By Stellenbosch University horticultural science department senior researcher Elmi Lötze

The efficiency of foliar calcium (Ca) applications to increase the Ca content of fruit - especially apples - is still a mystery. Adequate Ca levels are important for preventing a range of fruit problems including bitter pit in Golden Delicious and Braeburn apples. Calcium is also relevant in maintaining fruit firmness and general internal fruit quality and can reduce susceptibility of, for example, peaches to fungi infections. A range of factors need to be considered to ensure maximum efficiency for applied Ca that must reach the cells to impacts on fruit quality.

Timing is crucial

The time of application of foliar Ca depends on the aim of the application. Early on in the season, 21-63 days after full bloom (dafb), Ca penetrates a bigger proportion of the whole fruit (area to volume ratio), the penetration tempo is higher, and thus more Ca will be available for the fruit during the last phase of cell division and the first part of cell enlargement. Thus, the much needed Ca for strong cell walls and membranes can be supplied at this stage.

A different perspective will be to apply the sprays after 80 dafb, when the fruit is bigger and allows for more total Ca to be deposited onto the fruit. This is indeed what results when the total Ca content of the fruit is determined at harvest; but the penetration of the foliar product is still only about 5mm from the peel inwards which will mainly impact on defects/deficiencies that are initiated later during the developmental stage and occur on the outside of the fruit.

In the past, Ca burn occurred on fruit and leaves during early Ca application with some formulations. Now, new formulations and more recent papers show positive results with early season foliar applications.  Various papers reported on the lack of Ca uptake of foliar applications on apples around 70 to 80 dafb – rendering applications during this approximately 14-day period ineffective.

There has also been evidence of applying only six weekly Ca foliar applications from 21 dafb to Golden Delicious and Braeburn apples during consecutive seasons, with sufficient bitter pit control. However, most commercial producers still apply eight to 12 weekly foliar applications for fruit quality insurance which is motivated by the relative low cost of Ca products and high cost of possible defects, counteracting the possible saving on the number of Ca applications.

Absorption factors

The contribution of the Ca formulation toward efficient Ca uptake comprises the available active Ca (g.kg-1), the carrier molecule (chelate, salt, etc) and the size of the molecule. Salts like MainstayTM (200 g.kg-1) and Calcium nitrate (195 g.kg-1) contain much higher active Ca than chelated products Cal-Pro (65 g.kg-1) and Carbology Cal (56 g.kg-1). Reports showed a direct relationship between a higher active Ca component and final Ca concentration in studies on leaf penetration. Manni-PlexTM uses a sugar alcohol as a smaller carrier molecule which is of a different chemical composition than a chelate such as Ca EDTA. Smaller molecules like calcium chloride penetrated an apple peel disc quicker during 24 hours after application than calcium sulphate.  Molecule size is also relevant when considering penetration through stomata.

The latest information showed the maximum diameter of stomata on leaves as 4-4.8 nm which can limit stomatal penetration of bigger molecules. A scanning electron microscopy study on Ca penetration of tomatoes, 24 hours after application of foliar Ca, confirmed the differences between commercial formulations in terms of depth of penetration into the pulp and Ca concentration (ppm) in these areas.

Environmental conditions during application may enhance or reduce the efficacy of the product by influencing the penetration tempo. The point of deliquescence of Calcium chloride and Calcium nitrate is lower than of Ca acetate, lactate and propionate. Penetration during 24 hours on an intact apple peel was therefore faster in Calcium chloride and Calcium nitrate. At a low relative humidity, stomata tend to close, which reduces uptake of the foliar application. Dry, warm conditions will also lead to closed stomata and very quick drying of the applied solution, resulting in a decrease in penetration of the peel or leaf surface. In the worst case, applying Ca during temperatures above the ideal of 21°C can cause irreversible lenticels burn on fruit and also low Ca penetration.

Thus, when choosing your application program and product, all factors should be integrated for a final decision. And remember, in spite of satisfactory results from foliar Ca applications to increase fruit Ca concentration, it can never replace soil Ca applications, which replenish Ca reserves and supply the necessary Ca to the new buds, fruitlets and leaves during spring.

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