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Ch. 2, pp. 15 - 17
[Soils: soils | properties | classes | caliche | depth | components | pH ]

The effective depth of a soil for plant growth is the vertical distance into the soil from the surface to a layer that essentially stops the downward growth of plant roots. The barrier layer may be rock, sand, gravel, heavy clay, or a cemented layer (e.g. caliche). Terms that are used to express effective depth of soil are:
Very Shallow — surface is less than 10 inches from a layer that retards root development.
Shallow — Soil surface is 10 to 20 inches from a layer that retards root development.
Moderately deep — Soil surface is 20 to 36 inches from a layer that retards root development.
Deep — Soil surface is 36 to 60 inches from a layer that retards root development.
Very deep — Soil surface is 60 inches or more from a layer that retards root development.
Soils that are deep, well-drained, and have desirable texture and structure are suitable for the production of most garden or landscape plants. Deep soils can hold more plant nutrients and water than can shallow soils with similar textures. Depth of soil and its capacity for nutrients and water frequently determine the yield from a crop, particularly annual crops that are grown with little or no irrigation. Plants growing on shallow soils also have less mechanical support than those growing in deep soils. Trees growing in shallow soils are more easily blown over by wind than are those growing in deep soils.
Surface Features
Soils that have a lost part or all of their surface are usually harder to till and have lower productivity than those that have desirable thickness of surface soil. To compensate for surface soil loss, better fertilization, and other management practices should be used. Increasing the organic matter content of an eroded soil often improves its tillage characteristics, as well as its water and nutrient capacity. Erosion can be the result of running water or wind, or can be the result of land leveling during home construction. Whatever the cause, generous use of soil amendments, organic materials and necessary fertilizers can help speed the conversion of poor quality subsoil into high quality top soil.


Organic Matter
Organic matter in soil consists of the remains of plants and animals. When temperature and moisture conditions are favorable in the soil, earthworms, insects, bacteria, fungi, and other types of plants and animals use the organic matter as food, breaking it down into humus (the portion of organic matter that remains after most decomposition has taken place) and soluble nutrients. Through this process, materials are made available for use by growing plants. In addition, organic material has a very high cation exchange capacity, so nutrients are retained in plant-available form. The digested and decomposing organic material also helps develop good soil-air-water relationships.
In sandy soil, organic material occupies some of the space between the sand grains, thus binding these together and increasing water-holding capacity. In a finely textured or clay soil, organic material on and around soil particles creates aggregates of the fine soil particles, allowing water to move more rapidly around these larger particles. This grouping of the soil particles into aggregates or peds makes soil mellow and easier to work.
Organic matter content depends primarily on the kinds of plants that have been growing in a soil, the long-term management practices, temperature, and drainage. Soils that have native grass cover for long periods usually have a relatively high organic matter content in the surface area. Those that have desert or native forest cover usually have relatively low organic matter content. In either case, if the plants are grown on a soil that is poorly drained, the organic matter content is usually higher than where the same plants are grown on a well-drained soil. This is due to differences in available oxygen which is needed by the organisms that attack and decompose the organic material. The activity of soil microorganisms is temperature dependent. Soils in a cooler climate such as in Northern Arizona have more organic matter than those in the southern Arizona deserts where the climate is much hotter.
Water and AirTop
Water in the soil ultimately comes from precipitation (rain, snow, hail, or sleet), entering the soil through cracks, holes, and openings between the soil particles. As the water enters, it pushes the air out. Oxygen is taken up by roots for respiration. If air is unavailable for too long, the roots will die.
Plants use some water, some is lost by evaporation, and some moves so deep into the soil that plant roots cannot reach it. If it rains very hard or for a long time, some of it is lost through surface run-off.
When organic matter decomposes in the soil, it gives off carbon dioxide. This carbon dioxide replaces some of the oxygen in the soil pores. As a result, soil air contains less oxygen and more carbon dioxide than the air above the soil surface. Carbon dioxide is dissolved by water in the soil to form a weak acid (carbonic acid). This solution reacts with the minerals in the soil to form compounds that can be taken up and used as foods by the plants.
Plant NutrientsTop
Plants need 18 elements for normal growth. Carbon, hydrogen, and oxygen (come from air and water). Nitrogen is a major plant constituent. Although the atmosphere is 78% nitrogen, it is not directly available for plant use. However, certain bacteria that live in nodules on the roots of legumes are able to fix nitrogen from the air into a form available to plants. Beans, peas, and Mesquite and Acacia trees, and alfalfa, are examples of legume plants.
The other 14 essential elements are iron, calcium, phosphorus, potassium, copper, sulphur, magnesium, manganese, zinc, boron, chlorine, cobalt, nickel and molybdenum. These elements come from the soil.

With the exception of nitrogen, phosphorus, and iron there is usually a large enough quantity of each of these elements in Arizona soils for cultivation of crops. Irrigation and rain water also can contain ample amounts of some essential plant nutrients.

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