Toxicity of Aluminum to Coffee in Ultisols and Oxisols Amended with CaCO ₃ , MgCO ₃ , and CaSO ₄ ·2H ₂ O

Abstract A greenhouse experiment was conducted with six acid soils from southern Brazil to investigate the effect of available Al on growth and mineral nutrition of coffee ( Coffea arabica L.) seedlings. Coffee seedlings were grown for 7 months in pots containing soil treated with varying amounts of CaCO 3 up to twice the lime equivalent, and amounts of MgCO 3 and CaSO 4 ·2H 2 O equal to the lime equivalent. Leaf samples were collected immediately before harvesting the seedlings and analyzed for Ca and Al. At this time, soil was collected from each pot and analyzed for exchangeable cations and soluble ions. The chemical composition of the soil solution was used as input data for a computer program (GEOCHEM) to chemically speciate Al in the soil solutions. Shoot and root weights were correlated with KCl‐exchangeable Al of soil, percent Al saturation of soil, the concentrations of total Al (Al t ) and Al 3+ (calculated), and the activity of Al 3+ (calculated) in the soil solution. Growth reductions of the seedlings correlated best with the Al 3+ activity value. The toxicity threshold for the Al 3+ activity was approximately 4.0 × 10 −6 . Leaf Al concentrations likewise correlated best with Al 3+ activity. Threshold leaf Al concentrations of approximately 62 and 100 µg/g, respectively, were observed for reduction in root and shoot growth.