Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu and Emmalea Ernest, Scientist – Vegetable & Fruit Crops; emmalea@udel.edu
Calcium dominates exchange sites in soils and is rarely deficient in the soil. However, a number of calcium disorders can affect crops, even in well limed soils, including:
- Blossom end rot in tomatoes, peppers, and eggplants (see https://sites.udel.edu/weeklycropupdate/?p=15082) (Fig. 1)
- Blossom end rot in watermelons
- Watercore and glassiness in melons
- Internal leaf tipburn in cabbage
- Leaf tipburn and curd defects in cauliflower
- Internal browning of Brussels sprouts
- Leaf tipburn in spinach
- Leaf tipburn in lettuce (Fig. 2)
- Leaf tipburn and deformity in strawberry
- Internal browning, hollowheart, storage disorders, and poor skin set in potatoes
- Cavity spot in carrots
- Bitter pit, cork spot, cracking, internal brownspot, and water core in apples (Fig 3.)
- Hypocotyl necrosis in beans and other legumes
- Meristem death or distortion of new growth from meristems in many plants (cupped leaves, common in pepper transplant production)
Figure 1. Blossom end rot in tomato caused by calcium deficiency (Edward Sikora, Auburn University, Bugwood.org)
Figure 2. Lettuce tipburn caused by calcium deficiency (Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org)
Figure 3. Bitter pit in apple caused by calcium deficiency (University of Georgia Plant Pathology, University of Georgia, Bugwood.org)
Calcium is taken up in quantity from the soil by the plants from the undifferentiated area right behind the root tip. Once in the root, it moves in the xylem (water conducting vessels) and is distributed in the plant. Much of this movement in the xylem occurs by exchange. Calcium is attracted to the xylem wall and must be displaced by another ion (another calcium or other cation). This process is driven by transpiration and subsequent water movement through the xylem. Therefore, calcium movement is relatively slow compared to other nutrients that move easily in the transpiration flow. Calcium is not translocated in the phloem (plant food transport system) so it cannot move from one area of the plant to another.
Calcium has many roles in the plant from root growth control, to cell membrane function, to stomatal regulation. The main function that leads to the disorders listed above is in the formation of plant structure. Calcium is component of cell walls and the middle lamella which cements plant cells together. Calcium provides cross linkages in the pectin-polysaccharide matrix and adds to the structural strength of plant tissues. When insufficient calcium is present, plant tissues do not form properly and they may appear deformed and in severe cases may become necrotic – tissues may die or collapse.
Because calcium moves slowly through exchange in the xylem and is dependent upon water flow, disruptions in that flow can lead to localized deficiencies in calcium. Plant organs with low transpiration rates or that are rapidly expanding such as fruits and storage roots often do not receive enough calcium to support that growth. Growing tips and meristematic areas that are rapidly laying down new cells are also at risk for calcium deficiencies when water flow is interrupted. High humidity, drought, flooding (leading to roots shutting down), root injury, compaction, and root diseases can therefore lead to calcium disorders by the reduction of water flow and calcium exchange and movement in the xylem.
Competition from other cations such as magnesium (Mg2+), ammonium (NH4+), and potassium (K+) can also affect calcium (Ca2+) uptake and movement. In low pH soils, aluminum can interfere with calcium uptake and lead to deficiencies.
Control of calcium disorders starts with proper liming. This provides soil calcium and raises the pH to eliminate the effect of aluminum. The most important factors to control calcium disorders are to supply a steady rate of water (through a good irrigation program), limit root damage (such as root pruning by cultivation), provide a rooting area for plant that is free from compaction and waterlogging, and create a healthy soil environment that limits root disease potential. Above ground, planting at a spacing that allows for good air movement around the plant will also help. Control fertilizer programs to limit competition between calcium and other ions (use nitrate forms of nitrogen instead of ammonium forms for example). In addition, choose varieties that are less susceptible to these calcium disorders (varieties with very large or very long fruit are more susceptible to calcium deficiencies).
There have been mixed results with foliar application of calcium and these applications should be considered a supplement to help limit these disorders and not a correction for calcium deficiencies and good soil and water management. As stated before, calcium movement is limited so it will be difficult to get calcium to where it is needed by foliar sprays except when applied to active meristematic tissue. Calcium sprays have been effective in improving quality in crops such as apples. Sidedressed calcium has been shown to have positive effects on root crops such as potatoes, particularly in sandy soils. Calcium nitrate or chelated calcium applied through the drip irrigation system can help alleviate calcium disorders in some drip irrigated vegetables such as tomatoes.