Humankind is an environmental variable whose activities are affecting more and more of the planet and its soils. At present, humans appropriate over 40% of the Earth's net primary productivity (Rojstaczer et al., 2001) and use 60% of freshwater run-off (Postel et al., 1996). It is intuitive that this exploitation of resources results in a massive impact on terrestrial ecosystems. Thus, humankind influences ecological processes on a global scale, sometimes on a par with the role of climate, geological forces and astronomical variations. On this basis, it has been proposed that we are entering a distinct geological era, the ‘Anthropocene’, a period of intensive human industrialization and land change which started in the late eighteenth century, marked by the increase in atmospheric concentrations of carbon dioxide and methane (Crutzen, 2002). It has been estimated that today 83% of the ice-free land on Earth is affected directly or indirectly by humans (Sanderson et al., 2002). In summary, humans can be viewed as a distinct soil-forming factor, apart from other organisms that are not endowed with the ability to reason, due to the magnitude of their impact on the planet, and their cultural characteristics which drive their decision-making process (Amundson and Jenny, 1991).

In any natural environment, population growth reaches a limit imposed by factors such as light, space, nutrients, or water. Carrying capacity is the maximum number of individuals a particular habitat can support.

Soil is a dynamic natural body occurring in the upper few metres of the Earth's surface at the interface between the atmosphere, biosphere, hydrosphere and geosphere. A soil is both an ecosystem in itself, and a critical part of the larger terrestrial ecosystem. From the earliest perceptions of soils as the organic enriched surface layer to today's pedologic horizonation of profiles, there is a rich history of beliefs and understanding of this vital life-sustaining resource.

In Chapter 1 changes in perceptions of soils and their classification are explored. Chapter 2 describes some of the specific reactions that are components of the soil-forming processes that transform geologic materials into recognizable pedologic features and horizons. Solids, along with the liquids and gases that fill pore spaces between the solids, compose the three-phase soil system.

Chapter 3 treats the inorganic fraction of the solid phase, examining differences between primary minerals, derived directly from rocks, and secondary minerals, formed by pedogenic processes. Soil organic matter is discussed in Chapter 4. It is often a minor fraction of soil in quantitative terms, but exerts a major control on soil properties. Soil organic matter is complex, being a mixture of a multitude of different components. Organic matter may be tightly bound to clay surfaces by adsorption or physically protected by entrapment within aggregates. These associations modify the physicochemical and physical properties of the mineral phase and affect organic matter biodegradation rates.

The liquid phase of soil is an aqueous solution of solids and gases.

Naming genetic horizons

The genetic horizons are not the equivalent of the diagnostic horizons. Genetic horizons are used in profile descriptions to identify observations of soil profiles in situ. Diagnostic horizons are quantitatively defined features that are used to differentiate between taxa in soil classification systems. A diagnostic horizon may encompass several genetic horizons, and the pedogenic features implied by genetic horizon designations may not be sufficient to justify recognition of different diagnostic horizons. In genetic horizon nomenclature capital letters are used to represent the master horizons (Table I).

Where a substantial thickness is present between two master horizons a transitional or combination horizon may be described. Transitional horizons dominated by properties of one master horizon while having subordinate properties of an adjacent master horizon are designated by two master horizon capital letters. The first letter indicates the dominant master horizon characteristics and the second letter indicates the subordinate horizon characteristics. For example an AB horizon indicates a transitional horizon between the A and B horizon, but more like the A horizon than the B horizon. Also a BA horizon is a transitional horizon between the A and B master horizons but more like the B horizon than the A horizon. Other commonly designated transitional horizons include AE, EA, EB, BE, BC, CB, and AC.

Combination horizons are recognized where separate components of two master horizons are recognizable in the horizon and one of the components surrounds the other. Such combination horizons are designated as A/B, B/A, E/B, B/E or B/C. The first symbol designates the material of greatest volume in the horizon.