Blossom End Rot

Blossom End Rot Picture by John Cappadonna

By John Cappadonna – Permian Basin Master Gardener

Blossom-end rot is a common phenomenon with tomatoes and peppers and occasionally with squash and melons. You have probably heard that it is caused by a calcium deficiency. Most folks assume this means there is not enough calcium in the soil, and they add things like eggshells or Epsom salts to the soil because they saw it on the internet. While blossom-end rot is a calcium deficiency, it is a deficiency at the extremity of the plant. The fruit is the most extreme part of the plant, so that’s where blossom-end rot shows up.

The problem, at least here in West Texas, is seldom a calcium deficiency in the soil. Our soil contains plenty of calcium. If that’s the case, why is blossom-end rot such a common problem?

The answer lies with the relationship between the plant, water, and the nutrients in the soil. Plants take up all their nutrients from the soil with water, with the exception of CO_2, which is absorbed through pores in the leaves. For the plant to take up nutrients in water, they must be water-soluble. That means when they dissolve in water, they become ions. Without going too far down the chemistry rabbit hole, an ion is an atom or molecule with a net charge. For example, a calcium ion has a positive electrical charge. Water is also a unique molecule because it has two poles like a magnet; this is called dipolar.

When nutrients in the soil become dissolved in water, the ions are attracted to the poles of the water molecules, and they are dragged along with the water to every part of the plant. If the plant is not getting enough water, it also won’t get the nutrients that are dragged along with the water, and a deficiency in the extremities of the plant occurs.

In hot, dry West Texas, this is what most often occurs when your tomatoes or peppers get blossom-end rot. The simple solution is to increase the amount of water you are giving the plant. Try to increase the water somewhat gradually, or you can cause other problems with tomatoes like splitting. Of course, you can have a calcium deficiency in the soil, although it is unlikely here in West Texas. Don’t add anything to the soil blindly; it can cause problems. For example, Epsom salts or magnesium sulfate; it contains no water-soluble calcium, so it can’t possibly add calcium. Magnesium can combine with calcium ions in the soil, thus reducing the available calcium to the plant and making the problem worse. Many people swear they cured blossom-end rot by adding Epsom salts dissolved in water. It was the extra water that did the trick, not the Epsom salts. Likewise, adding eggshells does nothing in the short term. Eggshells are calcium carbonate, or CaCO3; this is not an ion, and so it can’t be taken up by water. Eggshells do eventually break down in the soil, but it may take years, so it does nothing in the short term.

To sum things up, if you get blossom-end rot, try increasing the water gradually over a week; this is by far the safest and most effective solution.

If you have any gardening questions, call the AgriLife office in Odessa at 498-4071 or in Midland at 686-4700.  Additional information and our blog for access to past articles are available at westtexasgardening.org.  Click on resources.