Tipburn and blossom end rot are symptoms of calcium (Ca) deficiency. Sometimes the first thought when a nutrient deficiency occurs is to add the nutrient that appears to be required to solve the issue. This isn’t always the best way to resolve the issue.
Calcium has a very important role in the formation of plant cell walls. If calcium cannot reach new cells in adequate time, cell wall formation cannot be completed. This can result in necrotic (dead) tissue in the leaves (tipburn) and fruit (blossom end rot) as a consequence of cell death. This is why calcium uptake from roots to leaves is a very important process.
Calcium movement in plants
The main force moving calcium through plants is water. When plants transpire, calcium moves. Calcium will only be present in plants when water is moving. Most cases of calcium deficiency in greenhouse crops are due to environmental conditions, not a nutrient deficiency directly related to the nutrient solution being applied.
Calcium deficiency triggers
There are several environmental conditions that can trigger calcium deficiency. The most common factors include:
Low relative humidity. When plants are exposed to low relative humidity levels, small pores in leaves called stomata close. Stomata are responsible for transpiration. Calcium movement depends entirely on stomata behavior. It is important to know optimum relative humidity levels for each crop and to keep the humidity levels as uniform and consistent as possible. The humidity in a greenhouse can be increased by running water through the evaporative cooling pads and/or by installing a fog system if necessary.
Lack of airflow over the crop. There needs to be air movement around the leaves to ensure continuous gas exchange. The airflow velocity around the plant leaves can be reduced as a result of the friction between the leaf surface and the moving air. This creates a boundary layer which is a layer of heavy air that can decrease gas exchange in plants. This reduction in gas exchange can impact calcium uptake by the plants.
This reduction in calcium uptake is common in greenhouse lettuce. Lettuce has a very tight leaf canopy. New leaves are usually exposed to a very dense boundary layer. Good airflow over the crop canopy is required to avoid tipburn. Installation of vertical fans is usually recommended to improve airflow in lettuce greenhouses. It is also important to maintain proper airflow in vertical farms. A 1 meter per second air velocity rate in each vertical layer is recommended for leafy greens.
High light intensity in vertical farms. With indoor farm production, there are many variables that need to be controlled to ensure good crop performance. Two variables that together can trigger tipburn are light intensity and the boundary layer. When plants are located close to the grow lights, the light intensity tends to increase, and space for air flow decreases.
If plants are exposed to the same photoperiod during the entire production cycle, the total daily light integral (DLI) tends to increase with time. Recent research demonstrated that in indoor vertical farms when plants are exposed to a DLI that exceeds 17 moles of light per square meter per day (mol·m-2·d-1) for more than three days, tipburn is triggered.
High VPD levels. Some crops, including tomatoes, show tip burn under high VPD levels. Transpiration from roots to leaves increases under high VPD environment. When VPD is too high for tomatoes, calcium uptake goes directly from the roots to the leaves, bypassing the fruit. This is why sometimes blossom end rot (calcium deficiency in fruit) occurs in tomato fruit, but no deficiency symptoms appear on the leaves.
Avoiding calcium deficiency
When calcium deficiency is seen in plants, make sure to check that the fertigation system is operating properly. If the fertilizer stock solution is maintained in multiple tanks, check all reservoirs to ensure the same solution levels so that all nutrients are being delivered uniformly to all crops.
But remember to always monitor environmental conditions before adding calcium to any crop. Excess calcium can cause other nutrient deficiencies. If the decision is made to apply foliar calcium, then this treatment is required during the whole production cycle to avoid calcium deficiency. Foliar calcium applications to prevent calcium deficiency might be avoided if the production environment is properly controlled.