Based on an intern project by Rachel Vecchio, Certified Master Gardener
THE REALITY: Blossom end rot is a disfiguring black spot on the bottom of a tomato. This problem can occur on peppers, eggplant, squash, and watermelon too. It can be a maddening blight for anyone who has invested the time and effort to grow tomatoes. Can eggshells help avoid this frustrating situation?
Tomato plants require a generous amount of soluble calcium to grow and develop normal fruit. Calcium in the garden soil is transported by water absorbed by the tomato plant’s roots. Conventional wisdom has held that blossom end rot is caused by calcium deficiency, which amendments like eggshells and lime can help prevent. However, that is not accurate. Blossom end rot is a result of nutritional deficiency that is caused by inconsistent and insufficient watering. A single healthy tomato plant can require up to 2-3 gallons per week. The damage to the fruit occurs when the lack of soil moisture limits the availability of calcium required for the tomato plant’s health and growth.
While most garden soils contain the minimum amount of calcium needed by tomatoes, there can be a possibility that there is not enough calcium available in the soil. In some cases the lack of CA availability is due to a highly acid soil. A soil test should be performed at least once every 3 years to measure the soil’s pH and nutrient levels. In general, soil pH between 6.5 and 7.0 has the appropriate acidity required for uptake of calcium and successful tomato growth. While eggshells do add calcium and micronutrients to the soil, they will not do much to correct soil acidity. If the pH is too low (too acidic), amendment with agricultural lime as recommended by a soil test is the best way to improve soil conditions for vegetable crops. If the need for additional calcium is indicated, the lime application will amend this as well. While the regular addition of eggshells to the garden can help with maintenance of a healthy levels of Calcium, it is important to remember that consistent watering is the key to avoiding this annoying problem.
“Blossom End Rot – Vegetables.” University of Maryland Extension, Home and Garden Information Center. https://extension.umd.edu/growit/blossom-end-rot-vegetables
Gillman, J. and M. Maynard. 2012. Decoding Gardening Advice. Portland: Timber Press.
Pavlis, Robert. 2014. “Blossom End Rot.” Garden Myths – Learn the truth about gardening. http://www.gardenmyths.com/blossom-end-rot/
Voyle, Gretchen. 2015. “Blossom end rot causes and cures in garden vegetables.” Michigan State University Extension.