Agriculture Reference
In-Depth Information
upper horizons for plant use. Clay minerals themselves hold more water than other
common inorganic soil components and thus store water for plant use. In addition
through their cation exchange capacity (CEC) clays hold plant nutrients in forms that
are available to plants.
There are other horizons in soil and horizon subdivisions that are used by soil
scientists to identify different soils. Some of these may have a dramatic effect on
plant and animal production. This is the case when the surface horizon is very low
in organic matter and there is a buildup of salts in these and other horizons. In some
types of environments soils can become very acidic and in others very basic, and
either of these conditions will also adversely affect productivity.
Another aspect of soil is its depth. We think of food crops as having roots that
explore the soil to a depth of 150 cm (
5 ft). This means that shallow soils may not
have enough rooting depth for profitable crop production. Although areas having
shallow soils may produce grass for pasturing animals, they may not be suitable for
crop production. Also compacted and stony horizons (Fig. 9.7) will restrict root
growth and thus may result in decreased yields of various crops. In Figure 9.8 there
is a layer of sand with a layer of gravel over it and a layer of sand on top of this.
Colored water added to the top layer of sand will stop at the gravel layer. This illustrates
the fact that any abrupt change in any characteristic of any horizon will tend to impede
the downward movement of water and roots through the soil profile.
9.6
SOIL INORGANIC COMPONENTS
Soil inorganic components come from rock. Rock is broken down into sand and silt-size
particles by physical forces such as tumbling in a river, grinding in ice such as glaciers,
Figure
9.8.
Water moving down through layers of various coarseness.
