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Is Your Lettuce Crop Starving For Calcium?
Tip burn is a common problem in lettuce crops lacking calcium. Young leaves can develop In lettuce, calcium deficiency often manifests as tip burn on developing leaves
By Erica Hernandez|
August 12, 2020
Tip burn is a common problem in lettuce crops lacking calcium. Young leaves can develop In lettuce, calcium deficiency often manifests as tip burn on developing leaves. Tip burn gives leaf margins a burned or crinkled appearance and will affect their appearance throughout development.
Calcium, though not a macronutrient, is essential for proper plant development. Plants absorb calcium both through their roots from nutrient solutions and through leaves and shoots during foliar applications. Xylem, the water-carrying network of cells within plants, transports calcium from the point of absorption upward through plant tissue.
Transpiration powers this upward flow as the plant photosynthesizes and releases water molecules into the environment. Calcium serves several important purposes within plants. Strong cell walls require calcium to be incorporated in their structure, with adequate supplies leading to well-formed, healthy leaves and stems. Several biological processes also employ calcium as a signaling molecule, keeping plant development on-track, or responding to changing environments.
Lettuce is a long-cultivated crop; calcium requirements have been well documented and the differences between field and greenhouse production understood. Hydroponic lettuce production calls for 100 to 150 ppm calcium throughout the growth cycle. The calcium must be supplied by the water rather than obtained from field soil. Tap water contains some calcium, but often not enough to hit this target.
Hydroponic producers may incorporate a calcium-nitrate-based fertilizer to provide both calcium and nitrogen to their crops. Calcium chloride at 1 pound per 100 gallons can also be applied as a foliar spray to help deliver calcium to leaves when transpiration is low.
Know the Symptoms
In lettuce, calcium deficiency often manifests as tip burn on developing leaves. Tip burn gives leaf margins a burned or crinkled appearance and will affect their appearance throughout development. The damage continues to affect new leaves until the cause of the deficiency is remedied. Damaged tissue can go on to become necrotic, providing a potential point of entry for secondary Botrytis infection. Overall, tip burn reduces the health, appearance, and shelf-life of lettuce heads and leaves.
Pinpoint the Root Cause
Identifying the cause of calcium deficiency in lettuce can be a tricky topic, as it is not always caused by a lack of applied calcium. Since water carries calcium through the plant as transpiration occurs, calcium movement is dependent solely on transpiration rates. It will not matter how much calcium you supply to the root zone of your crop if you are growing in a humid, low-transpiration environment. Conversely, overly dry conditions causing plants to close their stomates and cease transpiration may also lead to calcium deficiency.
Head lettuce is particularly susceptible to tip burn in humid environments. As lettuce heads develop, they enclose the apical meristem, or growing point, in an envelope of leaves. This creates a pocket of stagnant air, which can be resistant to outside air movement. Stagnant air cuts down on transpiration at the growing point, leading to less calcium being delivered and developing leaves becoming misshapen and tip burned. Alternatively, in high-light, low-humidity environments, lettuce will experience a surge in photosynthesis while closing off stomates to cut down on water loss. Again, the movement of calcium is impeded within the plant even though there may be enough at the root zone.
Remedying calcium deficiency first relies on identifying the cause. Step one is looking at the nutrient solution. Is there a high or low level of calcium in the base water? Is there a calcium source included in the recipe? Calcium is available over a wide pH range, so pH changes are often not necessary. Next, look at the environmental conditions. Appropriate airflow is essential, though it may require different equipment than other crops.
Successful lettuce growers often utilize vertical airflow fans over horizontal airflow. The downward direction of airflow allows air to be forced into and around the protected growing point of head lettuce. A gentle and constant flow is all that is necessary — high airspeeds can lead to stomatal closure and tip burn once more. Finally, make sure photoperiods and light levels are appropriate for lettuce. A daily light integral (DLI) of 17 mols/m2/d is ideal for lettuce production, with a photoperiod between 14 to 18 hours.
Erica Hernandez is a Controlled Environment Agriculture Technical Specialist at Griffin. After obtaining her advanced degree in Horticultural Biology from Cornell University, she came to work with the GGSPro team supporting greenhouse growers across the country. She works to bring proven horticultural techniques to growers and increase production efficiencies through knowledgeable advice.
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Lead Photo courtesy of Griffin
How To Avoid Calcium Deficiency In Controlled Environment Food Crops
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
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 plant cell wall formation. 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 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 is 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 airflow 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.
Excess humidity. Some crops including tomato show tipburn under high relative humidity environments. Transpiration from roots to leaves increases under high relative humidity levels. When the relative humidity is too high for tomato 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.