Agriculture Reference
In-Depth Information
Figure I.9 illustrates diagrammatically the distributions of the predominant terrestrial
clay mineral groupings throughout the world (Millot, 1979). While the boundaries are
considerably more complex than indicated, different environments clearly produce
characteristic suites of clay minerals.
In Antarctica and other deserts, almost no leaching or hydrolysis occurs; little clay
forms under these circumstances and nitrates and other soluble elements of aerosolic
origin may accumulate (Campbell and Claridge, 1987). Under conditions of moderate
leaching, mixed-layer minerals and vermiculites are typical. Where greater leaching
and hydrolysis pertain, but where some Si, Fe and Mg still remain, smectites occur.
Smectites also seem to form most readily where there is a clear alternation of wet and
dry seasons. Where very strong leaching and hydrolysis occur, kaolinite and halloysite
are typical and in the extreme, oxide minerals such as gibbsite may accumulate.
Even at more restricted geographic scales, climate is a dominant factor controlling
surface soil clay mineral composition. Folkoff and Meentemeyer (1985) have shown
that climate is the dominant factor determining the phyllosilicate clay assemblages
of the A horizons of soils in continental United States of America. However, a number
of soils were exceptions to this generality: the major group comprised older soils whose
clay mineral suites had been determined under previous climatic regimes while another
small group consisted of soils of different parent material composition. Clearly the balance
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