Man cannot exist without soil. His history is to a very large degree the story of its use and abuse; civilizations that have flourished have always been based on an intensive use of soils for food production. Thus, from historic times knowledge and experience about soils have been related primarily to agriculture. The importance of soils in supporting natural forests has been tacitly understood, but man’s use of such forests has been in general an exploitative one. Forests have provided shelter and a source of game, wood for fuel, construction, and industrial purposes. Forests are an integral part of the...

Green plants use energy from the sun and by photosynthesis convert carbon dioxide and water into carbohydrates. Soil provides anchorage for the plants and much of the raw materials – water, nutrients, and oxygen – necessary for plant growth. Thus, the scientific definition for soil is ‘the unconsolidated mineral material on the surface of the earth that serves as a natural medium for land plants’ (Soil Science Society of America, 1973); by extension, forest soils are those which support forest vegetation.

The soil is not just a storehouse of raw materials used by plants; it is a space in which...

When a soil profile is viewed in the field the most obvious features are those associated with the solid particles, but on closer examination it will be seen that the living components such as roots occur mainly in the spaces between the solid particles. It is also in these spaces that the water and air of the soil are present. Describing a soil’s texture and structure is very much like describing a building by observing the kinds of construction materials – wood, bricks, stone, and how they have been assembled, how large the building is, and the number of storie...

Colour is the most obvious property of any soil and historically the names that have been used to describe many soils do so by colour. For example, red and yellow podzolic, chestnut, terra rossa, black cotton, sols brun acide, and braunerde are names which have been used by soil scientists to describe different types of soils. To a large degree, colour of a soil reflects organic matter content and mineral composition. The amount and form of these materials are in turn affected by such properties as soil moisture, temperature and aeration; these properties control the rate and development of plant...

Water occurs in the pore spaces of soil in one or more of three phases, as a liquid, solid, or gas. Most studies of the movement and retention of water in soils have been concerned with it as a liquid, but water vapour and ice are forms of water which may be significant for shorter or longer periods in certain situations.

The water content of a soil is continually changing during a large part, if not all of the year and these changes in content result from additions and losses to the soil system. Within the soil pores which are...

One of the most important characteristics of soils, and, in fact, a diagnostic feature, is the presence of organic matter and organisms. Certain soils such as peats are almost entirely organic material, whereas in soils of arid areas the organic content may be small.

The green plants which a soil supports convert solar energy to organic matter by means of photosynthesis. The overall efficiency of the conversion is low; for example, Odum (1971) has estimated that the efficiency of conversion to net primary production is 0.1 per cent. This value is an average for the biosphere; similar estimates for crops...

The array of living and dead plant material occurring in any soil harbours and supports populations of plants and animals ranging in size from the microscopic bacteria to the larger mammals such as rabbits and gophers. The complexity and interwoven nature of many of these populations reflect not only the varied food base on which they rely for energy but also the environmental differences in temperature and moisture, as well as the change in physical habitat occurring from the upper soil surface down to the mineral subsoil layers. The varied nature of the soil segregates populations and activities; in turn,...

Soil chemistry is concerned with the nature, chemical composition, properties, and reactions of soils. The chemical composition of a soil will reflect to a large degree the geologic origin of the soil materials. A soil developed in limestone will be characterized by high levels of calcium and perhaps magnesium. Often the mineralogical state of the geologic materials may be altered by physical weathering and transportation forces such as ice, wind, or water. Tills deposited by ice show a mixing of minerals and rocks, whereas there is a sorting of particles both by size and density when they are moved by...

Soil fertility is defined as ‘the status of a soil with respect to the amount and availability to plants of elements necessary for plant growth’ (Soil Science Society of America, 1973). The definition implies that amount of growth or yield is a variable, dependent on the level of soil fertility but that many other factors such as type of plant and growing conditions may also significantly affect growth. Jacks (1956) distinguished three stages in the evolution of man’s use of soil: (1) the shifting cultivation stage when human activity has only an ephemeral effect on the soil; (2) soil-exhausting agriculture...

It is the nature of man to name, to categorize, and to attempt a systematic approach in any field of study; in effect to develop a ‘scientific’ classification. Classifications evolve in relation to increase in knowledge and new and different groupings developed from time to time in turn establish other relationships. What may have begun as a simple separation of objects to facilitate description or use can develop so that the arrangement itself provides insights about the objects and stimulates the development of new concepts about them.

All schemes of classification are man-made and therefore arbitrary and in a relatively...

A soil survey involves the systematic examination, description, classification, and mapping of soils in an area. Although soil classification is an integral part of any soil survey, they differ in that a survey deals with the areal distribution of soils and classification does not. This basic difference is reflected in the fact that the scale of mapping must be decided before a survey can proceed. Once that is done, the information may be coalesced into groups of smaller scale, but the maximum scale is that at which the work is originally undertaken. Soil geography is, in fact, the study of...

Apart from the matrix of soil materials, the most conspicuous components of forest soils are roots. Living and dead roots provide pathways for the movement of water, nutrients, and air in the soil and their presence modifies the soil to such an extent that they can be considered to have a major influence on soil profile development. Root development is modified and controlled by soil properties and the degree of control may vary with the tree species.

On a weight basis, roots constitute approximately 20 to 25 per cent of the total tree biomass but, because of their inconspicuous location...

Vast areas of the world are burnt annually by both man-set and natural fires. A large proportion of these areas is in forest cover extending from the equator to the limits of tree growth in both southern and northern hemispheres.

Fuel is necessary if burning is to take place and the living vegetation and accumulated organic material of the soil, primarily the forest floor, are the two fuel sources. In many natural forests plant species occur which are ecologically adapted to the occurrence of fire and some even depend upon it as a factor in their normal cycle of growth...

The fate of water, from the time of precipitation until it is returned to the atmosphere and is again ready to be precipitated, has been termed the hydrologic cycle (Soil Science Society of America, 1973). The properties of the soil plant system are of particular interest since they provide mechanisms which regulate interception, flow and storage of water in the cycle. It is also this system that is subject to the greatest modifications as a result of man’s activities.

Water is essential for life processes and, depending upon the particular concern of investigators or users of water, often only a...

Living organisms require a supply of nutrient elements and much of it is dependent on a cycling of these elements in the biosphere. Carbon and nitrogen are two elements whose prime source in the cycle is the atmosphere, whereas others such as calcium, phosphorus, and potassium enter the cycle from weathering of geologic materials. These latter are termed ‘mineral elements’ because their source is the minerals of the earth’s mantle.

A consideration of nutrient cycling is somewhat analogous to a discussion of economics. The size of the area which is taken into account can be of great importance. The economic...

When living organisms colonize an exposed portion of the earth’s surface, the processes of soil development begin. The geologic materials in which the soil processes are at work may already have been subjected to some degree of physical and chemical weathering. In many places soil development begins on materials which have been transported from their place of origin. Windblown loessial deposits, glacial till areas, and waterlaid sediments, while retaining certain attributes of the materials from which they arose, will display new properties reflecting their mode of transportation and deposition.

Exposure to the climatic forces, primarily of temperature and precipitation, together...

The very act of digging into a soil and examining its profile is to look at a form of historical record. The question of deciphering and understanding the nature of the record depends not only on knowledge of soil properties but also on a realization of their significance both present and past. It is for this reason that the nature of the geological surfaces in which the soil has developed are important. Not only do the geological materials comprise the ‘body’ of the soil in most instances but the time span during which they have been formed provides a broad...