Composition of Soils and Soil Materials
Soils comprise a mixture of inorganic and organic components: Minerals, air, water, and plant and animal material. Mineral and organic particles generally compose roughly 50 percent of a soil’s volume. The other 50 percent consists of pores – open areas of various shapes and sizes.
Networks of pores hold water within the soil and also provide a means of water transport. Oxygen and other gases move through pore spaces in the soil. Pores also serve as passageways for small animals and provide room for the growth of plant roots.
1. Inorganic Material
The mineral component of soil is made up of an arrangement of particles that are less than 2.0mm in diameter. Soil scientists divide soil particles also known as soil separates, into three main size groups: sand, silt, and clay.
According to the classification scheme used by the United States Department of Agriculture (USDA), the size designations are sand, 0.05 to 2.00mm; silt 0.002 to 0.05mm and clay, less than 0.002mm.
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Depending upon the rock materials from which they were derived, these assorted mineral particles ultimately release the chemicals on which plants depend for survival, such as potassium, calcium, magnesium, phosphorus, sulfur, iron, and manganese.
2. Organic Material
Organic materials constitute another essential component of soils. Some of this material comes from the residue of plants – for example, the remains of plant roots deep within the soil, or materials that fall on the ground, such as leaves on a forest floor.
These materials become part of a cycle of decomposition and decay, a cycle that provides important nutrients to the soil. In general, soil fertility depends on a high content of organic materials.
Even a small area of soil holds a universe of living things, ranging in size from the fairly large to the microscopic: earthworms, mites, millipedes, centipedes, grubs, termites, lice, springtails, and more. And even a gram of soil might contain as many as a billion microbes – bacteria and fungi too small to be seen with the naked eye.
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All these living things form a complex chain: Larger creatures eat organic debris and excrete waste into the soil, predators consume living prey, and microbes feed on the bodies of dead animals. Bacteria and fungi, in particular, digest com organic compounds that up living matter and reduce them to simpler compounds that plants can use for food.
A typical example of bacterial action is the formation of ammonia from nitrogen compounds called nitrites, and still other bacteria act on the nitrites to form nitrates, another type of nitrogen compound that can be used by plants. Some types of bacteria can fix, or extract, nitrogen directly from there and make it available in the soil.
Ultimately, the decay of plant and animal material results in the formation of a dark-colored organic matter known as humus. Humus, unlike plant residues, is generally resistant to further decomposition.
3. Water
Soil scientists also characterize soils according to how effectively they retain transport water. Once water enters the soil from rain or irrigation, gravity comes into play causing water to trickle downward.
Water is also taken up in great quantities by the roots of plants: Plants use anywhere from 200 to 1,000kg of water in the formation of 1kg of dry matter. Soils differ in their capacity to retain moisture against the pull exerted by gravity and by plant roots.
Coarse soils, such as those consisting mostly of sand, tend to hold less water than soils with finer textures, such as those with a greater proportion of clays.
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Water also moves through soil pores independently of gravity. This movement can occur via capillary action, in which water molecules move because they are more attracted to the pore walls than to one another. Such movement tends to occur from wetter to drier areas of the soil.
The movement from soil to plant roots can also depend on how tightly water molecules are bound to soil particles. The attraction of water molecules to others is an example of cohesion. The attraction of water molecules to other materials, such as soil or plant roots, is a type of adhesion.
These effects, which determine the so-called metric potential of the soil, depend largely on the size and arrangement of the soil particles. Another factor that can affect water movement is referred to as the osmotic potential.
The osmotic potential hinges on the number of dissolved salts in the soil. Soils high in soluble salt tend to reduce the uptake of water by plant roots and seeds. The sum of the metric and osmotic potentials is called the total water potential.
4. Air
In soil, water carries out the essential function of bringing mineral nutrients to plants. But the balance between water and air in the soil can be delicate. An overabundance of water will saturate the soil and fill pore spaces needed for the transport of oxygen.
The resulting oxygen deficiency can kill plants. Fertile soils permit exchange between plants and the atmosphere, as oxygen diffuses into the soil and is used by roots for respiration. In turn, the resulting carbon dioxide diffuses through pore spaces and returns to the atmosphere. This exchange is most efficient in soils with a high degree of porosity.
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For farmers, gardeners, landscapers, and others with a professional interest in soil health, the process of aeration – making holes in the soil surface to permit the exchange of air-is a crucial activity. The burrowing of earthworms and other soil inhabitants provides a natural and beneficial form of aeration.
