Groundwater as a Geomorphic Agent

Binghamton Geomorphology Symposium 13

R.G. LaFleur

• Éditeur: Routledge
• Année: 2020
• ISBN:
  9781000045680
• Édition: 1
• Sujet: Physical Sciences
• Sous-sujet: Geography

This book, first published in 1984, has both a geomorphic and a hydrologic message. It examines and analyses the role of groundwater in landscapes in a series of articles by authors of diverse backgrounds and experience.

1

Rates of soil
formation

John E. Foss and Antonio V. Segovia

Introduction

The rate of soil formation, or “soil age,” is of interest to many scientists because of the interpretations that can be made about the general age of landscapes and the various weathering processes occurring during soil development. Specifically, the rates of soil formation and the resulting soil profiles have been used in dating geologic events, interpreting archeological sites, evaluating geomorphic processes, determining past climatic conditions, and establishing tolerance rates (T factors) for agricultural erosion. In many cases, the preceding interpretations have been limited by the complexity of the soil formation process and lack of precise data on soil age for various geologic materials, landscapes and environments. Recently, however, numerous studies have elucidated the relationship between soil morphological, chemical, physical and mineralogical properties and soil age. This paper will describe the general aspects of soil formation and give specific examples of soil age studies on various geologic materials in New Jersey, Maryland, Virginia, Georgia and South Carolina.

General aspects

The formation of soil from geologic material is the result of complex, dynamic processes whereby changes are continually taking place. In soils on old landscapes, the changes may take place very slowly, but in young geological materials (e.g. glacial till or alluvium), changes may occur rapidly as vegetation becomes established and as environmental factors influence weathering of the materials. In most cases, 2000–3000 years are necessary to show significant profile development in humid temperate regions although organic matter accumulation or structural development may take place in 50 years or less.

Figure 1.1 illustrates the general process of soil formation from two types of geologic material. In the first case, soil has developed from limestone with the resulting soil profile showing A, B and C horizons. A soil profile 2 m thick may have required as much as 20 m of limestone to yield the parent material (non-carbonate mineral impurities such as silicates or iron and magnesium oxides, etc.) for this soil. The ratio of limestone to residual soil may vary depending on the impurities (non-carbonates) in the limestone. In a soil developed from loess, the profile shows A, B and C horizons resulting from soil formation processes and little change in volume from the initial material.

The rate of soil development and thickness of the profile will be greatly influenced by the nature of the geologic material. For example, soil developing in granite for 10 000 years in a cool, humid environment may be only a few centimeters thick, whereas a soil developed from loess for the same length of time may be 1 or 2 m thick. Figure 1.2 (Hall et al. 1982) shows some examples of soil thickness and length of weathering (soil age).

Figure 1.1 Generalized diagram showing conversion of geologic material to soil.

Figure 1.2 Diagram illustrating thickness of solum under different ages of soil and weathering environments (Hall et al. 1982). Data compiled from (1) Hay (1960), (2) Ahmed et al. (1977), (3) Foss (1974), Foss et al. (1978), (4) Leneuf and Rubert (1960), and (5) Owens and Watson (1979).

Although many interrelated processes and factors are involved in soil genesis, researchers have identified specific factors and processes dominating soil development. Figure 1.3 summarizes some of these major factors and processes. The factors of soil formation regulate the overall reactions occurring in the soil system whereas the specific processes (such as additions and losses) result in horizon formation.

Factors of soil formation

The factors of soil formation approach to soil genesis has been useful in explaining variability of soil systems. The equation (after Jenny 1941) for expressing the relationship between a soil or its properties and the soil formation factors is

S = f(C, O, G, R, T)

where S = an entire soil system or body or a single soil property, f = function of, C = climate, O = organisms, G = geologic or parent material, R = relief or topography and T = length of time for weathering. These factors explain the distribution of certain types of soils (e.g. prairie, strongly weathered or fragipan soils) or the variation in a single soil property such as pH, color or organic matter content.

The importance of time available for weathering is readily apparent in the characteristics of soils developed on different ages of geologic material. For example, soils developed on young alluvial sediments (< 200 years) will have little horizonation and show little effect of pedogenic processes, except for organic additions. In contrast, those soils developed on loess, till or coastal sediments of Pleistocene age in humid environments will generally show good horizonation and abundant evidence of pedogenic weathering processes. Certain diagnostic horizons are associated with the length of time for weathering. Weak argillic horizons, e.g., have been found to develop after 4000 years on alluvial sediments near Front Royal, Virginia (Foss 1974). Cambic horizons will develop in 50–100 years on river dredgings in Washington, D.C. (Stein 1978). In Pennsylvania, cambic horizons can develop in <200 years on alluvial sediments (Bilzi & Ciolkosz 1977).

Figure 1.3 Generalized diagram illustrating factors and processes involved in soil genesis. The processes of additions, losses, transformations and translocations have been described previously by Simonson (1959).

The increase in organic matter content of soils appears to be one of the first recognizable features of soil formation. Hallberg et al. (1978) found organic matter accumulation in the upper 10 cm of a 100 year old soil in Iowa to be equivalent to adjacent soils dated at 14000 years. Other studies have also demonstrated the rapid accumulation of organic matter in surface horizons in <60 years ...