Agriculture Reference
In-Depth Information
croorganisms, and physical forces associated with the change in state of water and
its movement act on loose soil particles. The particles are arranged to form larger
units called
aggregates
.
The formation of aggregates (also called
peds
) of various sizes and shapes de-
termines the porosity of the soil. Both features—peds and pores—have been used
to define soil structure, which in the broadest sense is denoted by the size, shape,
and arrangement of soil particles and aggregates; the size, shape, and arrangement
of the pores separating the particles and aggregates; and the combinations of ag-
gregates and pores to form different structures.
For optimum plant growth, the soil structure should have a predominance of
aggregates from 5 to 10 mm in diameter. The physical, chemical, and biotic forces
involved in structure formation and stabilization include the following:
• Swelling pressures generated through the osmotic effects of exchangeable
cations adsorbed on clay surfaces (box 3.1).
• Forces associated with the change in state of soil water. For example,
tension forces develop as water evaporates from soil pores, pulling particles
together. Unlike any other liquid, water expands on cooling from 4 to 0°C.
Because water in the largest pores freezes first, liquid water is drawn from
the smaller pores thereby subjecting them to shrinkage forces. But the
expansion of the water on freezing, and the buildup of ice lenses in large
pores, subject these regions to intense disruptive forces. For example, the
action of frost in cold climates breaks down the massive clods left on the
surface of a clay soil after autumn cultivation, producing a mellow “frost
tilth” of numerous small granules (fig. 3.1).
Friable tilth created by frost in
cultivated soil. Photograph by the
author. See color insert.
Figure 3.1
