LAUSR

Abstract The aim of the study was to examine the effect of use of silicon (Si), as silicic acid (H4SiO4), along with drip-fertigated nitrogen (N), as ammonium nitrate, on the vegetative and generative responses of dwarf ‘Red Delicious’ apple (Malus domestica Borkh.) trees. The study was conducted at a commercial orchard over a 3-year period after tree planting. Apple trees were planted on sandy loam soil at a spacing of 3.5 × 1.5 m. Drip-fertigated N was applied weekly at a solution concentration of 0.05% at the rate of 10–15 g plant−1 season-1, over a 12-week period, beginning at full bloom. Trees were fertigated with Si along with N at each fertigation cycle (the 12- treatments), every second cycle (the 6-treatments) or every third cycle (the 4-treatments) at rates of 180 mg, 90 mg and 60 mg Si plant−1 year−1, respectively. Trees fertigated with N but without Si served as the control. It was found that after the first growing season, soil pH values beneath the emitter declined from 6.1 to 5.4 at 0–20 cm in depth and 5.8 at 21–40 cm in depth and reached the lowest values of 4.4 at 0–20 cm in depth and 5.0 at 21–40 cm in depth in the last year of the study. In N-fertigated plots, soil acidification beneath the dripper did not occur up to 40 cm in depth. The trunk bark and leaf concentrations of N, phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), boron (B), iron (Fe), zinc (Zn) and copper (Cu) were not influenced by the Si treatments. However, the trunk and bark concentrations of P, K, Ca, and the leaf concentrations of N, P, K, Mg and Ca were dependent on tree age. The trunk bark and leaves of the trees in the 12-treatments contained decreased concentrations of manganese (Mn) and aluminium (Al) and increased Si. Compared to the control, only trees in the 12-treatments were less affected by internal bark necrotic disorder, had stronger growth and produced more apples. Fruit size was not influenced by Si fertigation. It is concluded that in ‘Delicious’ apple orchards fertigated with N, as ammonium or urea, the use of H4SiO4 at each fertigation event at rates as high as 15 mg Si plant−1 cycle−1 can be used to ameliorate negative plant responses to soil acidification beneath the drippers.