soil

The word “soil,” like many common words, has several

meanings. In its traditional meaning, soil is the natural medium for the growth of land plants, whether or not it has discernible soil horizons. This meaning is still the common understanding of the word, and the greatest interest in soil is centred on this meaning. People consider soil important because it supports plants that supply food, fibres, drugs, and other wants of humans and because it filters water and recycles wastes. Soil covers the earth’s surface as a continuum, except on bare rock, in areas of perpetual frost or deep water, or on the bare ice of glaciers. In this sense, soil has a thickness that is determined by the rooting depth of plants.

Soil in this text is a natural body comprised of solids

(minerals and organic matter), liquid, and gases that occurs on the land surface, occupies space, and is characterised by one or both of the following: horizons, or layers, that are

distinguishable from the initial material as a result of additions,losses, transfers, and transformations of energy and matter or the ability to support rooted plants in a natural environment. This definition is expanded from the 1975 version of Soil Taxonomy to include soils in areas of Antarctica where parthenogenesis occurs but where the climate is too harsh to support the higher plant

forms.

The upper limit of soil is the boundary between soil and air,

shallow water, live plants, or plant materials that have not begun to decompose. Areas are not considered to have soil if the surface is permanently covered by water too deep (typically more than 2.5 m) for the growth of rooted plants. The horizontal boundaries of soil are areas where the soil grades to deep water,barren areas, rock, or ice. In some places the separation between soil and non soil is so gradual that clear distinctions cannot be made.

The lower boundary that separates soil from the non soil

Underneath is most difficult to define. Soil consists of the

Horizons near the earth’s surface that, in contrast to the

Underlying parent material, have been altered by the interactions Of climate, relief, and living organisms over time. Commonly,

Soil grades at its lower boundary to hard rock or to earthy

Materials virtually devoid of animals, roots, or other marks of Biological activity. The lowest depth of biological activity, However, is difficult to discern and is often gradual. For Purposes of classification, the lower boundary of soil is

Arbitrarily set at 200 cm. In soils where either biological activity or current pedogenic processes extend to depths

greater than 200 cm, the lower limit of the soil for classification purposes is still 200 cm. In some instances the more weakly cemented bedrocks (paralithic materials, defined later) have been described and used to differentiate soil series (series control section, defined later), even though the paralithic materials below a paralithic contact are not considered soil in the true sense. In areas where soil has thin cemented horizons that are impermeable to roots, the soil extends as deep as the

deepest cemented horizon, but not below 200 cm. For certain

management goals, layers deeper than the lower boundary of the soil that is classified (200 cm) must also be described if they affect the content and movement of water and air or other interpretative concerns.

In the humid tropics, earthy materials may extend to a depth

of many meters with no obvious changes below the upper 1 or 2 m, except for an occasional stone line. In many wet soils, greyed soil material may begin a few centimetres below the surface and,in some areas, continue down for several meters apparently unchanged with increasing depth. The latter condition can arise

through the gradual filling of a wet basin in which the A horizon is gradually added to the surface and becomes gleyed beneath.Finally, the A horizon rests on a thick mass of gleyed material that may be relatively uniform. In both of these situations, there is no alternative but to set the lower limit of soil at the arbitrary

limit of 200 cm. Soil, as defined in this text, does not need to have discernible

horizons, although the presence or absence of horizons and their nature are of extreme importance in soil classification. Plants can be grown under glass in pots filled with earthy materials, such as peat or sand, or even in water. Under proper conditions all these media are productive for plants, but they are non soil here in the sense that they cannot be classified in the same system that is used for the soils of a survey area, county, or even nation. Plants even grow on trees, but trees are regarded as non soil.

Soil has many properties that fluctuate with the seasons. It

may be alternately cold and warm or dry and moist. Biological activity is slowed or stopped if the soil becomes too cold or too dry. The soil receives flushes of organic matter when leaves fall or grasses die. Soil is not static. The pH, soluble salts, amount of organic matter and carbon-nitrogen ratio, numbers of microorganisms,soil fauna, temperature, and moisture all change with the seasons as well as with more extended periods of time. Soil

must be viewed from both the short-term and long-term

perspective.

Buried Soils

A buried soil is covered with a surface mantle of new soil

material that either is 50 cm or more thick or is 30 to 50 cm

thick and has a thickness that equals at least half the total

thickness of the named diagnostic horizons that are preserved in

the buried soil. A surface mantle of new material that does not have the required thickness for buried soils can be used to establish a phase of the mantled soil or even another soil series if the mantle affects the use of the soil.

Any horizons or layers underlying a plaggen epipedon are

considered to be buried.

A surface mantle of new material, as defined here, is largely

unaltered, at least in the lower part. It may have a diagnostic

surface horizon (epipedon) and/or a cambic horizon, but it has no other diagnostic subsurface horizons, all defined later.

However, there remains a layer 7.5 cm or more thick that fails the requirements for all diagnostic horizons, as defined later, overlying a horizon sequence that can be clearly identified as the solum of a buried soil in at least half of each pedon. The recognition of a surface mantle should not be based only on studies of associated soils.