Geological materials exposed on the surface of the Earth will almost immediately begin to change in response to a variety of processes collectively referred to as "weathering". Weathering processes can be generally classified as one of two types, either "chemical" or "physical". Chemical weathering results in the destruction of the original minerals found in the rock and, from the remnants of these chemical compounds, the creation of new minerals. These new minerals, called "secondary" minerals, are relatively stable under surface conditions and tend to accumulate. Examples of these stable secondary minerals common in this area are clay and iron oxide (producing red colors). Physical weathering does not modify the mineral content of rock, but simply breaks the rock down into ever smaller fragments. Together, these small fragments of rock and the new minerals formed by chemical weathering accumulate on the Earth's surface to form a layer of "rotten rock" called the regolith.
The regolith mantles the majority of the land surface of the Earth, with the exception of areas where erosion is active enough to result in the exposure of bare bedrock. The regolith varies in thickness from a few centimeters to hundreds of meters depending on local geologic circumstances. The upper portion of the regolith is a component of one of the most interesting and important portions of the Earth : the soil. The soil is the intersection between all the major zones of the Earth. It is formed in the weathered lithosphere (regolith), the rocky portion of the Earth. It touches, and mixes with, the Atmosphere, the gaseous portion of the planet. It contains, due to rainfall, a portion of the Hydrosphere, the liquid portion of the Earth. Finally, it is the home for the majority of the Earth's Biosphere, the living portion of the planet. This combination of the four major zones results in the soil being the most dynamic, most active natural system on the Earth. In addition, the great majority of human activities, such as agriculture, construction and even just standing around, take place on the soil surface.
Soil Physical Properties: soil texture.
Soils are extremely diverse in terms of their appearance and properties. To help classify different types of soils we rely primarily on their "physical properties". Of these, one of the most important is soil texture.
Soil texture is a measure of the various particle-sizes found in the soil. Soil particle sizes are defined in terms of their diameters, as given in the following table:
|Gravels||> 2 mm|
|Sands||2.0 - 0.5 mm|
|Very coarse sand||2.0 - 1.0 mm|
|Coarse sand||1.0 - 0.5 mm|
|Medium sand||0.5 - 0.25 mm|
|Fine sand||0.25 - 0.10 mm|
|Very fine sand||0.10 - 0.05 mm|
|Silts||0.5 - 0.002 mm|
|Clays||< 0.002 mm|
|Coarse clay||0.002 - 0.001 mm|
|Colloidal||< 0.001 mm|
A variety of techniques can be used to determine the percentages of the different particle-sizes found in the soil, ranging from ultra-sophisticated computer-driven laser devices down to your fingers. For our purposes, we will just consider the Sands, Silts and Clays, ignoring the more detailed subdivisions.
Once the percents of each particle-size present in the soil have been determined, these numbers are used to classify the soil texture as one of twelve different "textural classes" on the Soil Textural Triangle. The textural triangle is a graph with three axes: sands on the bottom, silt on the right and clay on the left. The percentages of sand, silt and clay are plotted on this graph to determine the textural class. For example, say that a soil sample consists of 30% sand, 40% silt and 30% clay. Find the 30% sand point on the bottom of the graph. See how 30% is indicated by the dotted line moving up to the left. Now find the 40% silt point on the right. The silt line is dashed, and moves down to the left. Finally, find the 30% clay line on the left side of the triangle.
The solid clay line runs straight across the graph. If you have done things correctly, all three of these lines will intersect at one point. This point is in the area designated as "clay loam", which is the textural class of this particular soil.Check your use of the triangle and your answer.