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Soils - Part 2: Physical Properties of Soil and Soil Water

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Surface Area of Soil Separates

Clay is the smallest mineral particle in soil. Clay particles are the active portion of a soil, because chemical reactions occur at their surface. The chemical reactions control the adsorption and release of plant nutrients and many other chemicals in the environment. Sand and silt particles are much larger than clay and are quite inactive chemically because of their mineral composition and limited surface area.

Clay particles have about 1,000 times as much external surface area as the particles in an equal weight of sand. The effect of decreasing particle size on surface area can be illustrated with a deck of cards. Stacked together, the deck has only 25 square inches of surface area. When separated as individual cards, the deck has a surface area of nearly 1,000 square inches (Figure 2.4). As another example, consider a room in your home. Pack in as many basketballs or golf balls or pinheads as possible. Then multiply the number of balls or pinheads by the surface area of each. You will find that the total surface area of pinheads is substantially greater than for basketballs and golf balls.

Figure 2.4.  Subdivision of playing cards, like clay particles, increases surface size.

The large surface area of clay and its mineral composition make it the storehouse of plant nutrients. It is not surprising, then, that soils with more clay have more nutrients than sandy soils. Likewise, deep clayey soils have more nutrients than a clayey surface horizon above sandy subsoil horizons. Eroded soil that once had a clayey surface horizon and a silty or sandy subsoil will also have less nutrient-supplying capacity because of the loss of clay — fine, medium, and coarse.


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