What Soils Are

Roy W. Simonson.

Soil is continuous over the land surface of the earth, except for the steep and rugged mountain peaks and the lands of perpetual ice and snow.

Soil is related to the earth much as the rind is related to an orange. But this rind of the earth is far less uniform than the rind of an orange. It is deep in some places and shallow in others. It may be red, as soils are in Hawaii, or it may be black, as they are in North Dakota. It may be sand, or it may be clay.

Be it deep or shallow, red or black, sand or clay, the soil is the link between the rock core of the earth and the living things on its surface. It is the foothold for the plants we grow. Therein lies the main reason for our interest in soil.

The soil mantle of the earth is far from uniform, but all soils have some things in common.

Every soil consists of mineral and organic matter, water, and air. The proportions vary, but the major components remain the same.

Every soil occupies space. As a small segment of the earth, it extends down into the planet as well as over its surface. It has length, breadth, and depth.

Every soil has a profile a succession of layers in a vertical section down into loose weathered rock. The nature of the soil profile has a lot to do with the growth of roots, the storage of Moisture, and the supplies of plant nutrients. The profile also is basic to scientific studies of soil. The profile carries within itself a record of its history for those who learn to read it.

A SOIL PROFILE consists of two or more layers lying one below the other and parallel to the land surface. The layers are known as horizons. The horizons differ in one or more properties such as color, texture, structure, consistence, porosity, and reaction.

Soil horizons may be thick or thin. Some are no more than a fraction of an inch. Others are several feet thick. Few horizons are at either extreme. Generally they merge with one another and lack sharp boundaries.

Horizons in a profile are like the parts of a layer cake without the clear bands of frosting between them.

Most soil profiles include three master horizons, identified by the letters A, B, and C. Some soils lack a B horizon and are said to have AC profiles. When a soil is used without proper care, the A horizon and even the B horizon may be eroded away.

The combined A and B horizons are called the solum, sometimes the "true soil." Together they form the major part of a profile. They are also direct results of the processes by which soils are formed.

All of the master horizons may be subdivided in the scientific study of soils. Such subdivisions are identified by the proper letter plus a subscript number, thus: A₁, A₂, A₃, B₁, B₂, B₃. The subdivisions of master horizons provide clues to the processes of soil formation and are important to the use and management of soils.

The A horizon, the uppermost layer in the soil profile, often is called the surface soil. It is the part of the soil in which life is most abundant in such forms as plant roots, bacteria, fungi, and small animals. It is therefore the part in which organic matter is most plentiful.

Because it lies at the surface, the A horizon is also the part of the soil that falling rain reaches first. Hence it normally is leached more than are the deeper horizons. Most A horizons have lost soluble substances. Many have also lost some clay mineral particles finer than a pinpoint. (About 10 thousand clay particles of average size laid end to end would equal 1 inch.) They may also have lost iron and aluminum oxides, which soil scientists generally call sesquioxides. Iron oxide is the familiar rust on an old piece of steel. Aluminum oxide is the dark tarnish on aluminum kettles.

Two subdivisions of the A horizon are common in soil profiles, although only one of the two may be present in a given profile. If a soil has been formed under prairie vegetation, as in the Corn Belt, it has a thick, dark A, and lacks an A₂ horizon. If the soil were formed under forest cover in the same region, the A horizon has two distinct subdivisions, a thin, dark surface layer (the A, horizon) and a much thicker, lighter colored layer beneath it (the A₂ horizon). The A, and A₂ horizons can be recognized in many uneroded soil profiles in humid regions, but both exist in few soil profiles of dry regions.

The B horizon lies immediately beneath the A horizon and often is called the subsoil. Lying between the A and C horizons, it partakes of the properties of both. Living organisms are fewer than in the A horizon but more abundant than in the C horizon. Color is often transitional between those of the A and C horizons. The B horizon generally is harder when dry (and stickier when wet) than its neighbors. It is frequently higher in clay than either of them. It may have a blocky or prismatic structure, usually combined with greater firmness. Concentrations of iron oxide or aluminum oxides or both, usually in combination with organic matter, mark B horizons of some soils.

THE C HORIZON is the deepest of the three major horizons. It consists of the upper part of the loose and partly decayed rock beneath the A and B horizons. The rock material in the C horizon is of the same kind as that which now forms the bulk of the soil above it. The C horizon therefore is said to be the parent material of soils. It may have accumulated in place by the breakdown of hard rock, or it may have been moved to where it now is by water, wind, or ice. The C horizon has less living matter than overlying ones and is therefore lower in organic matter. It is commonly lighter in color than the A and B horizons. The C horizon in most soils is more like the B than the A horizon. Some profiles, however, lack B horizons. Such profiles usually consist of faint or distinct A horizons grading downward into C horizons. The differences between the A and C horizons may then be small, especially if the A horizon is faint.

The master horizons and their subdivisions, recognized in scientific studies of soils, are shown in the second diagram. This hypothetical profile cannot be found in nature. All the horizons and subhorizons in it do not exist in any actual soil. Yet some of the horizons are part of every soil on earth. Moreover, the kinds and arrangement of horizons in a profile are a record of what has happened to that soil since it began to form. This history has meaning to the fertility, tilth, and productivity of soils for plants useful to mankind.

SOIL FORMATION proceeds in steps and stages, none of which is distinct. They are like the overlapping fibers in a piece of string the eye can hardly tell where one fiber ends and another begins. Similarly, it is not possible to be sure where one step or stage in soil formation stops and another starts.

The two major steps in the formation of soils are accumulation of soil parent materials and differentiation of horizons in the profile. Each step can be thought of as consisting of several stages, which are hard to tell apart.

Some processes begin with the onset of rock weathering. Silicate clay minerals, for example, are formed and oxides are released from primary minerals, such as feldspars, before a rock disintegrates. Feldspars are minerals formed when molten lava crystallizes into hard rock, as happens deep in the earth. They consist of aluminum, silicon, and oxygen and one or more of calcium, potassium, and sodium. Silicate clay minerals are also mostly silicon, aluminum, oxygen, and hydrogen, but differ in atomic structure from feldspars.

Clay minerals may continue to be formed and oxides released within a soil as long as it exists. Minerals normally continue to decompose in a soil profile long after the distinct horizons form. Some processes in horizon differentiation, however, may begin only after there is a deep mantle of loose weathered rock. The two main steps in soil formation thus merge with one another.

STAGES in each of the major steps of soil formation, like the steps themselves, are far from being distinct.

The accumulation of soil parent materials follows from the weathering of rocks, which is slow, gradual, and continuous. It proceeds little by little from the time that the first changes take place in the solid rock mass. Many changes normally occur in a rock before it disintegrates. Decomposition of minerals usually proceeds long after a rock has disintegrated. Weathering continues without any sharp breaks. Thinking of it in stages is simply a way of looking at a continuous process one piece at a time.

Horizons in soil profiles may be considered faint, distinct, and prominent, although the stages in their differentiation are not clearly defined. Two or more faint horizons first appear when a soil profile begins to form. The earliest changes are small, and the horizons therefore are faint and hard to distinguish. If conditions are favorable, these horizons become distinct with the passing of time.

Additional horizons may also appear. Horizons change slowly and gradually as they become more and more distinct. These changes are like the steady upward movement of a barrel being rolled up an incline rather than like the jumps with which a boy goes upstairs.