The amount of clay in a soil indicates its physical and chemical properties. Clay contributes to soil plasticity, shrink-swell potential, and cation exchange capacity. (See discussion of nutrients for explanation of cation exchange capacity.) Total water held in the soil increases as the clay content increases; however, water available to plants does not necessarily increase. Clay is a mobile component in soils and moves downward with percolating water. The extent of this translocation often reveals the state or degree of horizon development. For example, many soils contain a relatively low amount of clay in the surface layer, a higher amount between the depths of 25 to 75 centimeters (10 to 30 inches), and a decreased amount below a depth of 100 centimeters (40 inches). The clay-enriched zone is designated as an argillic horizon, a key feature in the study of soil formation. Translocation of clay is further discussed in the section "Processes of horizon differentiation."

Figure 17 illustrates the clay distribution in pedons representative of Laidig, Leck Kill, Hagerstown, and Morrison soils. The contrasting curves indicate that the four soils represent different amounts of clay movement. The depth of soil is closely approximate to the length of the curve, because samples were collected and analyzed from pits dug to bedrock or dug as deep as machine would allow. The development of the argillic horizons in Leck Kill, Morrison, and Hagerstown soils differ in degree. Laidig soils show little clay translocation, and clay content is close to the boundary between coarse-loamy and fine-loamy textural family classes. The clay distribution of Hagerstown soils is close to that expected for the Paleudult classification (soils that have low base status on old, stable land surface). Silt and sand data show that Leck Kill and Hagerstown soils contain mainly silt in the remaining fine earth (particles less than 2 millimeters in diameter) whereas Laidig and Morrison soils contain mainly sand.

10/15/98