WHAT SOILS ARE 17 Ries Within Itself a Record of Its History for Those Who Learn to Read It
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WHAT SOILS ARE 17 ries within itself a record of its history for those who learn to read it. A SOIL PROFILE consists of two or What Soils more layers lying one below the other and parallel to the land surface. The layers are known as horizons. The Are horizons dififer in one or more prop- erties such as color, texture, structure, Roy W. SImonson consistence, porosity, and reaction. Soil horizons may be thick or thin. Soil is continuous over the land Some are no more than a fraction of an inch. Others are several feet thick. surface of the earth, except for Few horizons are at either extreme. the steep and rugged mountain Generally they merge with one an- other and lack sharp boundaries. peaks and the lands of per- Horizons in a profile are like the petual ice and snow. parts of a layer cake without the clear bands of frosting between them. Soil is related to the earth much as Most soil profiles include three mas- the rind is related to an orange. But ter horizons, identified by the letters this rind of the earth is far less uniform A, B, and G. Some soils lack a B hori- than the rind of an orange. It is deep zon and are said to have AG profiles. in some places and shallow in others. When a soil is used without proper It may be red, as soils are in Hawaii, care, the A horizon and even the B or it may be black, as they are in North horizon may be eroded away. Dakota. It may be sand, or it may be The combined A and B horizons are clay. called the solum, sometimes the "true Be it deep or shallow, red or black, soil." Together they form the major sand or clay, the soil is the link between part of a profile. They are also direct the rock core of the earth and the living results of the processes by which soils things on its surface. It is the foothold are formed. for the plants we grow. Therein lies the All of the master horizons may be main reason for our interest in soil. subdivided in the scientific study of The soil mantle of the earth is far soils. Such subdivisions are identified from uniform, but all soils have some by the proper letter plus a subscript things in common. number, thus: Ai, A2, A3, Bi, B2, 63- Every soil consists of mineral and The subdivisions of master horizons organic matter, water, and air. The provide clues to the processes of soil proportions vary, but the major com- formation and are important to the use ponents remain the same. and management of soils. Every soil occupies space. As a The A horizon, the uppermost layer small segment of the earth, it extends in the soil profile, often is called the down into the planet as well as over surface soil. It is the part of the soil in its surface. It has length, breadth, and which life is most abundant in such depth. forms as plant roots, bacteria, fungi, Every soil has a profile—a succession and small animals. It is therefore the of layers in a vertical section down part in which organic matter is most into loose weathered rock. The nature plentiful. of the soil profile has a lot to do with Because it lies at the surface, the A the growth of roots, the storage of horizon is also the part of the soil that moisture, and the supplies of plant falling rain reaches first. Hence it nor- nutrients. The profile also is basic to mally is leached more than are the scientific studies of soil. The profile car- deeper horizons. Most A horizons have 406157°—57 3 i8 YEARBOOK OF AGRICULTURE 1957 /. A single area of a soil type as it occurs in nature. At the right is an enlarged sketch of the profile with its major horizons. lost soluble substances. Many have also the subsoil. Lying between the A and lost some clay—mineral particles finer C horizons, it partakes of the proper- than a pinpoint. (About lo thousand ties of both. Living organisms are fewer clay particles of average size laid end than in the A horizon but more abun- to end would equal i inch.) They may dant than in the C horizon. Color is also have lost iron and aluminum ox- often transitional between those of the ides, which soil scientists generally call A and C horizons. The B horizon gen- sesquioxides. Iron oxide is the familiar erally is harder when dry (and stickier rust on an old piece of steel. Aluminum when wet) than its neighbors. It is fre- oxide is the dark tarnish on aluminum quently higher in clay than either of kettles. them. It may have a blocky or pris- Two subdivisions of the A horizon matic structure, usually combined with are common in soil profiles, although greater firmness. Concentrations of only one of the two may be present in iron oxide or aluminum oxides or both, a given profile. If a soil has been usually in combination with organic formed under prairie vegetation, as in matter, mark B horizons of some soils. the Corn Belt, it has a thick, dark Aj and lacks an Aj horizon. If the soil THE C HORIZON is the deepest of the were formed under forest cover in the three major horizons. It consists of the same region, the A horizon has two upper part of the loose and partly de- distinct subdivisions, a thin, dark sur- cayed rock beneath the A and B hori- face layer (the Aj horizon) and a much zons. The rock material in the C hori- thicker, lighter colored layer beneath zon is of the same kind as that which it (the A2 horizon). The Aiand Ajhori- now forms the bulk of the soil above it. zons can be recognized in many un- The C horizon therefore is said to be eroded soil profiles in humid regions, the parent material of soils. It may but both exist in few soil profiles of dry have accumulated in place by the regions. breakdown of hard rock, or it may have The B horizon lies immediately be- been moved to where it now is by neath the A horizon and often is called water, wind, or ice. The C horizon has WHAT SOILS ARE 19 less living matter than overlying ones con, aluminum, oxygen, and hydro- and is therefore lower in organic mat- gen, but differ in atomic structure from ter. It is commonly lighter in color feldspars. than the A and B horizons. The G hori- Clay minerals may continue to be zon in most soils is more like the B than formed and oxides released within a the A horizon. Some profiles, however, soil as long as it exists. Minerals nor- lack B horizons. Such profiles usually mally continue to decompose in a soil consist of faint or distinct A horizons profile long after the distinct horizons grading downward into C horizons. form. Some processes in horizon differ- The differences between the A and C entiation, however, may begin only horizons may then be small, especially after there is a deep mantle of loose if the A horizon is faint. weathered rock. The two main steps in The master horizons and their sub- soil formation thus merge with one divisions, recognized in scientific stud- another. ies of soils, are shown in the second diagram. This hypothetical profile can- STAGES in each of the major steps of not be found in nature. All the hori- soil formation, like the steps themselves, zons and subhorizons in it do not exist are far from being distinct. in any actual soil. Yet some of the hori- The accumulation of soil parent ma- zons are part of every soil on earth. terials follows from the weathering of Moreover, the kinds and arrangement rocks, which is slow, gradual, and con- of horizons in a profile are a record of tinuous. It proceeds little by little from what has happened to that soil since the time that the first changes take it began to form. This history has place in the solid rock mass. Many meaning to the fertility, tilth, and pro- changes normally occur in a rock be- ductivity of soils for plants useful to fore it disintegrates. Decomposition of mankind. minerals usually proceeds long after a rock has disintegrated. Weathering SOIL FORMATION proceeds in steps continues without any sharp breaks. and stages, none of which is distinct. Thinking of it in stages is simply a way They are like the overlapping fibers in of looking at a continuous process one a piece of string—the eye can hardly piece at a time. tell where one fiber ends and another Horizons in soil profiles may be con- begins. Similarly, it is not possible to be sidered faint, distinct, and prominent, sure where one step or stage in soil for- although the stages in their difí'erenti- mation stops and another starts. ation are not clearly defined. Two or The two major steps in the forma- more faint horizons first appear when tion of soils are accumulation of soil a soil profile begins to form. The earli- parent materials and differentiation of est changes are small, and the horizons horizons in the profile. Each step can therefore are faint and hard to distin- be thought of as consisting of several guish. If conditions are favorable, these stages, which are hard to tell apart.