How can irregular watering cause calcium deficiency symptoms in tomato fruit even when soil contains calcium?

Irregular watering can lead to calcium deficiency symptoms in tomato fruit, a condition known as blossom-end rot, even when the soil contains sufficient calcium.

This occurs because the uptake of calcium by the plant is directly dependent on the availability of water. When the water supply is inconsistent, the plant cannot absorb calcium in the quantities necessary to nourish its fast-growing fruit. This disruption in water availability inhibits the transport of calcium to the developing tissue at the blossom end of the tomato, resulting in localized necrosis or sunken, leathery, brown areas.1

Essentially, while the nutrient may be present in the soil, the plant's physiological mechanism for moving that calcium into the fruit is interrupted by moisture stress. This condition is typically temporary and often resolves once consistent water conditions are restored.1