Ask a Gardener - Calcium for Tomatoes

Question: Last year my tomatoes did not produce well and the fruit had blossom end rot. Was told the soil had a lack of calcium. Are egg shells a good source? How should they be applied? -LeRoy Leroy,

You’re exactly right. Blossom end rot is caused by a lack of calcium. The CSU Extension Office explains it this way, “Calcium is not very mobile within the plant and it is required in relatively large amounts when the fruit is rapidly growing. If demand for calcium exceeds supply, tissues lacking this mineral break down leaving a characteristic leathery, brown to black area on the blossom end of fruit such as tomato, pepper, squash, eggplant and watermelon.”

Calcium deficiency is usually caused by big changes in temperature or moisture. With our crazy Colorado weather, it’s no wonder it’s such a common problem. But there are some easy steps to take to protect against blossom end rot. First, make sure your soil is amended with B.O.S.S. or some other organic material. This will not only provide extra nutrition for your tomatoes, but also improve the physical properties of the soil, allowing water to move and drain freely. Thick, clay soils tend to smother and trap calcium. Mulching will also help regulate moisture. Be sure to give consistent water, especially when the tomatoes are growing and setting blossoms. Watering should always be done in the cool of the morning or evening. It is important to water based on need and not day of the week. Tomatoes rarely need daily water and they do need time to drain. The best way to see if your tomatoes need water is to

use the Ol’ Finger Test: Stick your index finger into the soil up to the second knuckle. If it feels wet at that depth, let it go and check again tomorrow. Watering is needed if the soil is still just slightly moist.

When you plant your veggies, adding some crushed up egg shells in the hole is a great way to boost the calcium. Many tomato fertilizers, like Espoma’s Tomato Tone, also contain extra calcium for the same reason. For more information on tomatoes, check out the Growing Tomatoes Fact Sheet on our website.

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