The Nature of the Soil Soil is the natural surface layer that contains both inorganic weathered debris and living matter. Soils are developed over a long period of time through the combination of physical, chemical, and weathering processes. The upper O and A horizons usually contain the most organic matter. Debris is moved down the profile producing distinct layers or horizons.

The Nature of the Soil Exposure of the solum and regolith beneath it, Little Tesque Creek, near Santa Fe, New Mexico. An idealized diagram showing soil profile with soil horizons.

Soil Horizons Most soils possess soil horizons – distinctive horizontal layers that differ in physical composition, chemical composition, or organic content or structure. A column of soil will normally show a series of horizons.

Soils of the World The soils of the world are classified into groups, orders and suborders. The following map shows the major areas of occurrence of the soil orders. The soil orders and suborders are correlated with global climate zones.

Key: Boralfs Spodosols Tundra Soils Udalfs Oxisols Highland (Icesheet) Ustalfs Vertisols Xeralfs Mollisols Ultisols Aridisols Soils of the World