Q. I grow tomatoes in pots and keep getting blossom-end rot. I know it is caused by calcium deficiency. Should I add some eggshells to the potting soil?
A. Blossom-end rot is a common problem in pots and in garden beds. Peppers, eggplants, summer squashes and melons can all get it, but tomatoes are the most susceptible.
Blossom-end rot is caused by insufficient calcium reaching the tissue when it is needed. It’s not contagious to other plants and doesn’t live in the soil. It starts as a small, dark spot on the bottom of the fruit or vegetable and can grow to cover the whole bottom half. You can eat affected vegetables after cutting off the yucky looking part.
Interestingly, adding more calcium is not always the solution. In garden tomatoes, blossom-end rot is typically caused by uneven watering, not a lack of calcium in the soil. The plant needs water to carry the calcium up out of the soil.
But if you’re growing tomatoes in potting soil, you likely do need to add calcium. The soilless mixtures commonly sold for pots often don’t have any calcium in them, nor do the typical fertilizers we use.
There are actually 17 nutrients that plants need in various amounts, from the common nitrogen, phosphorous and potassium (N-P-K on the fertilizer label) to iron, copper and, of course, calcium. Plants don’t do well with too little or too much of these nutrients. Much of the necessary nutrition can be found in our local soil and water. If you use a soilless potting mix (this is what is usually sold in the bags for container use) or use reverse osmosis or rainwater in a garden bed, you need to look for a fertilizer that adds these missing nutrients in addition to the standard N-P-K. Some liquid fertilizers that are labeled as being for tomatoes have these additions.
Burying pennies, eggs shells, nails and the like sounds like it would add some of the missing elements, but scientific studies show that these methods don’t work. The elements are leached much too slowly to have any effect other than to make us feel like we are doing something. There are some liquids you can buy to spray calcium on tomato leaves, but research doesn’t support their use on garden tomatoes, either.
You may find that blossom-end rot goes away after you try some of these unproven methods, but that is likely because it would have gone away anyway. Blossom-end rot tends to be a bigger problem earlier in the season. Your best defense against it is to water evenly, and for tomatoes planted in potting soil, use a fertilizer that contains micronutrients.