Included in the clay grade, smaller than 0.002 mm (0.000,08 in), are mineral soil colloids. Colloid particles are so small that they cannot be seen by optical microscope and will remain suspended indefinitely in water. Individual particles are in the form of thin flakes in the size range 0.001 to 1.0 microns diameter. Finely divided humus provides another class of soil colloids, which may be referred to as humus colloids, or organic colloids.
Unusual chemical properties of colloids result from their very vast surface area for a given weight. Colloids have the property of being electrically charged and can therefore attract and hold ions, the unit chemical particles of dissolved substances. Ions of calcium, magnesium, and potassium are known in soil science as bases. These bases may be given up by the colloids to plants, which require them for growth, by a process known as base exchange. On the other hand, the hydrogen ion in the soil solution makes for an acid condition. The concentration of hydrogen ions in the soil solution, relative to hydroxyl ions, is known as the pH of the soil, and is a measure of soil acidity or alkalinity. Hydrogen ions are held also by the soil colloids in exchange positions. In soils a value of 7.0 in the pH scale is neutral, whereas values are below 7.0 (4.0 to 7.0) in acid soils but above 7.0 (7.0 to 10.0) in alkaline soils.
Soil colloids also are useful in holding water in the soil. When present in large quantities colloids may make the soil sticky and tough so that it is difficult to cultivate.
Soil structure refers to the way in which soil grains are grouped together into large pieces held together by soil colloids. Irregular pieces with sharp comers and edges give a blocky or nutlike structure. More or less spherical pieces make granular and crumb structure. Some soils have columnar and prismatic structure, made up of vertical columns or prisms 0.2 to 4 in (0.5 to 10 cm) across. Platy soil structure consists of plates, or flat pieces, in a horizontal position. Soil structure influences the rate at which water is absorbed by the soil, the susceptibility of the soil to erosion, and the ease of soil cultivation.
Another constituent of the soil is soil air, which occupies the pore spaces of the soil when it is not saturated with water. Soil air has been analyzed and found to contain an excess of carbon dioxide, but a deficiency of oxygen and nitrogen.
Soil water, the water temporarily held in the soil, is in reality a complex chemical solution. It is dilute solution of .such substances as bicarbonate, sulphates, chlorides, nitrates, phosphates, and silicates of calcium, magnesium, potassium, sodium, and iron.
The term soil profile denotes the arrangement of the soil into layer like horizons of differing texture, colour and consistency. Soils are recognized and classified into broad groups on the basis of the parts of the profile that are present. Basically there are three parts to the soil profile. Horizons A and B represent the true soil, or solum; horizon C is the subsoil, or weathered parent body. Below this is the parent bedrock or other underlying rock, designated as horizon D. The A horizon in humid climates is composed of two very different parts. The upper, or A', horizon is rich in organic matter and is dark coloured. The lower or A2, horizon is a zone of leaching. The B horizon is usually a zone of accumulation of soil colloids and is dark in contrast to the A2 horizon above it.
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