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.
Physical and Chemical Rock Decay.—Experimental studies on the physical disintegration of rocks have been conducted by Hilger in Erlangen (compare page 32). He found limestone to be most resistant.
Mica-schist particles having diameters of from 4 to 6 mm. rapidly broke into smaller fractions. Sandstones were the least stable materials.
Chemical rock decay has been repeatedly imitated in vitro.
Daikuhara (5) leached powdered rocks with carbonated water for a period of 12 weeks. The order of decomposition was found to be as follows:
Basalt > Gneiss > Granite > Hornblende-Andesite
Optical examination is a common means of ascertaining relative degrees of weathering of minerals. Unfortunately, the reports found in the literature are not consistent, and it is difficult to formulate general rules. Fair agreement seems to exist for the following comparisons:
1. Quartz is one of the most resistant minerals.
2. Among the feldspars the plagioclases, especially their basic representatives, weather more readily than the potassium feldspars.
3. Biotites decay more rapidly than muscovites.
4. Amphiboles (e.g., hornblende) are not so resistant as pyroxenes (e.g., augites).
The fundamental difficulty of ascertaining relative rankings of chemical decay, even under controlled conditions, lies in the nature of the weathering process itself. It is a surface phenomenon, and the first step may be considered as an ionic exchange reaction
In the foregoing hypothetical potassium mineral, the K is exchanged for H. The resulting H surface is likely to become unstable (O— of the lattice becomes OH—, which tends to alter the bond strength), and the surface layer partly peels off. To arrive at quantitative orders of rock disintegration, it would be necessary to
compare pieces with equal surface areas. Since we know of no reliable method to determine surface area of alumino-silicates, the weathering series mentioned above can have no claim to finality.
In nature, the extent of chemical rock decay may be very deceiving. Niggli's (20) decomposed gneiss of San Vittore-Lumino, Misox (T = 10.9°C, N S quotient = 624) has all the appearances of a completely weathered rock; yet chemical analysis can detect little change other than hydration and oxidation.
Significance and Behavior of Cations.—The nature of the elements released during rock decay has a specific bearing on soil formation. Silicon and aluminum furnish the skeleton for the production of clay colloids; iron and manganese are important for oxidation-reduction processes, and they strongly influence soil color;
potassium and sodium are dispersing agents for clay and humus colloids, whereas calcium and magnesium have high flocculating powers and assure soil stability.
Acid igneous rocks contain considerable amounts of quartz and are rich in monovalent cations, whereas basic igneous rocks are high in calcium and magnesium contents. One would expect that these chemical and mineralogical differences would be reflected in the trend of soil formation. Indeed, Hart, Hendrick, and Newlands (9) in their studies on the soils of Scotland found that, under conditions of identical climate and topography, the basic igneous rocks produce brown earth and the acid igneous rocks produce podsolized soils.
It is a common saying that acid igneous rocks give rise to soils of good physical condition, whereas soils from basic rocks possess favorable chemical characteristics that ensure abundant plant growth.
Aside from the fact that well-known exceptions exist (e.g., serpentine soils), it is probable that such belief originated in regions where chemical weathering is not pronounced. In warmer climates with extensive leaching and removal of bases, the chemical influence of the parent rock is likely to be less marked. Cobb's work seems to support this conclusion.
Cobb (4) has published chemical analyses of igneous rocks and soils derived therefrom that were collected in the North Carolina section of the Piedmont Plateau. He arranged the data in the form of two development series as follows:
Cobb's analyses, recalculated to molecular values, are plotted in Figs. 31 and 32.
The ba values, or the base-alumina ratio that reflects the leaching of potassium, sodium, and calcium, and the relative
FIG. 31.—Comparison of relative base status of soils derived from acid and basic igneous rocks. (Cobb's data.)
accumulation of aluminum, show perceptible differences only in the earlier stages of weathering and soil formation. Basic igneous rocks and young soils derived from basic igneous rocks have higher 6a values than the granites and gneisses. The older and more mature soils from various types of parent material have almost identical ratios.
The silica-alumina ratios follow an entirely different course.
Owing to the presence of free silica (quartz), the acid igneous rocks have higher ratios than the basic magmas. Inasmuch as quartz dissolves at an exceedingly slow rate, the two curves are at no point coincident, as is the case with the ba ratios.
FIG. 32.—Silica-alumina ratios of soils derived from acid and basic igneous rocks.
(Cobb's data.)
Chemical Composition of Surface Soils.—Jensen (12) has arranged data on the chemical composition (hydrochloric-acid extracts) of Australian soils according to the geological characteristics
of the parent material (Table 11). For the two major groups, granites and basalts, he found that basaltic soils are higher in lime and phosphoric acid than granitic soils. The potassium and nitrogen contents could not be definitely correlated with parent material.
Composition of Rocks and of Soil Organic Matter.—Soil formation and development of vegetation are concomitant. As the soil features change, the plant cover adjusts itself accordingly. In very immature soils, the situation is different. Within a given climatic region, the growth of vegetation is mainly determined by the character of the parent material, whether limestone, igneous rock, sand deposit, or clayey shale. Not only type of vegetation but its chemical
composition as well is affected. This influence is reflected in the decomposition products of the vegetational debris or, in other words,
TABLE 11.—EFFECT OF PARENT MATERIAL ON CHEMICAL COMPOSITION OF
AUSTRALIAN SOILS (Jensen) (Averages)
CaO, per cent P2O5, per cent N, per cent K2O, per cent Region
Gra- Basal- Gra- Basal- Gra- Basal- Gra- Basal- nitic tic nitic tic nitic tic nitic tic soil soil soil soil soil soil soil soil Southern
Tableland. 0.125 0.306 0.100 0.226 0.149 0.125 0.122 0.273 West central
Tableland 0.135 0.262 0.105 0.170 0.100 0.221 0.113 0.115 Northern
Tableland 0.211 0.241 0.104 0.192 0.104 0.207 0.159 0.122
in the constitution of soil organic matter. A good example of this is given by Leiningen's (16) analyses of alpine humus soils (Table 12).
Humus layers developed on igneous rocks are much lower in calcium, but higher in potassium and phosphorus than those of calcareous origin. The nitrogen content shows little difference between the two types of rocks.
TABLE 12.—COMPOSITION OF ALPINE HUMUS DEVELOPED ON LIMESTONE AND ON ACID IGNEOUS ROCK (Leiningen)
Limestone (nine Igneous rock (five Constituents samples), per cent samples), per cent
CaO 2.75* 0.665
(0.71-5.88) † (0.17-1.19)
K2O 0.053 0.171
(0.021-0.088) (0.126-0.317)
P2O5 0.121 0.219
(0.045-0.187) (0.155-0.264)
N 1.77 1.71
(1.46-2.29) (1.37-2.00)
* Average.
† Extreme values in parentheses.
C. SOIL FORMATION ON SEDIMENTARY ROCKS