Pre-harvest calcium sprays and post-harvest dip: The calcium from pre-harvest sprays must move into fruit through their surfaces, and only a very limited increase in the concentration of calcium can be achieved in this way. Therefore, most of the necessary calcium must move into the fruit via the roots.
Calcium for apples
It has long been recognised in the apple industry that it is essential to apply extra calcium to fruit which are susceptible to calcium-related disorders such as bitter pit. The most effective and efficient method is to spray calcium regularly during the growing season.
Coupled with sap and fruit analyses, calcium sprays can effectively control bitter pit in susceptible varieties. Calcium sprays must be regarded as a top-up, not a substitute for poor management of trees.
Bitter pit
Bitter pit has long been a disorder in apple orchards of the world.
Bitter pit appears as small, brown, dried and sunken corky pits of collapsed cells. Most of the pitting is just below the skin and primarily at the calyx end of the apple.
Although bitter pit can develop in the orchard as tree pit, most of the pit develops after harvest and in storage as storage pit.
Foliar applications
Foliar applications of calcium mid to late in the season are preferable to early applications. The sprays must come in contact with the fruit, because calcium is not very mobile within the tree.
The amount of calcium that penetrates into a fruit can be increased by higher concentration of sprays and dip, although spray concentrations of calcium higher than 4 per cent may damage fruit and leaves.
Fruit takes up calcium while it is in solution, so you need to make sure that the sprays dry slowly. That is, apply the sprays during cool, humid weather with little wind.
Surfactants
Surfactants lower the surface tension of solutions, so help to spread calcium across the skins of the fruit.
Surfactants and penetrants can also change the chemical or physical nature of the cuticle, making this waxy layer more permeable to calcium.
Standard calcium spray
The standard calcium spray is calcium chloride or calcium nitrate.
The biggest problem with calcium chloride is that it can cause leaf burn, especially when sprayed on warm days.
There are numerous commercial sprays that contain calcium other than calcium chloride or calcium nitrate. However, many of them can be less effective, or not more effective, than are chloride and nitrate.
The benefit is proportional to the amount of calcium that you apply.
If you use the other sprays, make sure that you always apply the amount of calcium equivalent to the standard calcium chloride spray.
Calcium and stone fruit
Stone fruit do not usually suffer from low calcium. However, firmness and storage life of fruit have been improved for apricots sprayed with two calcium foliar sprays four weeks and two weeks before harvest.
Such sprays are important if fruit set is light, fruit is large and trees are very vigorous.
Based on sap tests, fertigation of trees with a liquid soluble calcium formulation have in some orchards eliminated the need for calcium sprays.
Post harvest dips
A post-harvest dip or drench of calcium is an effective way of putting extra calcium in apples.
A 20-second drench was found to add 60 ppm calcium to the fruit tissue, where disorders begin, and thereby reduce the development of post-harvest disorders.
Calcium sprays and dips will never be the cure-all for poor management of the orchard. When you have problems related to calcium in your fruit, there is something wrong in your orchard or the climate.
It might be pruning, irrigation, fertilisers, crop load or heat waves. Calcium sprays can reduce, not eliminate, calcium related disorders, while the rest have to be taken care of by improved management with Mother Nature’s cooperation.
See this article in Tree Fruit August 2013
