Issues involved in soil erosion

The erosion of soil and rock by water, wind, and ice is a natural process, commonly accelerated by human activity. Geological erosion over long periods of time has provided the parent material from which soil can be built. Such erosion by water and the subsequent deposition in flood plains is the origin of productive alluvial soils, but if natural processes are extreme or very rapid, as in large-scale landslides, these can cause human tragedy.

Human activity has greatly accelerated the rate of natural erosion. Expanding populations and their demand for land-based resources has led to rapid conversion of forest and grassland to rural, urban, industrial, and other uses. Such change commonly removes the protective land cover under which soils slowly develop their pedological and biological characteristics. The degree of hazard induced by such change varies a great deal with climate and topographic factors, and with the type of soil material exposed. Inappropriate methods of forestry/land clearing or agricultural land management, or overgrazing of pasture or savannah lands can lead to accelerated rates of soil erosion, which can threaten or undermine the sustainability of such land use (Riquier, 1982). About 50 years ago, serious erosion of farmland in North America, Africa, and Australia, for example, was apparent through the development of large gullies, or dust storms of increasing frequency and intensity. Examples of the history of this recognition are given by Bennett (1939).

This book is written for the scientist who works with soils, plants, or water resources, as well as for the student of soil physics. We have attempted to allow for the differing backgrounds of the soil scientist, the agronomist, the chemist, the physicist, the hydrologist, or the civil engineer, who may be concerned with soil physics, by defining a number of terms in soil science, physics, and mathematics that may seem elementary to some but can be unfamiliar to others. Similarly, we have treated units in some detail because the International System (SI) of Units that we use is not yet universally adopted in the field of soil physics. Here the differences in background are likely to be national rather than professional. To help in this we have, in many of the diagrams and tables, presented data in non-SI units as well. Units and conversion factors are listed in Appendix A. Vector notation is introduced to show the concise way in which flow equations are often expressed in the literature of soil physics, but knowledge of vector analysis is not required.

The central connecting theme is the state of water in a porous material. This is introduced early in the book in order to set out the principles of water retention that are basic to most of the topics that follow. The physical properties of soil are broadly dealt with in the opening chapter to provide an outline of the nature of the porous material with which water interacts and through which it moves.

Description and classification of soil

The relatively thin mantle of soil over the land surface of the earth is a porous material of widely varying properties. Its solid phase consists of the inorganic products of weathered rock or transported material together with the organic products of the flora and fauna that inhabit soil. Some of these products are recognizable remnants in the form of stones, sand grains, and leaf litter, but others like clay minerals and humus result from the profound chemical changes that occur in both inorganic and organic material during the process of soil formation. The resulting soil can range in texture from coarse sands to fine clayey materials and it can range in its organic content from a usual amount that is less than 5 per cent by weight to about 80 per cent in peaty soil.

These and other properties can differ greatly from place to place over the earth's surface and from top to bottom through the succession of horizons or layers that constitute the soil profile. Hence, it is useful to be able to recognize some order in the occurrence of soil materials in nature. The profile, extending from the soil surface to a depth that includes the zone explored by plant roots, forms the basis for studying soil distribution systematically. For a given set of conditions affecting its development, the profile exhibits predictable properties.

The movement of water through the soil, as discussed in Chapter 4, can sometimes be regarded as being in the steady state. There can then be no time-dependent change in the water content of the soil and a study of any particular zone of the soil presupposes that the fluxes of water across the boundaries of that zone can be specified, and that inflow and outflow just balance. This approach has been particularly useful in developing drainage theory, as will be shown subsequently in Chapter 6. The accurate satisfaction of the steady-state criteria may not be necessary if the time scale of interest is so long that short-term variations in water content of the soil, and lack of balance between inflow and outflow at the boundaries, can be smoothed over and eliminated on a longer scale of time. Rapid movements of water into and through the soil, however, cannot be described satisfactorily by the steady-state approach. Infiltration of rainwater or irrigation water through the soil surface and the descent of a wetting front into relatively dry soil is one of the basic natural processes. The life-environment of terrestrial plants includes this zone of intermittent or cyclical wetting of the soil. In this chapter the theory of infiltration of water will be described, together with the subsequent redistribution of water from wet to dry soil.

In revising Soil Physics, we have retained the original framework of the book, keeping soil-water relations as the central theme. Much of the updating that has been necessary is in chapters that bear on field aspects of the subject where there has been much activity in research.

The intervening years have seen the wide adoption of SI units and the acceptance in the plant sciences of a terminology for water retention and movement that is closely allied to that used in soil physics. These welcome trends have made our task easier than it was when we prepared the first edition of this book.

We acknowledge our indebtedness to the libraries of Flinders and La Trobe Universities and of the Division of Soils, CSIRO.

Absorption of water by roots

The movement of water through the soil to the plant and then through the plant to the atmosphere is the main theme of this chapter. Water is taken in by roots through or past their epidermal cells. There is some uncertainty about the actual path that the water follows in its passage to the xylem system near the axis of the root. Many terrestrial plants possess a mechanism whereby the root cortex selectively admits solutes of the soil water required for their mineral nutrition. The electrolyte concentration of their xylem sap is usually much less than the concentration in the soil water whence it was derived and which, of necessity, fluctuates markedly with varying soil moisture conditions. Water in the protoxylem of the roots then moves axially along them to the stem xylem, which forms the main part of the vascular system of the plant. The leaves form the terminus for this transpiration system. The change from liquid water to vapour occurs in their substomatal cavities whence the water vapour diffuses out through the stomata and carbon dioxide diffuses in to its sites of uptake. When the loss of water lowers the total potential of the water in the leaves relative to that of the water in the root cortex a gradient is created that favours the upward movement of water through the soil-root-stem-leaf continuum (Slatyer, 1967; Kramer, 1983).

Concern for the quality of water has increased a great deal in the last several decades, both in the developed and in the developing world. Deteriorating water quality has been prejudicial not only to human health, but also to the conservation of healthy natural ecosystems; these two aspects are rather intimately related in some regions. Since much of the fresh water on land flows over or through the soil, in which it has a wide range of residence times, soil must play a key role in determining its quality. In this chapter, we provide some of the basic physical knowledge needed to understand the role of soil in affecting water quality. In addition to the naturally occurring soil salts, consideration is also given to contaminant chemicals that are products of industry and society.

Soil characteristics are directly involved in the fate of widespread or nonpoint sources of pollutants. Furthermore, the soil is often used as the site of disposal of the waste products of industry and society, which can accumulate to thick deposits, as in landfills. Surface waters, such as streams, lakes, and reservoirs, tend to receive contaminants from water flow over soil or from eroded soil itself. Leaching through the soil profile tends to be the major path for chemicals that pollute groundwaters. In the longer term, polluted groundwater can also affect surface water quality.

Soil chemicals vary greatly in characteristics which affect their transport. Adsorption of a chemical to the soil constituents, either the mineral or the organic matter, tends to make erosive transport the mechanism of major importance.

Introduction

The living plants and animals that inhabit soil depend upon a supply of water and nutrients, an exchange of gases between the soil and the atmosphere, and the maintenance of suitable soil temperatures. Also, if roots are to extend and expand and if organisms are to move through soil, their activities will be affected by the structure and strength of the soil. With the exception of water and nutrient supply, which will be covered in Chapter 14, these physical aspects of the environment of roots and soil organisms form the subject of the present chapter.

Roots provide water, nutrients, and anchorage to plants. Although they may occupy only about 1 per cent or less of the soil volume, their penetration into the soil can be thorough. Rooting density decreases with depth in a manner that depends on the type and age of the plant and the soil conditions, including water distribution, structure, and strength.

The ability of plants to exploit the water and nutrients contained in a soil horizon depends largely on the concentration of roots there. For example, deep rooting makes plants less vulnerable to periods of drought. Roots can grow from 1 to 50 mm per day depending on the type of plant and the soil environment of structure, strength, temperature, aeration, and water content. Longitudinal growth occurs in a region extending about 1 cm back from the root tip.

Storage of water in soil

Within any given period the change, ΔW, in the amount of water stored in a zone between the surface and any particular depth in the soil can be represented as

Here the amounts added as precipitation, P, and irrigation, I, are balanced against the amounts lost in runoff from the surface, A, in underground drainage, D, and in evaporation from soil and plants (evapotranspiration), E, during the given period. Values can be negative for A when water runs on to the surface and for D when water comes to the root zone from below. Quantities are usually expressed in mm.

The amount of water in a soil horizon is given as a depth by

where z is the thickness of the horizon and θ, its volumetric water content, is the depth of water per unit depth of soil. A soil could ideally retain about 200 mm of water for plant use to a depth of 1 m. When such a soil is thoroughly wetted at the time of seeding, it retains enough water to enable a crop to grow to maturity provided none is lost to the atmosphere except through the crop. This has been demonstrated for maize in USA by Harrold et al. (1959), and it serves to illustrate the scope for conserving water in soil. Similarly, irrigation in Egypt before the nineteenth century relied upon the annual flood of the Nile without any supplemental watering.

Consistency

The state of soil – whether it is solid, plastic or liquid – is referred to as its consistency. Soil in the plastic state has the property of flowing, after a yield or threshold stress has been exceeded. It differs from a Newtonian fluid such as water which requires no yield stress for flow to occur, and it differs from soil in the solid state in that, under stress, it yields without fracturing. These differences provide the basis for simple tests to find the water content at the upper (wetter) and lower (drier) limits of plasticity of a soil that contains sufficient fine material to show plastic behaviour. The tests devised by Atterberg (1911) and modified by Casagrande are described fully by Sowers (1965), American Society for Testing and Materials (1984), and British Standards Institution (1975). For these tests, soil samples from which particles larger than 2 mm in diameter have been removed are remoulded into a paste after wetting. In standard engineering methods only material smaller than 0.4 mm is used.

The lower limit of plasticity is determined by finding the water content at which soil, rolled into a thread 3 mm in diameter, begins to crumble. This was called the rolling-out limit by Atterberg but is now more usually called the plastic limit. The upper limit of plasticity of a soil (called its liquid limit) is determined either by finding the water content at which a groove formed in the soil will just close under the influence of standardised impacts, or by the method preferred by the British Standards Institution using a cone penetrometer.

In the preceding editions, the application of soil physics to agriculture, hydrology, and engineering has been treated as an important part of the book. This is carried further in the third edition in which new chapters have been added on soil erosion and on the transport of chemicals in soil. Again, the treatment relies upon the underlying physical processes.