Evaluating the effectiveness of different strategies for calcium application on the accumulation of calcium in apple (Malus x domestica Borkh. 'Braeburn') fruit

Orchards displaying calcium (Ca) deficiency are a common phenomenon worldwide, despite the presence of sufficient Ca in the soil and the plant. A 3-year trial was conducted between the 2007 2008 and 2009 2010 growing seasons to evaluate the contributions of soil and foliar Ca applications to Ca concentrations in 'Braeburn' apple (Malus x domestica Borkh.) fruit. Ca(NO3)(2) (Calflo; Yara Africa, Fourways North, South Africa) was applied as six separate foliar sprays until run-off. Applications were made at 1-week intervals between approx. 21 - 70 d after full bloom (DAFB) at 6.75 ml l(-1). Soil applications of Ca (Tropicote (TM); International ASA, Oslo, Norway) at 300 kg ha(-1) were applied at fruit set, or after harvest, according to standard practice. Mineral analysis was conducted to assess the soluble Ca concentrations of whole fruit (without pips and stalks), to quantify the contribution of foliar sprays or soil-applied Ca. Fruit Ca concentrations were maintained at satisfactory levels (4.5 mg Ca 100 g(-1) FW) at harvest by applying a series of six foliar sprays early in the season (for all seasons) during the trial period. Fruit Ca concentrations at 80 DAFB were highest in the treatments with foliar applications of Ca. In 2009 - 2010, Ca concentrations in apple fruit were lowest (8.38 mg 100 g(-1) FW) for soil application of Ca at fruit set. Ca applications to soil after harvest in the previous season, and soil applications shortly after fruit set in the current season, did not significantly increase Ca concentrations in current-season fruit, providing soil Ca levels were above the minimum requirement for apple trees. A possible explanation is that apple trees regulate their uptake of Ca through the roots when soil Ca is available in sufficient quantities. This confirms the importance of active root growth for efficient Ca uptake by apple trees when applying Ca to the soil.