of better-tasting fruit.

Again, this is unnecessary unless you have a magnesium deficiency.

4. Epsom salts are highly soluble so you can’t overuse it.

Epsom salts are in fact highly soluble; however, unnecessary nutrients applied in excess typically become a pollutant as they will be washed out, landing in unwanted areas.

5. Epsom salts can help plants grow bushier.

This would be true if you have a magnesium deficiency but not true as a general all-purpose additive.

So how can you tell if you have a magnesium deficiency? Magnesium helps with the production of chlorophyll so plants grown in magnesium-deficient soil will lose their deep green color and their ability to photosynthesize. Therefore, if you are seeing yellow leaves with stunted growth, you may have a magnesium deficiency. But remember, this is not common in home gardens.

If you have concerns about a magnesium deficiency in your garden soil, before you start adding Epsom salts, you should get your soil tested by the OSU Extension (be sure and say you want your secondary nutrients tested also) but know, we rarely find a magnesium deficiency in residential soil tests. Best strategy … save your Epsom salts for your bath.

Garden tips

• Vigorous, unwanted limbs should be removed or shortened on new trees. Watch for forks in the main trunk and remove the least desirable trunk as soon as it is noticed.

• Most varieties of mums are more productive if “pinched back” now. Either pinch off with fingers or cut to remove an inch or so of limb tips above a leaf. This results in the growth of new limbs and a fuller plant. Do not pinch after mid-July or it will interfere with fall blooming.

• Watch for tiny, sap-sucking insects called aphids on roses, perennial flowers, shrubs and vegetables (especially tomatoes). They produce a sticky substance called “honeydew”. Many can be dislodged with a hard spray from your garden hose or 2 applications of insecticidal soap will usually greatly reduce any aphid damage to your plants.

• Crapemyrtles are one of the few shrubs that should be planted in the middle of summer. Growth of new roots of these plants occurs best with summer soil temperatures.