The best information is in the video – I lay it all out for you. Having said that, the text below should make it clear it’s either a watering issue or a temperature issue. Soil deficiencies are not likely so adding things is a waste of time (see bottom of page for myths)
There are typically two main causes of black spots on the bottom of your tomato. We call it blossom end rot btw.
If they’re on the bottom cluster only.
If you mainly see the problem on the first fruit truss (the bottom cluster of tomatoes) then it’s likely a too-cold temperature problem after planting. If the temperatures are too cold, the plant physiology doesn’t work (watch the video) and you’ll get BER.
The solution here is to plant later or protect your transplants from cool temperatures.
(As an aside, I note that Southern gardeners also have this problem when the air temperature gets too hot rather than too cold.)
If they’re on more than the bottom
If you mainly see the problem all over the plant, or sporadically up the plant, it’s likely a watering issue.
Tomatoes like their water. A tomato fruit is at least 95% water and if you skimp here, your fruit will show it. When the blossoms are pollinated and start setting fruit, the plant must move calcium to this young fruit. If there is not enough water in the ground to move the calcium then the developing fruit will have a black rotting section on its bottom while the top looks fine.
It is not the lack of calcium in the soil (so adding calcium isn’t the solution) but rather the lack of water (or cool temperatures) creating the problem. Regular and deep watering will prevent this.
It is also possible it’s a too-high-heat issue and the solution here is to slightly shade the plants.
Can I Add Something To The Soil To Make This Go Away
No. Not unless a soil test says your soil is magnesium deficient.
What Do You Recommend?
Plant later. Water better.
The Internet is so full of B.S. about adding useless things (from pennies to epsom salts to magic fertilizers) the real causes of the problem are lost.
- If only the lowest fruit truss has black bottoms, it was likely cold temperatures.
- If it’s fruit higher up as well, then it’s quite likely a watering problem.
Let Me Deal With a Few of the Myths Here
There is more bad advice going around on the net about growing tomatoes than just about any other plant. Here are a few of my favorites:
- Put a penny into the planting hole so the copper will help the tomato. Well, given that copper takes many years to degrade and that they stopped making pennies out of copper sometime in the early 1980’s, (it’s now a zinc alloy) I suspect this one is not true. 😉
- Add Epsom salts to the planting hole for great tomatoes. Well, Epsom salts are magnesium, a fairly common element. And yes, tomatoes do like magnesium but the only way this will help is if your soil is deficient in this element (most soils aren’t). The only way you’ll know is if you do a soil test. But this won’t stop some folks from doing it anyway. The question I have is how much magnesium is too much? 🙂 And a lack of magnesium isn’t one of the causes of B.E.R. – although it does have an impact on general plant health because of the leaf damage. Here’s an article with picture of a magnesium deficient tomato leaf.
- Add lime or calcium or eggshells to the planting hole to stop blossom end rot. Well, given the causes have little to do with a lack of calcium but more to do with the environment and gardening skills, this too is just another bit of “interesting” advice. Note most soils are not deficient in calcium. If you do a soil test and find you have a calcium deficiency, then add Ca directly and not by the small amounts you’ll get from egg shells. Compost the egg shells.
- Spray XX on the leaves. Well, there are two factors here. It seems leaves do indeed absorb many nutrients by leaf spraying. BUT, the research (at least the stuff I’ve read so far) seems to indicated the leaves don’t release the nutrients they absorb very well. In other words, if the nutrients come up from the roots, they’re distributed as the plant needs them. If they are sprayed/absorbed by the leaves, they tend to stay in the leaves. In this case, any liquid calcium sprayed would tend to stay in the leaf if it was absorbed by the leaf.
Last but not necessarily least, if you’re still convinced you need to add something to the soil, let me suggest a soil test. These can be arranged via your local agricultural or extension offices.
Contact local ag schools or universities with hort courses if you can’t find a government site.
Google “soil tests for XXXX” where XXXX is your city, state or province.
Do ask them to check for calcium levels.