Soil Mineral Composition

This is the solid inorganic matrix, which consists of clay (crystalline mineral particles <2 ^m in size), silt (soil mineral particles of 2-50 ^m), sand (soil mineral particles of 50 ^m to 2 mm) and gravel (soil mineral particles >2 mm). Depending on the proportions of each mineral fraction, a soil is classified into textural types. This changes both the physical and chemical properties in the soil and consequently affects its biological properties. The physical and chemical properties of the mineral component are closely linked to soil texture and structure, two key factors in decomposition, nutrient release and fertility for plant growth. Texture refers to the percentage of clay-silt-sand proportions in the soil (Fig. 2.2). Various proportions have been given names, referred to as a soil texture types. Knowing the soil type is a useful indicator of how workable or malleable the soil is. Sandy soils are friable and clay soils are heavy and difficult to work, especially when wet. The structure of the soil refers to how these soil mineral components aggregate into larger units. The natural aggregates, called peds, vary in size and shape with depth and a variety of parameters (mineral soil chemical composition, mechanical and physical conditions, climatic regime and organic composition). The extent of plant rootlets, fungal hyphae, cell filaments and secretions from living organisms all affect the size of peds and soil structure (Fig. 2.3). Secretions and organic matter tend to make mineral soil components clump more and increase mean ped size and stability. The rootlets, hyphae and other filaments form a mesh through the peds and hold the peds together.

The properties of clays are different from those of larger mineral components. Rocks, stones, gravel, sand and silt are just incrementally smaller size fractions of primary minerals. They are the result of

Lutum Silt Zand

Fig. 2.2. Soil mineral particle composition and resulting soil texture. The soil texture triangle places soils into categories based on the percentage composition of clay, silt and sand. To read the triangle, find the intersection of any two components. An example is illustrated (dotted line) with a soil that was composed of 65% silt, 30% sand and 5% clay. This triangle was proposed by the Soil Survey Staff (USA) and has since come into general use (see Soil Survey Staff, 1998).

Fig. 2.2. Soil mineral particle composition and resulting soil texture. The soil texture triangle places soils into categories based on the percentage composition of clay, silt and sand. To read the triangle, find the intersection of any two components. An example is illustrated (dotted line) with a soil that was composed of 65% silt, 30% sand and 5% clay. This triangle was proposed by the Soil Survey Staff (USA) and has since come into general use (see Soil Survey Staff, 1998).

1 nm 10 nm 100 nm 1 ^m 10 |im 100 |im 1 mm 10 mm 100 mm clays silt sand gravel rocks large molecules bacteria hyphae yeasts nanoflagellates cercozoan lobose amoebae conose amoebae filopodia testate amoebae ciliates nematodes invertebrates root hair fine roots cells

Fig. 2.3. Size distribution of soil mineral particles and interstitial organisms (measurements along a logarithmic scale).

mechanical erosion (breaking and fragmenting) of the parent primary minerals. When primary minerals (such as quartz, feldspars, micas and ferromagnesians) are chemically weathered, they produce secondary minerals. This erosion of primary minerals is referred to as weathering. Secondary minerals mostly result from chemical reactions of the primary minerals with water and dissolved ions (hydrolysis, oxidation, hydration and dissolution) (Fig. 2.4). The secondary minerals which form are clays, and consist of crystalline aluminosilicates, other crystalline minerals and various free oxides, such as precipitates of the soluble crystals of monosilicic acid (H4SiO4). These minerals have crystalline or amorphous forms with chemical and physical properties different from those of the parent primary minerals. Clays have negatively charged sites with hydroxyl ions or oxygen and will adsorb cations such as K+, Mg2+, Ca2+, Fe3+ and Al3+ in their hydrated forms. The charge of some clays depends on the pH of the soil solution. In tropical soils and those derived from volcanic rock, the charges of clays are more variable. The hydrated cations that adhere on to the surface of clay particles are strongly held, especially if the clay anion charges are high. The hydrated cations further from the clay surface are held less strongly. Thus the former are not available to the soil water solution, but the more weakly held cations are exchangeable with soil water. Only the exchangeable cations which can be released to the soil water are available to soil organisms. This difference between firmly held and exchangeable cations on clay surfaces is described by the Gouy-Stern or Gouy-Chapman double layer model. The quantity of exchangeable cations held by clays is important to determine what proportion are available to living organisms. Clays also interact with the nucleic acids, amino acids, glycoproteins and polysaccharides from the litter (Cheshire et al., 2000).

Biotite mica (K, Fe, Mg)Si4O10

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