Aluminum Organic Matter in Acid Soils: Buffering and Solution Aluminum Activity

The variation of solution Al 3+ activity with pH, the effect of suspending salt solution cation on Al 3+ activity, and the buffering intensity were determined for salt solution suspensions of three soil samples and an Al substituted peat that contained one equivalent of Al 3+ for each equivalent of carboxyl groups. The soil samples used were a surface and a subsurface horizon of an Inceptisol from New York and a surface horizon of an Oxisol from Brazil. The soils were equilibrated for periods of up to 35 days in 0.02 N solutions of the chlorides of K + , Na + , Ca 2+ , Mg 2+ , Cu 2+ and La 3+ with the addition of acid or base. In the pH range 3.9 to 4.9, plots of pH vs. pAl were linear with ΔpAl/ΔpH<2.5, significantly different from the value of 3 predicted by Al(OH) 3 solubility. At a given pH, Al 3+ activity was a function of the solution cation. The Al substituted peat in 0.02 N CaCl 2 gave a pH vs. pAl plot very similar to that of the surface horizon of the Inceptisol. Also, the addition of partially rotted leaf humus to the Inceptisol subsoil caused a reduction of Al 3+ activity at any given pH even after 35 days of equilibration. It was concluded that hydrolysis of organically bound Al is a major source of buffering in the pH range of 4 to 5 for dilute salt suspensions of acid soils and that exchange of Al ions from organic matter exchange sites controls the relationship between pH and Al 3+ activity in acid soils that have a low amount of permanent charge CEC relative to the quantity of organic matter.