role in determining mineralogical, chemical, textural, and structural properties of
soil. For instance, sandstone of bedrock usually produces relatively infertile,
sandy-textured soils.
One of the hallmarks of Earth’s physical system is the incomprehensible scope
of time. Parent material is morphed into soils because of availability and exchange
of mass and energy between Earth’s surface and its atmosphere. Physical and
organic weathering agents penetrate the parent material. With great amounts of
time there can be significant remineralization and translocations of materials up
and down through the soil profile. Given enough time, the soil can become physi-
cally unlike the parent material. As with rocks on continental surfaces, soil ages do
not approach the age of Earth. However, there is a wide range of ages. Some soils
have had hundreds of millions of years to evolve, while some can be dated to the
withdrawal of continental glaciers a few thousand years ago, and others are merely
tens or hundreds of years old.
Climateis the most important factor in a worldwide sense. Witness the slow
chemical processes of tundra soils that are palpably dissimilar from those creating
deep, infertile soils found in the tropical rainforest. Climate exchanges of mass
and energy with underlying soils determine the rates of physical and chemical
Soils 311A map showing the global distribution of soil types, based on the USDA soil taxonomy system. (U.S.
Department of Agriculture)