Bag Epsom salt: Adding magnesium can harm fruits, flowers

May 14, 2023

Q. I have been watching YouTube garden videos and they suggest adding Epsom salt around all of my roses for better blooms and around my tomato plants to prevent blossom end rot. Is this a good practice or can adding Epsom salts be harmful to the soil?

A. Epsom salt contains magnesium sulfate (MgSO4) and have been touted as a cure-all for many garden problems, and as an additive to increase garden productivity.

But there's no evidence to indicate that Epsom salt increases garden productivity unless your soil is deficient in magnesium. It's not a good idea to add any supplements to your soil, except the macronutrients nitrogen and sometimes phosphorus, without first having your soil tested for deficiencies. If your soil is found to be deficient then Epsom salt can be a good source of magnesium — but only use if a soils test indicates a deficiency.

In fact, adding Epsom salts to soils with already-adequate magnesium levels can cause excess minerals to percolate into ground water and cause contamination. This can happen with the addition of any excess fertilizer or nutrients that cannot be utilized by the plant immediately.

As for the claim that Epsom salts can help with blossom end rot, this is not true. Blossom end rot is not caused by a lack of magnesium; it's due to a lack of calcium.

Specifically, it's a lack of calcium present in the cells of the developing fruit, usually caused by inconsistent watering throughout the day. Calcium needs water to move within the plant cells and without calcium in the developing cells, the blossom end of the fruit becomes damaged and turns brown and leathery.

Excess magnesium can actually prevent plants from taking up calcium from the soil, causing even worse blossom end rot in tomatoes.

If you suspect your soil is deficient in magnesium then look for symptoms on older leaves of your roses or other plants. Deficient plants will show yellowing leaf edges and eventually nearly white leaf veins, with the rest of the leaf remaining green — a "Christmas tree pattern," as the University of California Integrated Pest Management website calls it.

Only when a large number of leaves are affected should you spray with a dilution of Epsom salts and water, and then in a ratio of a pinch of salts to a gallon of water. Too concentrated a spray can cause leaf scorch.

Soils that are deficient in magnesium are usually sandy and have a pH lower than 6.2. In most soils magnesium deficiencies can be cured by raising the soil pH to around neutral (7.0). This can be done by incorporating dolomite lime into the soil. Once the pH is closer to natural, magnesium is made available to the plant.

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The best practice is to avoid adding extra chemicals to your soil, only enriching your soil with organic matter — finished compost, well-rotted manure, etc. — several times a year. This can help keep soil pH near neutral and will feed soil biota that help plants with nutrient uptake.

The Shasta Master Gardeners Program can be reached by phone at 530-242-2219 or email [email protected]. The gardener office is staffed by volunteers trained by the University of California to answer gardeners' questions using information based on scientific research.