Q: I have read that you should fertilize tomato plants with Epsom salt. When should you add it, and how much should you add?
A: We don’t generally recommend using Epsom salt in the garden. Epsom salt can provide magnesium and sulfur, but there’s no need to add these elements unless you’ve had a soil test showing that your soil is deficient in these minerals. It’s not likely that ordinary garden soil would be deficient. Unnecessary additives that are not taken up by plants — including Epsom salt — can contaminate ground water.
Adding Epsom salt to the soil tomatoes are growing in can actually promote blossom-end rot, a truly disappointing garden woe. The tomatoes start to bear fruit and then rot on the bottom. Blossom end rot is caused by a calcium deficiency in the plants. Too much magnesium in the soil — which you get from Epsom salt — can prevent the plant from taking up enough calcium.
So what fertilizer should you use? That depends on your soil. If you added some well-rotted manure before planting, you may not need any fertilizer at all. You likely don’t need any phosphorous, and too much nitrogen will make the plants get bushy but produce less fruit. The only way to know for certain how much of each nutrient to add is to have a soil test. It’s an easy process: You fill out a form (available here: soiltest.cfans.umn.edu ), send it in with $17 and a sample of your soil, and get back a recommendation from the University of Minnesota Soil Testing Lab.
A lot of claims are made about Epsom salt’s utility in the garden. You can find websites — and even seasoned gardeners — recommending it for everything from fertilizing to killing weeds. Typically, there is a more effective, less damaging product that would be a better choice.
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Written by U of M Extension Master Gardeners in St. Louis County. Send questions to features@duluthnews.com.
