RESPONSE OF APPLE-TREES TO FERTIGATION-INDUCED SOIL ACIDIFICATION

Summerland McIntosh apple trees (Malus domestica Borkh.) on M.9 rootstock were fertigated for the first 3 yr after planting with all combinations of two rates of N (23.5 and 47.0 g tree(-1)) and three rates of P (0, 17.5 and 35 g tree(-1)). During this period, soil pH (0.01M CaCl2) beneath and near the drip emitters declined from 5.8 to < 4.0. Rate of fertigated N had few measurable effects on the trees. In contrast, fertigated P increased first-year tree vigour, second-year tree yield, and sometimes leaf Ca and Mg and leaf and fruit P concentrations. None of these nutrient concentrations were near critical values. Fertigated P also decreased leaf concentrations of K and Cu, which reached deficiency after 3 yr, and did not ameliorate the decline in tree vigour attributed to soil acidification. Increased Mn concentration in leaves was the most distinctive plant indicator of soil acidification, while decreased leaf K was the most important basic cation affected. Fruit Ca was not affected by the decrease in soil Ca availability.