NRES 201 Lecture 24 Outline of Last Lecture I Importance of soil temperature II Solar radiation and soil heating III Thermal properties of soils IV Soil temperature control Outline of Current Lecture I What is a colloid II Particle size and surface area III Importance of the clay fraction IV Clay minerals V Organic matter Current Lecture What is a colloid Any substance finely dispersed in a gaseous liquid or solid medium such that individual particles o are not visible in an ordinary microscope o do not settle out Examples Fog Smoke Milk Particle size and surface area Subdivision increases surface area Types of clay colloids Silicate colloids o Crystalline clay minerals Major type in most soils o Noncrystalline clay minerals Derived from volcanic ash Important in Andisols Nonsilicate colloids o Hydrous oxides of Fe or Al Important in Ultisols and Oxisols o Organic matter humus Most important in the A horizon These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Fundamental aspects of clay minerals Layered structures classified as phyllosilicates Two types of building blocks o Tetrahedral sheet o Octahedral Sheet Ionic adsorption o Negative charges attract positive ions cations o pH dependent charge Due to ionization of external OH groups Ionization increases with increasing pH Major source of charge for 1 1 minerals o Permanent charge Due to isomorphous substitution unaffected by pH major source of charge for 2 1 minerals Types of clay minerals Kaolinite o A 1 1 layer silicate o Unit layer charge 0 Montmorillonite o 2 1 layer silicate o Common in Illinois Soils Vermiculite o A 2 1 layer o Limited swelling o Common in Illinois Illite o A 2 1 layer o Low swelling o Common in Illinois Chlorite o 2 1 1 layer o Nonswelling o Common in Illinois mostly Alfisols Soil clays are usually intergrades with intermediate structures and properties Soil clays are often interstratified with vertical stacking of different minerals
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