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Ch. 2, pp. 2 - 4

[Soils: soils | properties | 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.

Composition of Soil by Volume
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 decomposed.
Soil Profile
Soil Horizons or LayersTop
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.

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