Effects of application of calcium fertilizers on fruit quality of Fuji apple cultivar

Eight-year-old trees of Fuji apple cultivar (Malus domestica cv. Fuji) were used as material for calcium fertilizing experiment of different combinations. Results showed that total calcium, exchangeable calcium, and calcium in fruit were significantly improved by soil application of CaSO4, while application of 3.5 kg of CaSO4 per plant reached the most significant effect among all treatments. Applying CaSO4 at fruitlet stage and in fruit enlargement period with amount of 3.5 kg per plant improved fruit calcium concentration significantly. Calcium fertilizer applied at different stage improved fruit calcium concentration significantly, no matter whenever it was applied together with organic fertilizer used in autumn or amino acid calcium sprayed in spring. The highest level of 179.17 mg/kg was obtained in the treatment of S(2+1.5)AM, when 2 kg CaSO4 were applied and 3 times spraying amino acid calcium in Spring, and 1.5 kg CaSO4 applied in Summer together with Special-for-apple organic fertilizer applied in Autumn. Special-for-apple organic fertilizer applied in Autumn improved fruit calcium concentration, as well as storage calcium in roots and branches, with the best effect appeared at amount of 10 kg per plant in Autumn plus CaSO4 at 1 kg Per plant in Spring. The calcium improvement effect decreased while the fertilizer amount reached to 15 kg of organic fertilizer or more than 3.5 kg of CaSO4 per tree. A positive correlation was found between Ca/Mg ratio in soil and fruit, exchangeable calcium, and fruit calcium concentration, while a negative correlation was found between N/Ca ratio in fruit, fast-acting phosphorus, and fast-acting potassium in soil, and fruit calcium concentration. N/Ca ratio and Ca/Mg ratio in fruit differed obviously among different fertilizing treatments. The incidence of apple bitter pit disease during storage was very low when N/Ca17 and Ca/Mg≥7 at harvest. The best calcium fertilizer combination seemed to be organic fertilizer applied in Autumn and Winter together with calcium fertilizer, calcium sprayed at fruitlet stage, and soil applying calcium fertilizer in fruit enlargement period.