SOILS AND FERTILIZERS: SOILS
Ch. 2, pp. 2 - 4
[Soils: soils |
classes | caliche
| depth |
components | pH
|Soil - The unconsolidated mineral and
organic matter on the immediate surface of the earth that serves
as a natural medium for the growth or land plants.
A desirable surface soil in good condition for plant
growth contains approximately 50% solid material and 50% open or
pore space. The mineral component is usually made up of many
different kinds and sizes of particles, ranging from those visible
to the unaided eye to particles so small that they can only be
seen with the aid of a very powerful (electron) microscope. This
mineral material comprises about 45% to 50% of the total volume.
Organic material makes up about less then 5% of the volume and may
contain both plant and animal residues in varying stages of
decomposition. Under ideal or near-ideal moisture conditions for
growing plants, soil pore spaces contain about 25% air and 25%
water based on the total volume of soil.
Most Arizona soils developed under desert or scrub
vegetation. However, because of the wide variation in elevation
and climatic conditions found throughout the state there are wide
differences in the types of soil profiles found. Interestingly
only tropical soils are not found within Arizona. The percentage
of mineral matter and organic matter in a cubic foot of surface
soil varies from one soil to another, and within the same soil.
Most Arizona soils have very low levels of organic matter, usually
less than 1% by weight. This is due to the slow rates of organic
matter production under arid conditions and the rapid rate of
decomposition of organic matter when the warm soils are moistened.
Content of organic matter is usually higher in soils that have
not been cultivated over long periods of time. Soils that are
tilled frequently or thoses with relatively small amounts of plant
residues are usually lower in organic matter. Plowing and tilling
the soil increases the amount of air in the soil, which increases
the rate of organic matter decomposition. Soils with poor drainage
or high water tables usually have a higher organic matter content
than those which are well drained, because water excludes air from
the soil mass.
Since pore spacesare filled with either air or water,
the amount of air in a soil at a particular time depends on the
amount of water present in the pore spaces. Immediately after a
rain, there is more water and less air in the pore spaces.
Conversely, in dry periods, a soil contains more air and less
water. Increasing organic matter content usually increases
water-holding capacity, but adding undecomposed organic material
reduces water capacity until the material has partially
Soil Horizons or Layers
Many soils have two or more distinct layers or horizons. The principal
horizons (collectively called the soil profile) are: A, surface
soil; E, the subsurface; and B, the subsoil. Beneath the soil
profile lies: C, the parent material; and R, rock, which may be
similar to that from which the soil developed. Many soils in
Arizona have developed in water-deposited (alluvium), wind-
deposited (aeolian silt or sand), or gravity transported material
(colluvium). When soil horizons are present , they usually
differfrom one another in color, texture, consistency, and
structure. In addition, there are usually considerable differences
in chemical characteristics or composition.
The surface and subsurface are usually
the coarsest layers. The surface soil contains more organic matter
than the other soil layers. Organic matter gives a gray,
dark-brown, or black color to the surface horizon, the color
imparted depending largely upon the amount of organic matter
present. Soils that are highest in organic matter usually have the
darkest surface colors. The surface layer is usually most fertile
and has the greatest concentration of plant roots; plants obtain
much of their nutrients and water from the surface soil. Any human
activity which removes or degrades the surface soils is very
serious considering the relatively higher quality of this horizon.
The subsoil layer is usually finer and firmer
than the surface soil. Organic matter content of the subsoil is
usually much lower than that of the surface layer. The subsoil
supports the surface soil and may be considered the soil
reservoir, providing storage space for water and nutrients for
plants, aiding in temperature regulation of the soil, and
supplying air for the roots of plants.
The subsoil can also present serious problems for those
wanting to use the soil to grow plants. These include coarse sandy
or gravelly layers, hardpans or caliche layers. Caliche is a
specific type of naturally occurring hardpan layer up to six feet
thick which is cemented with calcium carbonate. Corse sandy or
gravel layers are droughty and may not supply adequate moisture
for growing plant roots.
The bottom horizon, or parent material, is
decomposed rock or other transported material that has acquired
some characteristics of the subsoil and retained some
characteristics of the rock or other geological material from
which it weathered. It is not hard, like rock, but may show the
form or structure of the original rocks or layering if it is in a
water-laid deposit. The parent material influences soil texture,
natural fertility, rate of decomposition (and thus rate of soil
formation), alkalinity, depth, and in some cases, topography (or
lay of the land) on which the soil is formed.