A greenhouse experiment was carried out with 16 columns of an undisturbed Oxisol that had sufficient subsoil acidity to restrict root growth of a wide variety of crop plants. The objective was to determine the effects of surface applied CaCO3, CaSO4·2H2O, and water on subsoil pH and exchangeable Al, Ca, and Mg. Eight soil columns were treated with CaCO3 or CaSO4·2H2O at rates equal to 0.25 and 1.50 × the lime equivalent (KCl‐extractable Al). The irrigation treatments consisted of trickle irrigation applied at 8.94 and 17.88 mm day−1 for 6 months. These treatments were superimposed on the amendment treatments. Observations included volume and composition of drainage water during the course of the experiment and chemical composition of the soil column by depth increments once the irrigation treatments were completed. Soil analysis included pH, cation exchange capacity (CEC), exchangeable cations, and composition of saturation extracts of soil. Effects of CaCO3 treatments were observed only in the upper 20 cm of the profiles irrespective of irrigation and fertilizer treatments. The CaCO3 treatments increased soil pH, CEC, and exchangeable Ca while decreasing exchangeable Mg and exchangeable Al; and CaSO4·2H2O treatments reduced the level of exchangeable Al and Mg throughout the 100‐cm depth profiles while increasing the level of exchangeable Ca. Soil pH and CEC were unaffected by the latter treatment. Based on the effectiveness of CaSO4·2H2O in reducing exchangeable Al and Mg while increasing exchangeable Ca, the combination of dolomitic lime and gypsum appears to be an appropriate amendment treatment for Oxisols with toxic concentrations of available Al.
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 CaCO3 up to twice the lime equivalent, and amounts of MgCO3 and CaSO4·2H2O 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 (Alt) and Al3+ (calculated), and the activity of Al3+ (calculated) in the soil solution. Growth reductions of the seedlings correlated best with the Al3+ activity value. The toxicity threshold for the Al3+ activity was approximately 4.0 × 10−6. Leaf Al concentrations likewise correlated best with Al3+ activity. Threshold leaf Al concentrations of approximately 62 and 100 µg/g, respectively, were observed for reduction in root and shoot growth.
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