Epsom salt is helpful for growing strong tomato plants and getting good yields. It is considered a micronutrient, though, so you don’t want to overdo it.
When using Epsom salt, you don’t need as much as you might think. The idea is to provide a small but mighty boost with a modest amount of Epsom salt. There are different ways it can be applied, and different times to apply it.
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What Does Epsom Salt Do for Tomato Plants?
Epsom salt is a good source of certain minerals. It has the advantage of being cheap and widely available. Namely, Epsom salt is a source of magnesium and sulfur (Epsom salt is magnesium sulfate).
Most gardeners look to Epsom salt as a way to boost magnesium. Magnesium is an important micronutrient that supports cellular and tissue growth. It is crucial for good immune health (read: strong plants with good defenses and disease resistance).
Magnesium is also important in helping tomatoes access other nutrients, or working as a partner to those minerals and nutrients.
Read more about the things Epsom salt can do for your tomato garden here.
When and How to Apply Epsom Salt to Tomato Plants
There are three main times when you might apply Epsom salt to tomato plants:
- At planting
- During blossoming
- As needed throughout the growing season
A small word of warning:
It is possible for soil to have too much magnesium. Too much of what is usually a good thing can create other problems. So, it is important to know when it’s wise to apply it, and at what rate you should apply Epsom salt to your tomato garden (read: how much Epsom salt!).
At Planting or Seeding Time
Many people swear by applying Epsom salt when they plant their tomatoes. Some also mix a little bit into the soil when they are starting or seeding their tomatoes.
If you are growing your tomato transplants in a good potting soil, it should be balanced and already have good magnesium and sulfur.
If you are reusing old soil (which generally is not recommended, but is sometimes done), then it could be smart to add back in some magnesium to replace what is depleted.
The same is true of tomatoes that are being grown in containers -- especially if you are reusing some or all old soil to grow them in.
The most common way and the most common time to apply Epsom salt for a planting boost is to place the salt in the planting hole when you move your tomato transplants out into the garden.
You do not need a lot of Epsom salt to boost your tomatoes at planting time. Mostly, the amount you apply is a bit of a boost and bit of insurance to provide strong cellular support, but without overwhelming the soil and causing an imbalance (that might lead to other problems).
The right rate of application of Epsom salt at planting time for tomatoes is
- 1 cup per 100 square feet of garden or
- 1 tablespoon per planting hole
Clearly, the amount is small. Most people find it easiest to measure by simply dumping a tablespoon of Epsom salt into the bottom of the planting hole. Broadcasting one cup over a 10x10 garden area (100 square feet) is also easy, and you won’t have to worry about one more thing to do at each planting hole.
At Blossom Set
Another time when you might want to apply Epsom salt is when you see blossoms starting to set. This is intended to help the plants hold their blossoms better, preventing blossom drop, and therefore increasing the odds of successful pollination and of fruiting. This, in turn, increases tomato yields.
To apply Epsom salt to boost blossoms and performance:
- Apply 1 tablespoon per plant
- Apply either as a dry top or side dressing or as a water drench
To apply Epsom salt as a dry powder or top or side dressing:
- Apply the Epsom salt by sprinkling it in a circular area around the base of the plant
- Target the soil where the roots are under the plant (an area that extends about two feet all around the tomato plant)
- Spread this light layer evenly over the entire surface
- Avoid making large piles or clumps of salts
- You can use a hand rake to scratch the salt into the soil, but don’t dig too far down; you don’t want to harm your tomato’s roots
- Water will dissolve and deliver the Epsom salt down into the soil and down into the roots
To apply via watering drench:
A drench is the fastest way to deliver Epsom salt to a tomato plant’s roots. In a drench, the salt is pre-dissolved and will soak down into the soil to the roots much faster.
A drench is easy to make and easy to apply:
- Mix one tablespoon of Epsom salt per gallon of water
- Mix one gallon of water per tomato plant
- Stir to dissolve the Epsom salt in the water
- Then water the plant with the drench (one gallon of drench per plant)
During the Growing Season
One of the best ways to use Epsom salt for tomatoes is as an in-season boost for poorly producing, yellowing plants.
Yellowing tomato plants or yellowing leaves are not always because they need more magnesium, but it is a common cause of poor, yellowing plants.
When tomato plants don’t have enough magnesium, they cannot photosynthesize well, and they do not produce enough chlorophyll. The plants won’t have enough energy and sugars, which are produced through photosynthesis.
A lack of chlorophyll shows up as yellow leaves. When it is caused by a magnesium deficiency, the leaves will usually have green veins or ribs, but the rest of the leaf will look yellow, mottled, and sickly -- almost like a variegated plant’s leaves.
This is especially common in gardens with poor soils or sandy soils. Very light soils and chalky soils can have this problem, too.
Low magnesium can be a problem in tomato gardens that typically have good soil but have experienced overwatering or high rainfall. Too much rain or water can leach -- essentially wash -- magnesium out of even good garden soil.
For container plants, magnesium can also get rinsed out of the soil with regular and repeated waterings; there is no source for new magnesium unless you give it to container plants, and there is no backstock to tap into for container tomatoes, like tomatoes on the ground have.
When your mature plants show signs of magnesium deficiency and yellowing, the best way to apply Epsom salt is as a foliar spray or drench.
To apply Epsom salt as a foliar spray:
- Mix one tablespoon of Epsom salt into one gallon of water
- Apply all over the tomato plant, focusing on the leaves
- Spray until each leaf is dripping and wet
- A large garden sprayer works best for this type of application (just make sure it was not previously used for herbicides or anything potentially harmful to the tomato plant)
To apply as a top or side dressing:
You can also apply dry Epsom salt as a fertilizer as a top dressing or side dressing.
- Apply at a rate of 1 tablespoon per foot of plant height, per plant
- Sprinkle over the surface of the soil in a single layer, without large piles or clumps, for two feet around the tomato plant (the circumference of the root area)
- You can work the Epsom salt lightly into the top of the soil with a small hand rake
- Only scratch the Epsom salt into the soil’s surface; do not disturb the plant’s roots
Look for Signs of Magnesium Deficiency, and Don’t Overuse Epsom Salt on Tomatoes
It’s best to use Epsom salts with care so that you don’t cause a buildup or imbalance of magnesium in the soil. This could interfere with the presence and uptake of other minerals and nutrients in the soil.
The key is not to overuse Epsom salt. Get to know the signs of magnesium deficiency in tomatoes, so you'll know when an application might be warranted.
Consider sending out a sample of your tomato garden soil to be tested. This will tell you if your soil is magnesium deficient. Most home test kits will not measure magnesium and other minerals, but you can buy mail-in kits where you just need to bag up some soil and send it off for a full soil report. These cover more than just the typical nitrogen, potassium, phosphorus, and pH of at-home test kits, but they are still reasonably priced.
While magnesium is essential for plant performance, photosynthesis, and nutrient uptake and utilization, too much of this good thing can also impede absorption of some essential nutrients and minerals (like calcium).
Balance is important in the garden. So, apply according to the dilution and distribution amounts listed here. That should make magnesium and sulfur available, without going too far overboard to upset the soil balance. If you follow these guidelines for timing and application, Epsom salt should be a boost for your tomato garden, not a bust.
