ALLOPHANE AND HUMUS-ALUMINUM IN SPODOSOLS AND ANDEPTS FORMED FROM THE SAME VOLCANIC ASH BEDS IN NEW ZEALAND 1

We examined the colloid materials in two Spodosols (Tihoi and Mangorewa) and two Andepts (Taupo and Oturoa) formed from the same volanic ash beds, using a combination of chemical and instrumental techniques. The dominant colloid material in all horizons of the Vitrandepts was allophane, except for the A horizon of the Taupo soil, which contained humus-Al complexes and opaline silica and had low pH (H2O) and high amounts of organic matter. Humus-Al, kaolin, and 2:1 layer silicates were the main colloidal materials in the two Haplo-humods, except in the C horizon of the Tihoi soil and the 2Bsh and 3Bsh horizons of the Mangorewa soil, which contained allophane. In these exceptions the release of Al from primary minerals is considered to exceed the accumulation of organic matter, thus allowing the formation of allophane. Allophane formation in the soils studied tends to be related to both the (Al + Fe)/C atomic ratio and soil pH (H2O). The presence of significant amounts of opaline silica, even in the A horizon of the Spodosol where intensive leaching occurs, suggests that the limiting factor in allophane formation, both in the Spodosols and the Andepts, is the concentration of Al, rather than Si, in soil solution.