By Nora Doonan, UConn Soil Science Graduate Student, Department of Plant Science & Landscape Architecture
It can be frustrating to put time and energy into growing tomatoes, only to find that your tomatoes have developed blossom-end rot. Blossom-end rot is a physiological disorder that commonly affects tomato plants, characterized by a black, sunken spot on the fruit. This disorder is caused by insufficient calcium as a result of nutrient and water management challenges. Blossom-end rot can also affect peppers, eggplants, squash, and watermelons. In tomatoes, it is commonly seen on larger and elongated varieties, which can have a greater demand for calcium. Calcium is an essential plant nutrient that is required for proper growth, especially for the development of cell walls in the tomato fruit. When a fruiting tomato plant experiences calcium deficiency, the cell wall can collapse and show the appearance of blossom-end rot. Soil tests can reveal if your soil has sufficient levels of calcium and can determine if your soil pH is optimal for nutrient availability.
Oftentimes, the soil is sufficient in calcium levels, but blossom-end rot still occurs. Blossom-end rot can be closely associated with soil moisture that is too wet or too dry. The soil should never dry out completely. It is important to water regularly to ensure that plants receive 1 to 2 inches of water a week. A cycle of dryness followed by overwatering is only promoting calcium deficiency in the plant. If soil is too saturated, there may be insufficient oxygen levels in the soil that can affect the roots’ ability to take up water and nutrients.
One method of maintaining optimal soil moisture is to mulch tomato plants with an organic mulch, such as wood chips or straw, after the soil has warmed up. This will help to prevent excessive evaporation and drying of the soil. Another method is to plant tomatoes in a mound of soil that is high in organic matter or compost. Organic matter can retain soil moisture and provide adequate drainage.
There are other factors that can affect calcium uptake by the plant, such as high concentrations of ammonium, potassium, and magnesium in the soil. This can be caused by over fertilization. Specifically, over fertilizing with nitrogen can promote excessive production of leaves in the tomato plant, which can preferentially distribute calcium to the leaves rather than the fruit.
Calcium movement in the plant is dependent on transpiration, which is the loss of water to the atmosphere from plant leaves, stems, and flowers. Calcium is taken up through the roots and relies on water flow through the xylem for transport through the plant. This movement of water through the xylem is driven by the process of transpiration. Calcium is not redistributed from the leaves to the fruit. In addition, the overfertilization of ammonium, potassium, and magnesium can allow for competition in root uptake of calcium because they are all positive cations. Damages to roots, caused by fertilizer burns or cultivation, can also be a factor in impairing nutrient and water uptake. To prevent blossom-end rot through fertilization practices, apply fertilizer as recommended by a soil test and apply nitrogen fertilizers in the form of nitrate, rather than ammonium.
Any affected fruits can be removed from the plant, as they will continue to drain moisture and calcium for development from other fruits. A foliar application of calcium can be made to the leaves when the fruit is young, but it is not the most effective method, since calcium in the leaves is relatively immobile. The goal is to prevent the need for foliar application through water and nutrient management across the tomato plant growth stages. Hopefully these methods will allow you to prevent blossom-end rot starting at beginning stages of growth, so that you can enjoy a bounty of beautiful, red tomatoes.
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