Soil

Plants have adapted to the globe's variety of soils and can grow in almost every soil and under all variations of weather, yet plants grow better in some places than others, especially in places where nutrients are most readily available from the soil.

The tropical belt around the earth's equator contains the globe's "oldest" soils. Under heavy rainfalls and high temperatures, most nutrients have leached out of these soils. They generally contain high levels of iron oxides, which is why most tropical and subtropical (lateritic) soils are red in color. Yet many tropical soils are able to support rich, dense forests because organic matter is readily available on the surface of the soil as tropical vegetation falls to the ground and decays quickly. When tropical forests are cleared, the hot sun and heavy rains destroy the exposed organics, leaving very hard, dry soil that is poor for cultivation.

Soils in desert regions are usually formed from sandstone and shale parent rocks. Like tropical soils, desert soils contain little organic matter, in this case because the sparse rainfall in arid regions limits plant growth. Desert soils are generally light in color and shallow. Desert subsoils may also contain high levels of salts, which discourage plant growth, and rise to the surface under rains and irrigation, forming a white crust as the water evaporates.

Tundra (a Finnish word meaning "barren land") soils, dark mucky soils, cover treeless plains in arctic and subarctic regions. Below the A horizon lie darker subsoils, and below that, in arctic regions, lies permafrost. While these soils are difficult to farm because of their high water content and because permafrost prevents plant roots from penetrating very deeply, tundras naturally support a dense growth of flowering plants.

Below the great flat plains of the Midwestern United States and the grassy plains of South Africa, Russia, and Canada lie deep layers of black soil atop a limestone-like layer, which has leached out of the soil into the subsoil. These soils are termed chernozem soils, a term that comes from the Russian word for "black earth." These soils are highly productive.

The most productive soils for agriculture are alluvial soils, which are found alongside rivers and at their mouths, where floods bring sediments containing sand, silt, and clay up onto the surrounding lands. These are young soils high in mineral content, which act as nutrients to plants.