Chemical Properties of Soil
Soil Colloids
Soil colloids are the finer size fractions of the soil (clay and organic matter), which are considered as the most chemically active portion of the soil because of their large surface area and the chemical structure of the materials involved. The liquid phase called soil solution is composed of dissolved inorganic minerals, salts, and organic compounds and is responsible for nutrient transport. Some of the colloidal complexes of soils are inorganic minerals, whereas others are organic colloids. The inorganic colloidal complex of soils is mostly fine clay particles that display thin plate-like bodies. The organic colloidal complex of soils is small particles of organic matter that are infrequently resistant to change and appear as humus. Because of their small size soil colloids have a large surface area per unit mass and its volume ratio.
Types of Soil Colloids
The colloids most important in soils can be grouped in four major types, each with its particular composition, structure, and properties. There are four major types of colloids present in soil.
Crystalline Silicate Clays
These clays are the dominant type in most soils (except in Andisols, Oxisols, and Histosols. Their crystalline structure is layered much like pages in a book. Individual units link together to form “sheets” of tightly bound and tightly bonded silicon, aluminum, and oxygen atoms, which combine to form the layers which give the clay its characteristic crystalline structure (Figure 5.1).
Non-Crystalline Silicate Clays
These clays also consist mainly of tightly bonded silicon, aluminum, and oxygen atoms, but they do not exhibit ordered, crystalline sheets. The two principal clays of this type, allophane and imogolite, usually form from volcanic ash and are characteristic of Andisols (Section 3.1). Allophane and imogolites have both positive and negative surface charges (i.e., variable charge), but their capacity to fix cations varies with pH.
Iron and Aluminum Oxides
These properties are present in many soils but are expressed in highly weathered subtropical and tropical soils (e.g., Alfisols, Ultisols, and Oxisols). These clays are usually amorphous, i.e., without a clearly defined shape or form.
Humus
Humus or organic colloidal fraction is important in nearly all soils, especially in the upper parts of the soil profile (Chap. 7). The humus colloids are not crystalline. They are composed basically of carbon, hydrogen, and oxygen rather than of silicon, aluminum, iron, oxygen, and hydroxyl groups. Humus colloids consist of modified lignins, polyuronides, and other constituents composed of carbon, hydrogen, and oxygen, along with minor amounts of nitrogen, phosphorous, sulfur, and other elements.
Properties of Colloids
Due to their minute size and chemical nature, colloids acquire some properties entirely different from the larger fraction of the soil. Certain characteristics also assume considerable importance such as size, shape, shape, and surface area; surface charge; plasticity; cohesion; swelling and shrinkage; and flocculation. The properties of soil colloids are all surface phenomena and their strength depending on the amount and nature of the interaction by the colloids.
Size, Shape, and Surface Area
The organic and inorganic soil colloids are extremely smaller in size less than 0.001 mm or less than 1micrometer in diameter. These particles cannot be viewed using an ordinary light microscope but can be visualized only with an electron microscope. The shape of colloidal clay particles is seen as spherical, crystalline, and or amorphous.
Surface Charge
Humus and clay mineral cations and anions are adsorbed on their surfaces because of permanent negative charges of clays formed by isomorphous substitution. For most soil colloids, negative charges dominate, although some mineral colloids in very acid soils have a net positive charge. The negatively charged soil colloids attract hundreds of positively charged ions or cations such as hydrogen (H⁺), aluminum (A13⁺), potassium (K⁺), sodium (Na⁺), calcium (Ca2⁺), and magnesium (Mg2+) to satisfy negative charges. These cations exist in the hydrated state (surrounded by a shell of water molecules), but for simplicity the cations (e.g., Ca2⁺ or H⁺) will appear as non-hydrated forms.
Plasticity
Plasticity is the capacity of clay particles to be easily molded when it is moist or wet. Soils that contain greater than 15 percent clay show a plasticity manner. This property may be due to the plastic-like nature of the clay particles.
Swelling and Shrinkage
Swelling is the process of expansion of crystal layers that occurs because of water moving between crystal layers. Swelling is considered by water attracted to ions and adsorbed by clays and water entered in soil pore spaces soils.
Flocculation
Flocculation refers to a process where soil elements are held together. Flocculation refers to a process during which soil particles dispersed in a solution contact and adhere each another, forming clusters, flocks, flakes, or clumps of a larger size.
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