Mechanical Structure

Tillage normally is used to modify the structural organization of the soil by mechanically loosening the soil surface. This loosening results in a reduction of bulk density and an associated increase in the porosity and hydraulic conductivity. Water retention is enhanced and root proliferation is encouraged to exploit available soil water and nutrients. Although beneficial, the physical soil properties associated with these freshly tilled surfaces are only temporary because rainfall and other mechanical actions tend to modify them. The rate at which the permeability, porosity, and surface structural change is a function of the soil texture, plant cover, previous management, and rainfall characteristics [55]. Amelioration of soil microstructure also can be achieved unnaturally by the application of soil structural conditioning substances.

Besides the addition of organic matter to the soil by crop-residue incorporation and organic manuring (force yard manure, organic wastes, sludges), which are converted to cementing humic substances by soil microorganisms, the main soil stuctural conditioners include the following:

• organic by-products,

• polyvalent salts,

• various synthetic polymers.

Polyvalent salts, such as gypsum, bring about flocculation of the clay particles, whereas the organic by-products and synthetic polymers act as cementing substances, binding the soil particles into more or less stable aggregates. Trivalent iron compounds may be included in the polyvalent salt group. Flotal and Glotal act as flocculating agents in slightly acid soil environments. They pass from sol to gel, changing pH and level of dehydration and can become stable cementing agents of soil particles [56].

Temporary stability can be achieved by using either oil- or rubber-based stabilizers or conditioners, or polyfunctional polymers, which develop chemical bonds with the mineral constituents of the soil. Although often too expensive for general agricultural use, they are helpful at special sites such as sand dunes, road cuttings, embankments, and stream banks, to provide a short period of stability prior to the establishment of a plant cover.

Soil structural conditioners fall under the following groups:

• those that render the soil hydrophobic and therefore decrease infiltration and increase runoff; and

• those that make the soil hydrophilic, favoring infiltration and, consequently, decreasing runoff.

For instance, hydrophilic conditioners based on bitumen, asphalt, and latex emulsion, can be used temporarily for controlling erosion and for soil sealing in relation to water harvesting. Other applications include the increase of infiltration rate and/or the prevention of crusting. However, the critical factor is that the size of aggregates that are produced may be so small that the infiltration rate remains low. A more effective improvement of the infiltration rate can be achieved with hydrophilic conditioners. In this case, the soil aggregates should be at least 2 mm and, ideally, larger than 5 mm [57].

Considerable interest has been shown in combining polyacrylamide conditioners with polysaccharides, which are biodegradable and swell in presence of water, thereby increasing the water-holding capacity of the soil.

Polyurea polymers contain a mixture of hydrophobic propylene oxides in a proportion determined according to the degree of hydrophilicity or hydrophobicity required. They have been used successfully, especially in dune stabilization [58].

Numerous other materials have been tested in soil stabilization and structural conditioning. Most are too expensive for generalized agricultural use, except with high-value crops such as vegetables and flowers.

Organic products, such as slurries and/or sludges, when free of inorganic and/or synthetic organic pollutants, could be disposed beneficially for soil improvement and environmental cleansing. Unfortunately, they are unpleasant to handle, create odors, and in the absence of special soil incorporation equipment, can be applied only in small quantities.

Other products obtained by processing different kind of wastes (including selected urban wastes) using either wet microbial processing or incineration, also can be used conveniently to give temporary stability to soil structure of road embankments and/or construction sites in urban areas.

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