Aluminum solubility control in different horizons of a Podzol

Aluminum extractability and solubility were investigated in detail in six h orizons of a Typic Haplohumod (FAO: Haplic Podzol) from southern Switzerlan d. Pyrophosphate and oxalate extractions as well as successive acid leachin g indicated that in the Ah, (AE), and Bh horizons reactive Al is mainly bou nd to soil organic matter, whereas in the Bs, BC1, and BC2 horizons it is o f inorganic nature. In the latter three horizons, infrared (IR) spectroscop y and transmission electron microscopy (TEM) revealed the presence of imogo lite, Batch equilibrium experiments at 20 degrees C in the pH range of appr oximately 3.5 to 5.5 showed that the podzol profile can be divided into two parts of different Al solubility control. In the Ah and (AE) horizons, Al solubility was found to be controlled by complexation reactions to soil org anic matter, Kinetic studies with samples of the Bh, Bs, BC1, and BC2 horiz ons showed that ion activity products with respect to both Al(OH)(3) and im ogolite, (HO)(3)Al2O3SiOH, reached a constant value after reaction times of 16 d. For pH >4.1, the compilation of all data revealed pAl + 0.5 pSi = 3. 05 pH - 7.04 (r(2) = 0.99) and pAl = 2.87 pH - 8.07 (r(2) = 0.99). These da ta could be shown to be consistent with either Al solubility control by imo golite-type material (ITM) with a log :*K-s(0) = 6.53 +/- 0.09, which disso lves incongruently, or a simultaneous equilibrium with ITM and hydroxy-Al i nterlayers of clay minerals, For pH <4.1, data indicated solubility control by a 1:1 aluminosilicate, e.g., poorly crystalline kaolinite.