Soil is like a layer cake. It has several layers, each of which has distinguishing characteristics (fig. 7.26) and which can vary enormously in thickness from one habitat to another. The "frosting" of this cake is the surface litter covering the soil, typically only a few centimeters thick and consisting of fallen leaves. Just beneath the surface litter, the uppermost layer of soil usually extends 10 to 30 centimeters below the surface. In most fertile soils, this topsoil is not acidic and contains 10% to 15% organic matter, which gives this layer a dark color. The layer below the topsoil consists of larger soil particles than those in the topsoil and extends 30 to 60 centimeters below the soil surface. This layer, called subsoil, usually contains relatively little organic matter and is therefore lighter in color than topsoil. In many regions, this layer contains large amounts of minerals washed by rainfall from the topsoil. Mature roots commonly extend into this layer, where minerals accumulate. The next lower layer often is referred to as the weathering bedrock, because it consists primarily of rock fragments. This layer usually lacks organic matter. In chapter 19, you learn about the formation of soils and how nutrients cycle through soils.
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