Effect of Low Soil pH on Aluminum Availability and on Mortality of Cherry Seedlings

Abstract

Levels of Al in leaf samples from 250 cherry orchards and the relationship between soil pH and Al levels in soil, roots and stems, and growth of 16-year-old sweet cherry (Prunus avium) trees and 1-year-old mazzard cherry seedlings were investigated. Leaf samples from commercial orchards contained 21 to 500 μg of Al per g of dry weight. Soil pH in the sweet cherry orchard was as much as 3 pH units below the recommended range (6.5 to 7.0) for optimal growth, with the concentration of available Al increasing from approximately 0.1 to 2.4 meq per 100 g with decreasing pH below pH 5.5. Levels of Al in the root system were proportional to its availability in the soil. Levels of Mg and Ca in roots decreased while Mn and Zn increased significantly (P ≤ 0.05) with increasing Al concentrations. Changes in the concentrations 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 meq of Al per 100 g) observed in the field were simulated by applying aluminum chloride to potted seedlings in three greenhouse experiments. Seedlings receiving calcium chloride served as controls (experiment 4). All seedlings planted in soil below pH 4.7 died within 4 weeks, with or without Al or Cl treatment, and seedling mortality increased as Al treatment increased. Root (P ≤ 0.05) and plant growth in general decreased with decreasing 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, however, root growth was reduced (P ≤ 0.05) compared with the controls. The level of Al applied to the soil and the concentration of Al in stems of cherry seedlings was highly correlated (P ≤ 0.001). The concentrations of macro elements in the stem (P ≤ 0.05 to 0.001) and Mn and Zn increased (P ≤ 0.001) with increasing Al concentrations. The interaction effect between the level of Al applied to the soil and soil pH on seedling mortality was highly significant (P ≤ 0.001). While there was a significant interaction between soil pH and the level of Cl applied to the soil, the level of available Ca did not increase over treatments that received Al. Overall, the data suggest that low soil pH could result in seedling death in part by increasing the absorption of Al into plants to toxic levels.