purely descriptive definition of the C horizon to include the concept of parent material. They define the C horizon as parent material. In selecting lower strata of a soil profile as parent material, the implicit assumption is made that the A and B horizons were derived from material that is identical with that of the C horizon. Numerous cases are known where such an approach would lead to erroneous results. If a shallow layer of loess is deposited on diorite rock, A and B horizons may readily form in the loessial material, whereas the igneous substratum may remain practically unaltered. It would be fallacious to designate the diorite as C horizon. Or, let us suppose that a chernozem soil is transformed into a podsol as a consequence of drastic climatic changes. Should we give the attribute parent material to the C horizon of the podsol or to the original chernozem? In view of these logical and practical difficulties, we prefer to define parent material as the initial state of the soil system and thus avoid special reference to the strata below the soil, which may or may not be parent material. The selection of the initial state of the soil system follows most logically from considerations of time functions (see page 45).
In the introductory chapter, soil was defined in terms of its properties as follows:
F(cl', o', r', s1, s2 , s3, • • • ) = 0 (2) Accordingly, parent material, herein defined as the initial state of the soil system, and designated as π, may be mathematically
expressed as follows:
F(cl' 0, o' 0, r' 0, s1' 0, s2' 0, s3' 0, • • • ) = 0 (8) The two equations are identical except that the affix zero indicates that in the case of parent material we are dealing with the initial value (at zero time of the soil formation) of each soil property.
As Overstreet* points out, Eq. (8) may at times lead into difficulties, because, logically, it would not permit comparative studies of rock weathering in widely different climatic regions. We might wish to investigate the decomposition of two granites of
* Private communication.
identical properties, one placed in the arctic region, the other near the equator. In those localities, the two rocks are, strictly speaking, not identical, because they differ, among other things, in temperature, which affects some of their properties, such as the refractive indexes of the mineral components. To overcome this shortcoming, we shall introduce an extension of Eq. (8) by stating that parent material is the initial state of the soil system referred to some arbitrary standard state (e.g., normal temperature and pressure). Thus, the two granites mentioned above become identical if referred to the arbitrarily chosen temperature of 18°C. Parent material so defined will be designated as p. In practice, the distinction between π and p is rather subtle and, in most cases, may therefore be neglected.
Determination of Parent Material.—It is only under special circumstances that the lower strata of a soil profile permit an exact quantitative evaluation of parent material as defined in the previous section. The exact composition of the initial state of a soil may be determined if the history of the soil, or, more precisely, the time functions of its properties are known. Such studies are restricted to soil families of known age. Another avenue of approach is given by soil-climate relationships, particularly soil-moisture functions. An illustration will be presented on page 121 in which the lime-rainfall curves of Alway are extrapolated to zero rainfall. The resulting value of 17.2 per cent CaO represents the calcium content of the parent material. Naturally, the reliability of such a determination depends entirely on the degree of accuracy of specific soil-property-climate functions. Generally speaking, the exact evaluation of the composition of the parent material involves considerable speculation and is the source of much uncertainty in the elucidation of soil-forming processes.
Weathering and Soil Formation.—Some of the leading pedologists of today lay great stress on the distinction between weathering processes and soil-forming processes. The former are said to be geologic; the latter are pedologic. Weathering includes solution, hydrolysis, carbonization, oxidation, reduction, and clay formation.
Among the soil-forming processes, the following are listed:
calcification, podsolization, laterization, salinization, desalinization, alkalization and dealkalization, formation of peat and poorly drained soils, including gleization (33). Since these are merely special types of chemical processes, their separation into geologic and pedologic groups appears neither convincing nor fruitful. The entire issue might be dismissed as being of purely academic interest, were it not for the fact that a practical consequence is involved, namely, the
determination of parent material.
Pedologists who distinguish between geologic and pedologic processes do not regard granite, basalt, limestone, and consolidated rocks in general as parent materials. They maintain that only the weathered portions furnish material for soil-building purposes, and only these deserve the name "parent material." To mention a case in point, it has been contended, that the relationship between clay content of soil and annual temperature, to be discussed on page 150, refers to a geologic and not to a pedologic problem, because it deals with the formation of parent material rather than of soil. Whatever the merits of this new approach may be, it frustrates the climatic theory of soil formation. Since weathering is controlled by moisture and temperature, it follows that the formation of parent material also becomes a function of climate. Parent material could no longer be treated as an independent variable and therefore would cease to be a soil-forming factor. All soil property-climate functions to be discussed in the following chapters become meaningless, because
they would have to be considered as the result of complex
combinations of soil-forming and parent-material-forming processes.
A most serious difficulty for the drawing of sharp distinctions between weathering reactions and soil-forming processes is presented by the continuation of weathering during soil development. Not only soil but also parent material would become a function of time. We could no longer speak of soil-maturity series (e.g., Shaw's family, Bray's sequences), because such a concept presupposes constancy of parent material. It is true, of course, that for a given development series such as
Rock —> Weathered rock —>Immature soil —> Mature soil our definition of parent material as the initial state of the soil system permits the choice of either rock or weathered rock as the starting point. But, once the initial state has been chosen, it must be treated as a constant and not as a variable. Such is possible only if climatically controlled weathering reactions are included among the soil-forming processes.
Should an arbitrary differentiation between geologic and
pedologic processes be insisted upon, the functional definition of soil, given in Chap. I, would readily lend itself to such purpose. We may formulate: Any reaction taking place in soils that is functionally related to soil-forming factors [Eq. (4)] is a soil-forming process.
Accordingly, natural phenomena such as volcanic eruptions, depositions of loess, and sedimentation in lakes and rivers are not soil-forming processes. They build up parent material and are classified as geologic phenomena. Likewise, any chemical reaction occurring in rocks that is not conditioned by Eq. (4) is arbitrarily excluded from the field of pedology.
Difficulties Encountered in Measuring Soil-Parent Material Functions.—For the study of the factor parent material the following equation is used
S = f (p) cl, o, r, t ,. . . (9) The greatest obstacle in evaluating quantitatively Eq. (9) lies in the task of assigning numerical values to different types of parent materials. We have no way of recording the properties of a diorite or of a sandstone in a single comparable figure. Such a number would have to embrace chemical composition, mineralogical constitution, texture, and structure of a rock. All we hope for in functional analysis of parent material is a correlation between soil properties and specific rock properties such as lime content, permeability, etc.
A notable attempt in this direction has been made by Prescott and Hosking (23), who correlated the mean clay content (from 0 to 27 in.
depth) of red basaltic soils from eastern Australia with the mineralogical composition of the parent basalt. Rocks that were composed of from 51 to 58 per cent feldspars (orthoclase, albite, and anorthite) yielded soils containing from 54 to 55 per cent clay. If the total feldspar percentage rose to from 62 to 68 per cent, the content of clay amounted to from 63 to 76 per cent. The true nature of Prescott and Hosking's correlation is somewhat masked by variations in rainfall values.
In his report "Soils of Iowa," Brown (2) has established quantitative relationships between soil nitrogen, soil organic matter, and parent material. The data presented in Table 10 pertain to the Carrington series, a group of soils derived from glacial till, which to some extent has been modified by the presence of loess. Variations in
the composition of this parent material are expressed by differences in soil texture. For surface soils, we may write the following equation:
Nitrogen = f (texture) cl, o, r, t, . . . (10) The constant soil-forming factors cl, o, r, and t are specified as follows:
cl = climate of Iowa (approximately identical for entire series), o = prairie (now cultivated),
r = gently undulating to rolling,
t = unknown, but presumably the same for the entire series.
In the foregoing equation it is assumed that the texture of the surface soil defines the parent material of the surface soil. Brown's comparisons are summarized in Table 10. Both nitrogen and carbon increase as the soils assume a heavier texture. The mean values for N and C are significant inasmuch as the variability within soil types is considerably less than between textural groups.
Types of Parent Materials.—In Figs. 28, 29, and 30 are shown the distribution of various types of parent materials within the United States. These maps were constructed from Marbut's "Distribution of
TABLE 10.—THE AVERAGE NITROGEN AND CARBON CONTENT OF SOIL
TYPES OF DIFFERENT CLASSES OF THE CARRINGTON SERIES
(Surface soils, 0 to 6 ⅔ in. depth)
Nitrogen, Organic carbon,
Texture of surface soils per cent per cent C/N
Sand 0.028 0.40 14.1
Fine sand 0.043 0.58 14.5
Sandy loam 0.100 1.25 12.5
Fine sandy loam 0.107 1.32 12.5
Loam 0.188 2.21 12.2
Silt loam 0.230 2.68 11.7
parent materials of soils" in his Soils of the United States in the Atlas of American Agriculture, Part III. According to Marbut,
No attempt has been made to make it accurate in detail. In considerable areas, there may be some legitimate difference of opinion as to the source and character of the materials, such, for example, as on the plains of southern Idaho and parts of central Oregon and Washington. In central Texas, the western part of the area of residual accumulations from sandstones and shales contain areas of Great Plains materials and sands. The distribution of loess has been extended over areas about which there is no universal agreement.
Notwithstanding these and many other areas of detail about which there is no universal agreement, the maps represent a mass of useful information.