Calcium in the “soil-plant” system of apple orchard when using nitrogen and potash fertilizers

The experiment was carried out to study the relationship between the soil calcium conditions and the content of this element in fruit and leaves of apple trees at ground fertilization with nitrogen and potassium. The experimental orchard is located in the forest-steppe zone of the Central Russian Upland (Oryol region). The soil of the orchard is Loamy haplic luvisol with a slightly acidic reaction and a high content of organic matter. Fruit plantations are represented by apple trees of ‘Sinap orlovsky’ variety on rootstock 54-118 planted in 2013. Ammonium nitrate and potassium chloride were used as soil fertilizers and applied early in spring at doses of N30K40, N60K80 and N90K120. Annually from 2016 to 2019, the content of exchangeable and water-soluble Ca compounds in the orchard soil and the total element content in leaves and fruits were determined. With the annual application of nitrogen and potash fertilizers the changes in the conditions of apple tree calcium nutrition occured already in the first years after treatments. The changes lay in increasing the calcium mobility and subsequent decrease in the element’s concentration in the root zone. At the same time, there was an annual decrease in the calcium status of fruits. The Ca content in fruits was positively correlated with the content in the soil of exchange (r = 0.43; P<0.05) and water-soluble (r = 0.51; P