Calcium nitrate is an inorganic compound and a highly soluble source of two nutrients necessary for plant growth: calcium (Ca) and nitrate nitrogen (\(\text{NO}_3^-\)). For gardeners and commercial growers of tomatoes, this compound is an excellent fertilizer choice. Its high solubility ensures that both calcium and nitrogen are immediately available for plant uptake. The dual-nutrient delivery system of calcium nitrate makes it an effective tool in tomato production, supporting vegetative growth and fruit quality.
The Role of Calcium and Nitrogen in Tomato Health
Calcium is a stationary nutrient, moving upward through the xylem tissue and incorporated directly into new cell structures. Its primary function is to serve as a building block for strong cell walls, particularly within the middle lamella, which glues plant cells together. Adequate calcium supply maintains the structural integrity of the plant, promotes healthy root growth, and improves the firmness and shelf life of the harvested fruit.
The nitrogen component is present in the nitrate form (\(\text{NO}_3^-\)), which is readily absorbed by the roots. Nitrate nitrogen is preferred because it is mobile in the soil and quickly supports vegetative functions, unlike ammonium-based nitrogen which can interfere with calcium uptake. This readily available nitrogen drives photosynthesis, protein synthesis, and overall rapid growth, ensuring the plant develops the necessary foliage to support heavy fruit production.
Preventing Blossom End Rot
Blossom End Rot (BER) is the most common reason tomato growers turn to calcium nitrate. This physiological disorder manifests as a sunken, dark, leathery spot on the blossom end of the fruit, rendering the tomato unusable. BER is not caused by a pathogen but is a direct result of a localized calcium deficiency within the developing fruit tissue.
The deficiency often occurs even when there is sufficient calcium in the soil, because the nutrient’s movement into the fruit is restricted by environmental factors. Inconsistent soil moisture and high temperatures restrict the plant’s ability to transport calcium to the fruit via the transpiration stream. Since calcium is primarily translocated to areas with high transpiration, the fruit, which has a low transpiration rate, becomes calcium-starved during periods of rapid growth.
Calcium nitrate prevents BER by ensuring a rapid and steady supply of soluble calcium at the root zone. Providing this calcium during fruit expansion helps build strong cell walls in the fruit tissue. Using a highly soluble form of calcium helps the plant overcome the transport limitations that cause the deficiency, significantly reducing the incidence of this disorder.
Application Methods and Timing
Calcium nitrate can be applied using several methods depending on the scale of the operation. The most common method is a soil drench, where the dissolved fertilizer is watered directly into the root zone. This ensures the calcium is available for uptake by the roots, which is necessary for long-term structural use.
For large-scale operations, fertigation is highly efficient, introducing soluble calcium nitrate directly into the drip irrigation system. Home gardeners typically use about four tablespoons of the granular product dissolved per gallon of water for a soil drench. Foliar sprays offer a quick, temporary boost but are not a substitute for soil application, as calcium is poorly re-translocated from the foliage to the fruit.
Prophylactic application is more effective than treatment after symptoms appear. Growers should begin applying calcium nitrate early in the season, typically when plants reach about 12 inches in height or when the first flowers appear. Regular applications should continue throughout the growing season, often every two to four weeks, to maintain a consistent supply. It is important to follow dilution rates strictly, as excessive nitrogen can lead to excessive vegetative growth or cause “burn” damage to the roots and foliage.
Soil and Nutrient Interactions
The effectiveness of calcium nitrate is closely tied to the chemical environment of the soil, particularly its pH level. Calcium absorption is complicated in highly acidic soils, which can lock up the nutrient and make it less available to the tomato plant. Conversely, the use of calcium nitrate can help balance soil pH, making the environment more favorable for overall nutrient uptake.
Calcium availability is also affected by the presence of other positively charged ions, or cations, in the soil solution. This phenomenon is known as nutrient antagonism, where excessive amounts of nutrients like potassium (K) or magnesium (Mg) compete with calcium for absorption by the plant roots. Even if a soil test shows high calcium levels, an imbalance with these competing cations can still result in a functional calcium deficiency within the plant.
The nitrate form of nitrogen in calcium nitrate is beneficial because it minimizes antagonism compared to ammonium-based fertilizers, which actively interfere with calcium uptake and can increase the risk of BER. The high solubility of calcium nitrate allows it to quickly bypass some of these soil challenges, ensuring the rapid delivery of calcium to the root zone. However, this same solubility necessitates careful management to prevent the nutrient from leaching out of the soil with excessive watering.