Does Epsom salt stop tomato blossom end rot? It’s one of the most repeated tomato tips out there, so let’s get straight to the answer.
No. Epsom salt won’t prevent blossom end rot. University of Minnesota Extension goes a step further: adding too much magnesium to your soil can keep calcium from getting into your plants, which makes blossom end rot even worse.
Epsom salt has a long reputation in the garden, and Minnesota Extension notes it is touted as a common garden cure-all.
Gardeners sprinkle it around tomatoes or drop a spoonful into the planting hole, hoping for healthier plants. Here’s what it really does, what actually causes blossom end rot, and what helps.
What Is Blossom End Rot?
Blossom end rot shows up on the blossom end of the fruit, the bottom end away from the stem.
Clemson Extension describes water-soaked spots that enlarge and turn black, and University of Maryland Extension says the blemishes may grow until the entire bottom of the fruit becomes dark, shrunken and leathery.
It is not a disease or caused by an insect pest, according to UF/IFAS Extension Duval County. It is a physiological disorder.
Iowa State University Extension says it isn’t caused by a pathogen, though secondary fungi and bacteria may enter the dead tissue and cause it to rot. Because it isn’t an infection, it doesn’t pass from plant to plant, and fungicide or insecticide sprays won’t treat it.
Iowa State says it is most often found on tomatoes, and Maryland Extension lists peppers, eggplant, pumpkin, squash and watermelon too.
It typically appears when plants are growing rapidly and often affects the first fruits to develop. Maryland also says it rarely appears on cherry tomatoes and most often on large plum- or paste-type tomatoes.
For identification, prevention and treatment in more depth, see our main guide to blossom end rot.
What Causes Blossom End Rot?
Blossom end rot is a calcium problem, but usually not a soil-calcium problem. Minnesota Extension says most Minnesota soils already have plenty of calcium for garden plants, and this kind of calcium deficiency signals a water-transport issue in the plant.
UF/IFAS says Florida’s native soils often have plenty of calcium, and irrigation water often has more.
Here’s why. Calcium enters the plant only through the actively growing root tips and moves into the fruit along with water, Minnesota Extension explains. UF/IFAS puts it simply: if watering is erratic, calcium supply to the developing fruit is too.
UF/IFAS names three main causes:
- Inconsistent watering. Calcium travels up through the plant in water.
- Too much nitrogen. Excess nitrogen promotes leaf growth and redirects water, and the calcium it carries, from the fruit to the leaves. The ammonium form of nitrogen can also inhibit calcium uptake.
- Calcium the plant can’t get. Excess soil magnesium or potassium, too little soil moisture to move calcium, or damaged root tips can keep it out of reach.
Other extension sources add to the list. UF/IFAS Extension Baker County includes excess soil moisture, damage to the root system and high salinity.
Iowa State names root pruning during cultivation, and Clemson lists rainy or cloudy weather with high humidity, cool temperatures, and too much ammoniacal nitrogen, potassium or magnesium fertilizer.
Related reading: Tips On Deep Watering Your Tomatoes
Why Epsom Salt Can Make Blossom End Rot Worse
Epsom salt is magnesium sulfate. It has no calcium in it, so it can’t supply what the fruit is missing. Minnesota Extension is blunt: blossom end rot is caused by a calcium deficiency and not a magnesium or sulfur deficiency, so Epsom salts will not prevent it.
The bigger issue is competition at the roots. Clemson’s Home & Garden Information Center says that if soil magnesium levels are already sufficient, too much magnesium inhibits calcium uptake.
The more magnesium in the soil, the less likely calcium will be absorbed, and that can worsen blossom end rot on tomatoes, peppers and eggplant. UConn Extension explains that when ammonium, potassium and magnesium are over-applied, they compete with calcium for uptake at the roots, because they are all positively charged ions.
Salt is the other concern. Clemson notes that Epsom salt is a magnesium salt, and that too much can build up around the root zone and block the plant’s ability to take up water.
That’s why the old habit of dropping a spoonful of Epsom salt into every planting hole isn’t a safe default. Clemson’s tomato fact sheet says not to apply Epsom salt to garden soil unless a recent soil report indicates a magnesium deficiency.
It won’t make your plants more productive, either. Minnesota Extension says there’s no evidence that Epsom salt makes peppers and tomatoes more productive unless your soil is magnesium-deficient. More on that in our guide to what Epsom salt does and doesn’t do for tomatoes.
When Does Epsom Salt Make Sense?
Only when a soil test shows your soil is short on magnesium. Minnesota Extension says Epsom salts can be a good source of magnesium, but to use them only if a soil test indicates a magnesium deficiency.
In Minnesota home gardens, it says, those deficiencies are most likely on sandy, low-pH soils. Clemson adds that magnesium levels in most garden spaces, especially those amended with organic matter, are generally already sufficient.
Get that test from an accredited lab. Clemson says at-home soil test kits can give inaccurate or inconsistent results, that your county Extension office can help you get a test, and that the results will include fertilizing recommendations if adjustments are needed.
Two more cautions from Minnesota Extension: spraying Epsom salt solutions on plant leaves can cause leaf scorch, and excess magnesium can add to mineral contamination in water that percolates through the soil.
What about palms? Yellowing palms can often be a magnesium issue. UF/IFAS Gardening Solutions says magnesium deficiency in palms mainly results from improper fertilization and can show as bright yellowing along the margins of older leaves; if a palm needs supplemental nutrition, it recommends a palm fertilizer that includes magnesium. That’s a palm question, though, not a tomato one.
How To Prevent Blossom End Rot
Prevention is the whole game here, because UF/IFAS says once a fruit has blossom end rot, you can’t fix it.
These steps come straight from extension guidance:
- Test your soil at the start of the season. UF/IFAS and Minnesota Extension both recommend a soil test and fertilizing based on its results. Clemson recommends liming according to the soil report to bring the pH to 6.5 and provide adequate calcium.
- Water consistently. Minnesota Extension says to keep the soil evenly moist, with adequate water but not too much. Iowa State suggests watering weekly in dry weather and says tomatoes need about 1 to 1.5 inches of water a week; UConn Extension says 1 to 2 inches.
- Mulch. Iowa State says mulch around tomato plants conserves and maintains uniform soil moisture.
- Go easy on nitrogen. Iowa State says to avoid large amounts of nitrogen, Maryland says to avoid high-nitrogen fertilizers like urea, and Clemson advises against ammoniacal nitrogen for side-dressing.
- Protect the roots. Don’t cultivate too close to the plant base, Minnesota Extension says.
- Skip extra magnesium and potassium unless a soil test calls for them. Clemson says they compete with calcium for uptake.
- Pick off damaged fruit. Iowa State says removing affected fruit lets the plant channel its resources into the rest.
We like growing tomatoes in raised beds with raised-bed soil for better control over soil quality. UF/IFAS notes that adding calcium to the soil may be a solution especially in a raised bed or container, so a soil test is worth doing there too.
Related reading: What’s The Deal On Tomato Flower Dropping?
Do Calcium Sprays Help?
Not much, according to most of the sources. UF/IFAS Extension Duval County says calcium solutions on the leaves are generally ineffective because the plant can’t move calcium from the leaves to the fruit, and UF/IFAS Baker County says foliar calcium is not effective at reducing blossom end rot.
UConn says a foliar spray can be used when the fruit is young but isn’t the most effective method, and Maryland says a calcium chloride spray may offer temporary relief.
So put your effort into watering and feeding, not spraying. If a soil test shows calcium is low but the pH is already right, Clemson recommends working gypsum (calcium sulfate) into the soil before planting.
The Bottom Line
Epsom salt isn’t a blossom-end rot fix, and on soil that already has enough magnesium, it can make the problem worse. Test your soil, water evenly, mulch, and use less nitrogen. Save the Epsom salt for a magnesium deficiency that a soil test has confirmed.
Have you battled blossom end rot? Tell us what turned it around in your garden.
Frequently Asked Questions
Does Epsom salt stop blossom end rot?
No. University of Minnesota Extension says blossom end rot is caused by a calcium deficiency, not a magnesium or sulfur deficiency, so Epsom salts will not prevent it. Epsom salt is magnesium sulfate. Minnesota Extension adds that too much magnesium in the soil can keep calcium from entering your plants and make blossom end rot worse.
Can Epsom salt make blossom end rot worse?
Yes, when soil magnesium is already sufficient. Clemson’s Home & Garden Information Center says too much magnesium inhibits calcium uptake and can worsen blossom end rot on tomatoes, peppers and eggplant. Clemson’s tomato fact sheet says not to apply Epsom salt to garden soil unless a recent soil report shows a magnesium deficiency.
How much Epsom salt should I use for tomatoes?
None, unless a soil test shows a magnesium deficiency. The University of Minnesota Extension says to use Epsom salts only if a soil test indicates a deficiency, and Clemson says magnesium levels in most garden spaces are generally sufficient. Clemson also says lab test results include fertilizing recommendations if adjustments are needed. Minnesota warns that spraying Epsom salt on leaves can cause leaf scorch.
What causes blossom end rot on tomatoes?
Not enough calcium reaching the developing fruit. UF/IFAS Extension names three main causes: inconsistent watering, too much nitrogen, and soil calcium the plant can’t take up, for example because of excess soil magnesium or potassium, dry soil, or damaged root tips. Iowa State University Extension says it most often results from erratic watering.
Can you save a tomato with blossom end rot?
That fruit can’t be fixed, UF/IFAS Extension says, but you can protect the ones still developing. Iowa State University Extension says to pick and discard affected fruit so the plant can channel its resources into the rest. Then water consistently, mulch and get a soil test. UF/IFAS says calcium sprays on the leaves are generally ineffective.