EFFECT OF LOW SOIL-PH ON ALUMINUM AVAILABILITY AND ON MORTALITY OF CHERRY SEEDLINGS

Levels of Al in leaf samples from 250 cherry orchards and the relation ship between soil pH and Al levels in soil, roots and stems, and growt h of 16-year-old sweet cherry (Prunus avium) trees and 1-year-old mazz ard cherry seedlings were investigated. Leaf samples from commercial o rchards contained 21 to 500 mu g of Al per g of dry weight. Soil pH in the sweet cherry orchard was as much as 3 pH units below the recommen ded range (6.5 to 7.0) for optimal growth, with the concentration of a vailable Al increasing from approximately 0.1 to 2.4 meg per 100 g wit h decreasing pH below pH 5.5. Levels of Al in the root system were pro portional to its availability in the soil. Levels of Mg and Ca in root s decreased while Mn and Zn increased significantly (P less than or eq ual to 0.05) with increasing Al concentrations. Changes in the concent rations of P, K, Fe, Cu, and B were not significant. Soil pH (3.9, 4.7 , and 7.0) and Al levels (0 to 27 meg of Al per 100 g) observed in the field were simulated by applying aluminum chloride to potted seedling s in three greenhouse experiments. Seedlings receiving calcium chlorid e served as controls (experiment 4). All seedlings planted in soil bel ow pH 4.7 died within 4 weeks, with or without Al or Cl treatment, and seedling mortality increased as Al treatment increased. Root (P less than or equal to 0.05) and plant growth in general decreased with decr easing pH. Increasing Al treatment had little effect on root growth at pH 3.9 and pH 4.7 (at which little growth took place). At pH 7.0, how ever, root growth was reduced (P less than or equal to 0.05) compared with the controls. The level of Al applied to the soil and the concent ration of Al in stems of cherry seedlings was highly correlated (P les s than or equal to 0.001). The concentrations of macro elements in the stem (P less than or equal to 0.05 to 0.001) and Mn and Zn increased (P less than or equal to 0.001) with increasing Al concentrations. The interaction effect between the level of Al applied to the soil and so il pH on seedling mortality was highly significant (P less than or equ al to 0.001). While there was a significant interaction between soil p H and the level of Cl applied to the soil, the level of available Ca d id not increase over treatments that received Al. Overall, the data su ggest that low soil pH could result in seedling death in part by incre asing the absorption of Al into plants to toxic levels.