In most soils of the humid tropics, kaolinitic topsoil horizons overlie more gibbsitic horizons. This arrangement cannot be produced simply by leaching. Quantitative measurement of the turnover of chemical elements in the litterfall in an Amazonian ecosystem indicates that the forest cycles a significant amount of elements, particularly silicon. As a result, fluids that percolate through topsoil horizons already contain dissolved silicon. This effect keeps silicon from being leached down and may account for the stability of kaolinite in the soil upper horizons. The soil mineral composition is thus maintained by biological activity.
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