Soil

Soil is the topmost layer of the Earth's surface, overlaying bedrock, except in those regions where bedrock is exposed, or those regions where the soil is overlain with glaciers.

In the planet's more fertile regions the topmost layer of the soil, called topsoil, is composed of smaller fragments of minerals, mixed with organic elements - bits of dead plants, dead animals, and living micro-organisms. These organic elements absorb and retain moisture, which the plants growing in these fertile regions rely on.

Dry desert areas largely lack those organic elements.

Bogs and marshes form in poorly drained areas, lack the mineral components, and the plants that grow there are adapted to the high water level, and the acidity of that water.