Excerpts Introduction to soil physics book

Page 8

Under certain conditions, arid-zone soils may even accumulate the more readilysoluble salts of sodium (NaCl and Na2CO3) and of potassium. Such soilsare prone to excessive salinity, in extreme cases of which they become practicallysterile. Irrigated soils in poorly drained river valleys of arid regions areparticularly liable to undergo the process of salination. When such soils areleached of excess salts, they must be treated with soil amendments (such asgypsum) to replace the exchangeable sodium ions with calcium, lest thesodium ion cause dispersion of the clay and the breakdown of soil structure.

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The traditional method of characterizing particle sizes in soils is to dividethe array of particle diameters into three conveniently separable size rangesknown as textural fractions or separates, namely, sand, silt, and clay. Theactual procedure of separating out these fractions and of measuring their proportionsis called mechanical analysis, for which standard techniques havebeen devised. The results of this analysis yield the mechanical composition ofthe soil, a term often used interchangeably with soil texture.

An essential criterion for determining soil texture is the upper limit of particlesize that is to be included in the definition of soil material. Some soils containlarge rocks that obviously do not behave like soil, but, if numerous, might affectthe behavior of the soil in bulk. The conventional definition of soil materialincludes particles smaller than 2 mm in diameter. Larger particles are generallyreferred to as gravel, and still larger rock fragments, several centimeters in diameter,are variously called stones, cobbles, or — if very large — boulders.The largest particles that are generally recognized as soil material are designatedsand, defined as particles ranging in diameter from 2000 m (2 mm) to50 m (USDA classification) or to 20 m (ISSS classification). The sand fractionis often further divided into subfractions such as coarse, medium, and fine sand.Sand grains usually consist of quartz but may also be fragments of feldspar,mica, and, occasionally, heavy minerals such as zircon, tourmaline, and hornblende,though the latter are rather rare. In most cases, sand grains have moreor less uniform dimensions and can be represented as spherical, though they arenot necessarily smooth and may in fact have quite jagged surfaces (Fig. 3.2).That, together with their hardness, accounts for their abrasiveness.

The next fraction is silt, which consists of particles intermediate in sizebetween sand and clay. Mineralogically and physically, silt particles generallyresemble sand particles. However, since the silt are smaller, particles havea greater surface area per unit mass, and are often coated with strongly adherentclay, silt may exhibit, to a limited degree, some of the physicochemicalcharacteristics generally attributed to clay.The clay fraction, with particles ranging from 2 m downwards, is the colloidalfraction. Clay particles are characteristically platelike or needlelike in shapeand generally belong to a group of minerals known as the aluminosilicates. Theseare secondary minerals, formed in the soil itself in the course of its evolution from

the primary minerals that were contained in the original rock. In some cases,however, the clay fraction may include particles (such as iron oxide and calciumcarbonate) that do not belong to the aluminosilicate clay mineral category.

Because of its far greater surface area per unit mass and its resulting physicochemicalactivity, clay is the fraction with the most influence on soil behavior.Clay particles adsorb water and hydrate, thereby causing the soil to swell uponwetting and to shrink upon drying. Clay particles typically carry a negativeelectrostatic charge and, when hydrated, form an electrostatic double layerwith exchangeable ions in the surrounding solution. Another expression ofsurface activity is the heat that evolves when dry clay is wetted, called the heatof wetting. A body of clay will typically exhibit plastic behavior and becomesticky when moist and then cake up and crack to form cemented hard fragmentswhen desiccated.The relatively inert sand and silt fractions can be called the soil “skeleton,”while the clay, by analogy, can be thought of as the “flesh” of the soil.Together, all three fractions of the solid phase, as they are combined in variousconfigurations, constitute the matrix of the soil.

You are also required to read the following topics:

Electrostatic Double Layer Cation Exchange Capacity (CEC) and Ion Exchange (page 58 to 62 in Hillel’s Book)