Calcium is integral for building strong cell walls and maintaining the structural integrity of developing tomato fruit. When tomatoes cannot access or distribute enough of this element, it quickly leads to a common physiological disorder. Understanding the role of calcium helps establish appropriate management strategies for optimal growth and fruit development.
Identifying and Preventing Blossom End Rot
The primary concern driving calcium application is the disorder known as Blossom End Rot (BER). This condition is identifiable by a distinct symptom: a water-soaked spot that appears on the end of the fruit furthest from the stem, which is the blossom end. This spot quickly enlarges, turning dark brown to black, and develops a sunken, leathery texture as the fruit matures.
Blossom End Rot is not a disease caused by a pathogen, but a physiological disorder resulting from a localized deficiency of calcium. The damage begins early in the fruit’s formation, often when the tomato is about half its final size. Without sufficient calcium to strengthen cell membranes and walls, the tissues at the blossom end collapse and die.
The plant’s inability to move enough calcium into the fruit, rather than a lack of calcium in the soil, is the more frequent cause of this issue. Tomato fruit does not transpire as readily as the leaves, which means calcium delivery via the water-transporting xylem is less efficient to the fruit’s distal end. Therefore, the most effective prevention strategies focus on ensuring a steady, uninterrupted supply of available calcium to the roots.
Long-Term Soil Amendments for Calcium
A foundational, long-term approach to calcium management involves preparing the soil before planting. Proactive soil amendment ensures a reservoir of calcium is readily available throughout the growing season. This method is preferable to waiting for symptoms to appear, as it addresses the root environment where calcium uptake occurs.
Two common soil amendments provide a slow-release source of calcium: dolomitic lime and gypsum (calcium sulfate). Dolomitic lime is used when a soil test indicates a low pH, typically below the optimal range of 6.2 to 6.8 for tomatoes. The application of lime not only supplies calcium but also raises the soil pH, which unlocks the availability of other nutrients.
Gypsum is the recommended amendment if the soil pH is already correct or slightly alkaline, as it supplies calcium without significantly altering the pH. The chosen amendment should be worked thoroughly into the top 8 to 12 inches of soil in the fall or pre-planting in the spring. Since these amendments are slow-acting, they are not a rapid cure for existing Blossom End Rot, but a preventative measure for the crop.
Determining the Frequency of Foliar Calcium Sprays
Foliar sprays offer a reactive, temporary solution for plants showing early signs of Blossom End Rot. Calcium is largely immobile once incorporated into plant tissue, meaning spraying the leaves will not correct fruit that is already damaged. Instead, the goal is to provide immediately available calcium to the newly developing fruit, which can absorb it directly through its surface.
Once a tomato plant begins to set fruit, or if early symptoms of BER are noticed, a schedule can be implemented. Applying a calcium solution, such as calcium chloride or calcium nitrate, directly to the foliage and young fruit is recommended every five to ten days. A common concentration uses about two level tablespoons of 96% calcium chloride mixed into one gallon of water.
This application should be repeated for three to four cycles on new growth and forming fruits until environmental conditions stabilizing calcium uptake have been corrected. To prevent leaf burn, especially with calcium chloride, the spray must be applied during the cooler parts of the day, such as late evening or on cloudy days. This allows the solution to be absorbed before rapid evaporation. Foliar treatments are considered a “stop-gap” measure, as they treat the symptom on new fruit rather than fixing the underlying problem in the soil or uptake mechanism.
Environmental Conditions That Impact Calcium Uptake
Even with sufficient calcium in the soil, environmental factors can severely restrict the plant’s ability to absorb and transport it, leading to Blossom End Rot. Calcium moves almost exclusively through the xylem, the plant’s water-conducting tissue, a process driven by the rate of transpiration, or water evaporation from the leaves. Anything that disrupts this steady flow of water compromises calcium delivery to the fruit.
Inconsistent soil moisture is the single most common environmental factor that inhibits calcium uptake. Both overly dry conditions and excessively wet soil fluctuations stress the roots and interrupt the uniform movement of water and calcium into the plant. Maintaining uniform soil moisture through consistent irrigation and the use of mulch is often more effective than simply adding more calcium.
Extreme temperature fluctuations also impact the transpiration rate. Very high heat causes rapid, excessive transpiration from the leaves, pulling calcium away from the fruit. Cold soil temperatures can slow root growth and reduce the initial uptake of calcium. Excessive application of nitrogen fertilizer, particularly in the ammonium form, can induce a calcium deficiency by competing for uptake sites or stimulating rapid growth that outpaces the calcium supply. Proper environmental management, including aiming for a soil pH of 6.5, is the most effective way to ensure the plant utilizes the calcium already present.