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Aluminum Organic Matter in Acid Soils: Buffering and Solution Aluminum Activity¹

ABSTRACT

The variation of solution Al³⁺ activity with pH, the effect of suspending salt solution cation on Al³⁺ 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 Al³⁺ 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 0.02N solutions of the chlorides of K⁺, Na⁺, Ca²⁺, Mg²⁺, Cu²⁺ and La³⁺ with the addition of acid or base. In the pH range 3.9 to 4.9, plots of pH vs. pAl were linear with pAl/pH<2.5, significantly different from the value of 3 predicted by Al(OH)₃ solubility. At a given pH, Al³⁺ activity was a function of the solution cation. The Al substituted peat in 0.02N CaCl₂ 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 Al³⁺ 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 Al³⁺ activity in acid soils that have a low amount of permanent charge CEC relative to the quantity of organic matter.

¹ Contribution of the Dept. of Agronomy, Cornell Univ., Ithaca, N.Y. 14853. Agronomy Paper no. 1273. This work was partially supported by AID tac-1104.

² Former Graduate Research Assistant, presently Asst. Prof. of Soil Science, Univ. of Minnesota, St. Paul, MN 55108; Asst. Prof. of Soil Science, Cornell Univ.; and Group Leader, Research, J. M. Huber, Co. Huber, GA. 31040, respectively.

Received for publication October 2, 1978. Accepted for publication February 22, 1979.

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