Fundamentals of Soils

John Gerrard

• Editore: Routledge
• Anno: 2014
• ISBN:
  9781134688456
• Edizione: 1
• Argomento: Physical Sciences
• Categoria: Geography

Fundamentals of Soil provides a comprehensive and engaging introduction to soils and the workings of soil systems. This text is the only one of its kind to provide an attractive, lively and accessible introduction to this topic. Featuring learning tools within each chapter, such as summaries, essay questions and guides for further reading, the text is also highly illustrated with useful tables, boxes and figures. Covering all key areas of study at an introductory level, subjects covered include:
· Soil properties
· Soil processes
· Controls on soil formation
· Soil classification
· World soils
· Soil patterns
· Soil degradation.

1

The Nature of Soil

What is Soil?

There are many different opinions as to what constitutes soil, and there is no commonly agreed definition. Soil is the superficial covering of most of the land area of the Earth and varies in thickness from a few millimetres to many metres. One definition is that soil is weathered material at the Earths surface which may or may not contain organic matter and often also contains air and water. A more all-embracing definition is that soil is a natural body composed of minerals, organic compounds, living organisms, air and water in interactive combinations produced by physical, chemical and biological processes. This would be a genetic soil definition. The primary components of soil are inorganic materials, mostly produced by the weathering of bedrock or other parent material, various forms of organic matter, gas and water required by plants and soil organisms, and soluble nutrients used by plants. These constituents differ from the parent material in their morphology, physical, chemical and mineralogical properties, and their biological characteristics.

There is often confusion with the term regolith. According to Bates and Jackson (1987), ‘regolith’ is the layer or mantle of fragmented and unconsolidated rock material, whether residual or transported and of highly varied character, that nearly everywhere forms the surface of the land and overlies or covers the bedrock. It includes rock debris of all kinds, volcanic ash, glacial drift, alluvium, loess and other aeolian deposits, vegetal accumulations, and soil. Oilier and Pain (1996) use ‘regolith’ as a composite term for a variety of earth materials that may be closely related. These materials are weathered rock, which, if it is in place, is called saprolite, residuum, which is weathered material that has been disturbed and moved, transported surficial sediments, chemical products in the nearsurface environment, soils, and miscellaneous products, including volcanic ash and lag gravels.

Soil has a definite three-dimensional organisation. In a vertical direction, soil constituents may be arranged into horizons of mineral and/or organic constituents of variable thickness. A simple subdivision into four horizons is often made (Figure 1.1). The processes creating such horizons are examined in Chapter 3 and their detailed characteristics in Chapter 5. Soil grades at its lower margin to hard rock or materials usually devoid of the marks of biological activity. Thus, the lower limit is normally the limit of biological activity and, according to Soil Survey Staff (1975, 1994), this generally coincides with the common rooting depth of native perennial plants. However, the bottom of the genetic soil may not coincide with this rooting depth, being either shallower or deeper depending on circumstances. In the recent Australian classification, the term ‘pedologic organisation’ is used (McDonald et al., 1990). This concept includes all changes in soil material resulting from the effect of the physical, chemical and biological processes that are involved in soil formation. Whatever approach is adopted, the lower limit of a soil is always difficult to establish, and this confusion draws attention to the somewhat arbitrary distinction between soil and regolith. A solution to the dilemma may be to use the term ‘solum’ for the genetic soil developed by soil-building processes. This is the manner in which the term ‘soil’ will be used in this book.

Figure 1.1 Basic subdivision of a soil profile.

In lateral directions, soil varies in response to variations in the major soil-forming factors such as climate, relief and parent material. Soil forms part of the landscape, and the term ‘soilscape’ is sometimes used. This is implicit in the concept of soil geomorphology (Gerrard, 1992a), which is an assessment of the genetic relationships between soils and landforms. Soil responds to and influences environmental processes and conditions. It is also very vulnerable. The top part of the soil profile is constantly being removed, added to or generally reworked. Also, there are continual additions and removals within the soil as a result of vertical and lateral water movement. Soil is a vital natural resource and is the medium within which most agriculture takes place. Thousands of years of agriculture and settlement mean that there are very few ‘natural’ soils left on the Earth’s surface, and there are many areas of the world where soils are being degraded by human activities. Thus it is very important to understand the basic characteristics of soils and the way in which those characteristics are shaped by specific processes.

Soils as Systems

Soils are the result of the interaction of many processes both within and outside the soil body, and it is necessary to treat soils as open systems, losing and receiving material and energy at their boundaries. Thus:

soils are complex open process and response systems. As such they continuously adjust by varying degrees, scales, and rates to constantly changing energy and mass fluxes, thermodynamic gradients and other environmental conditions, to thickness changes, and to internally evolved accessions and threshold conditions. Consistent with these facts is the notion that disturbance and change is a natural, predictable consequence of all soil evolving processes.

(Johnson and Watson-Stegner, 1987: p. 363)

A great many models of soil formation have been suggested, but they tend to fall into one of three general approaches. These are the functional factorial approach (Dokuchaev, 1898; Jenny, 1941), the system-process flux approach (Simonson, 1978), or some synthesis of both approaches (see Chapter 4). Although models of soil formation vary, they are all based on the notion that soils function as open systems. The essential characteristics of systems are listed in Box 1.1.

Box 1.1

Characteristics of Systems

1	Systems possess boundaries, either real or arbitrary.
2	Systems possess inputs and outputs of energy and matter crossing these system boundaries.
3	Systems possess pathways of energy transport and transformation associated with matter within the system.
4	Within systems, matter may be transported from place to place or have its physical properties transformed by chemical reaction or change of state.
5	Open systems tend to attain a dynamic equilibrium or steady state in which rate of input of energy and matter equals rate of output of energy and matter, while storage of energy and matter remains constant.
6	Where input or output rates of an open system change, the system tends to achieve a new dynamic equilibrium. The period of change leading to the establishment of the new equilibrium state is a transient state and the period of time involved will depend on the sensitivity of the system.
7	The amount of storage of energy and matter increases (decreases) when the rate of energy and material flow through the system increases (decreases).
8	The greater the storage capacity within the system for a given input, the less is the sensitivity of the system.

Systems may be classified in several ways. Morphological systems are the instantaneous properties that, in their organisation, form the observable part of reality. Soil as a morphological system would be what is called the soil skeleton. The skeleton is composed of the relatively stable and not easily translocated mineral grains and resistant organic bodies. Cascade systems are composed of a chain of subsystems that are linked dynamically by flows of mass or energy. In soil, this is represented by the soil plasma, that part of the soil capable of being moved, reorganised and concentrated. It is the active part of the soil and includes all material, mineral or organic, of colloidal size and the relatively soluble material. Integration of the morphological and cascade systems produces process–response systems, which represent the totality of the soil system. It is worth mentioning a fourth type of system, which is a control system, where there is some external control such as when soil is managed for cultivation.

The operation of soil systems can be examined in varying amounts of detail. At the simplest black box level the entire system is examined as a unit with no consideration of internal structure or what happens within the system. An example of this approach would be the measurement of precipitation as input into the soil and water emerging as output at the soil base without considering pathways, stores or lags. At a grey box level, a partial view of the system is examined. At this level, the soil body might be recognised as a potential regulator of water movement and perhaps also as having a storage capacity. The most complex and most realistic treatment of the soil body is as a white box, where as many of the regulators, stores and flows as possible are identified and analysed. Extending the water movement analogy, movement in individual soil layers would be considered at the white box level of analysis. The soil column can be considered to be a series of compartments through which there is continuous movement of material. Material is removed from the system by vegetation uptake, erosion and drainage, and is added to the system by atmospheric inputs, and from animal and vegetation activity. It is also recognised that additions and removals can occur from, and to, adjacent soil columns. The ways in which particular soils are formed can only be considered by a white box approach.

The open system nature of soils and the way in which systems react to change (see items 5 and 6 in Box 1.1) can be...