Introduction to Soil Chemistry
Introduction to Soil Chemistry
Introduction to Soil Chemistry
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soil naming 17<br />
Under most conditions little organic matter is eluviated down through the<br />
soil. However, many soils have dark or black thick upper horizons, which are<br />
high in organic matter. These horizons are found in soils developing under<br />
high water conditions and grass vegetation where the grass and associated<br />
roots die each year and contribute organic matter <strong>to</strong> the profile <strong>to</strong> a depth of<br />
0.5m depending on the type of grass and other environmental conditions.<br />
When soil is saturated with water, the soil environment becomes reducing,<br />
and under these conditions iron is reduced <strong>to</strong> the ferrous [Fe(II)] state.The soil<br />
color becomes lighter and more yellow. Under reducing conditions soil also<br />
develops variations in color called mottling or gleying. Thus any soil horizon<br />
description, which includes a “g” designation, indicates that the soil is under<br />
reducing conditions for a significant period of time during the year. It might be<br />
expected that mottling or gleying would occur only in the lower horizons;<br />
however, it can occur anywhere in a soil profile, even quite near the surface.<br />
Iron in the ferrous state is more soluble than iron in the ferric state; indeed,<br />
in the ferric state it is insoluble in most soil conditions . Under reducing conditions<br />
ferrous iron may be leached out of soil, leaving it gray in color. This is<br />
the origin of the term “gleying”.<br />
Some types of vegetation and environmental conditions result in acid producing<br />
litter on the soil surface. Under these conditions organic matter decomposition<br />
products (humic, fluvic acids, etc.) can eluviate and be deposited<br />
deeper in the soil. In Figure 1.4 the litter in the Oi horizon produces acid,<br />
which allows the illuviation of aluminum, iron, and organic matter decomposition<br />
products in<strong>to</strong> the B horizons <strong>to</strong> form the Bhs horizon. The leaching of<br />
aluminum, iron, and organic matter out of an area of the soil profile results in<br />
the horizon becoming light gray or white, giving rise <strong>to</strong> the potential development<br />
of an albic horizon.<br />
To some extent the description of a soil profile gives an indication of some<br />
of the chemistry and chemical conditions occurring in that profile. This in turn<br />
provides the researcher and analyst with information about the types of compounds<br />
and species likely <strong>to</strong> be found and the conditions necessary <strong>to</strong> isolate<br />
them [3].<br />
1.4. SOIL NAMING<br />
Various different soil naming systems are used throughout the world. In all of<br />
these systems the horizons and their subdesignations vary somewhat according<br />
<strong>to</strong> the different classification systems. In the United States the United<br />
States Department of Agriculture (USDA) has developed the USDA <strong>Soil</strong><br />
Taxonomy system (simply referred <strong>to</strong> as <strong>Soil</strong> Taxonomy), which recognizes 12<br />
soil orders. The United Nations, through its Food and Agriculture Organization<br />
and United Nations Educational, Scientific and Cultural Organization<br />
(FAO-UNESCO) and the International Society of <strong>Soil</strong> Science, has a system<br />
that includes 26 soil groupings. There are many other systems, including those