Agriculture Reference
In-Depth Information
field soil has horizons, which differentiate one soil from another; see Figure 9.7. The
surface horizon, called the A horizon, is darker in color and has more organic matter
than lower horizons. The lower horizons have more clay and are often redder in
color. Some soils are very young and so have little horizon development, while older
soils have extensive horizon development. Generally, soils that are not too young or
old are best for crop production, although all types of soils, except Gelesols as
described above, are used for crop and animal production. Gelesols are used by
grazing animals that are hunted for meat.
Organic matter has a dramatic effect on the surface horizon. It helps form the A
horizon, and in the case of Figures 9.1 to 9.3 this A horizon is termed an Ap horizon
because it has been plowed. Organic matter also aids in a soil's ability to produce
plants and support animals. It slowly decomposes, releasing plant nutrients in forms
that are available to plants. Organic matter also increases the amount of plant available
water that a soil can hold. It does this by two mechanisms: one is its intrinsic ability to
hold water and the second is its involvement in improving soil structure. Soil with good
structure, enhanced by organic matter, can hold more water, especially water available
to plants. Organic-matter-enhanced peds are larger than soil particles (sand, silt, and
clay), making them harder to move by water or air. Thus, they make soil less suscep-
tible to wind or water erosion. Organic matter can also adsorb materials that might be
toxic to plants or animals and render them nontoxic.
On the other hand the higher clay content in lower horizons is also beneficial. This
increased clay layer is called the B horizon or in many cases a series of B horizons.
These can often be identified by simply feeling the difference in texture between the
surface A horizon and the lower B horizons, which would be the clayey subsoil in
Figure 9.3. Clay retards the downward movement of water, thus holding it in the
Figure
9.7.
Soil profiles showing several different types of conditions. The Ap horizon is an A
horizon that is plowed. The lowest horizon in each is the water table. Compacted and stony
horizons restrict the downward extension of roots.
