Mythbusters: Epsom Salt and Tomatoes

Jesse Fulbright, MSU Liberty County Extension

Memorial Day seems to be a traditional time to put out some of tender garden plants in our area, such as tomatoes.  There are always little traditions or ways of planting that are passed down in families or make their way into popular culture.  Some of these are steeped in research-based techniques, while others may be great in areas of the country but not necessarily here.  One method of planting tomatoes that I hadn’t heard until recently was about adding Epsom salts to the hole when planting your tomatoes.  Some people will say that adding Epsom salt prevents blossom end rot in tomatoes.  It’s time to play myth buster and debunk this method of gardening. 

According to a North Dakota State University Extension publication, adding Epsom salt leads to more blossom end rot.  Blossom end rot is caused by a deficiency of calcium, or something that might be more common in our area, infrequent watering.  Typically, we have enough calcium in our garden soils for tomatoes and peppers.  What can happen though is that we get busy during the summer and our watering habits fall by the wayside.  As plants are watered, and then not for periods of time and then watered again, calcium will be deposited only as far as the water gets and then it doesn’t get picked back up again. 

Photo courtesy of Michigan State University Extension

Now, back to the topic of Epsom salt.  Epsom salt contains magnesium sulfate, no calcium at all.  Adding Epsom salt to the soil may create more rot since magnesium and calcium compete for uptake into the plant.  The more magnesium in the soil, the less chance that calcium will be absorbed.

So, what can we do to prevent blossom end rot?  As I mentioned, typically we have enough calcium in the soil, so we don’t focus on the soil in this instance.  Instead, focus on watering.  The uptake of calcium depends on the uptake of water.  Irrigate regularly.  Avoid the extremes of waterlogged soil and droughty soil.  You might consider mulching around your plants to maintain consistent levels of moisture in the soil.

When weeding, consider cultivating shallowly.  Don’t damage the roots of your vines.  We need these roots to absorb calcium.  Also, avoid overfertilizing, especially with ammonium nitrogen fertilizers such as ammonium nitrate and most complete fertilizers such as 10–10–10.  Ammonium competes with calcium for uptake.  Calcium nitrate is a better choice.

Vines should be green but not lush.  Lush vines are more likely to suffer rot since actively growing leaves take calcium from the vine before the fruits get it.  As a general rule, don’t side dress a vine until after its first fruits set.

Calcium sprays might or might not help.  Mix 4 tablespoons of calcium nitrate per gallon of water.  Spray fruits, not leaves, two to three times a week.  The key time is when tomatoes are dime-sized or smaller.