Chapter 23: Soils
If people had a longer life and were more observant of what is below their feet perhaps they would appreciate that soils (Fig. 1) are living things and that they demonstrate many of the properties that we associate with organisms. They are dynamic entities that develop through time in an orderly and predictable way and are characterized by a number of processes that involve matter and energy transformations. However, they do not have boundaries in space or time and, since they are not discrete, they do not reproduce. In an ecological sense, they are communities possessing a variety of living components that collectively, in conjunction with physical processes, carry out a variety of functions. Observers often consider soils to be an abiotic medium in which organisms grow, but the actions of the organisms in the soil are often so intertwined with the physical processes occurring there that it is pointless to try to separate them, other than to acknowledge that the relative importance of the activities of organisms in the soil vs. the physical processes varies greatly between different types of soils.
Although most consider soils to be an assemblage of debris (i.e., ‘dirt’), soils are better understood if viewed as being composed of the four basic elements that the Greeks and other cultures described: earth, air, water and fire. The earth is a solid material, derived both from inorganic sources, ultimately rock, and from organic sources, pieces of material that are derived from living things. Water pervades the soil as its adhesive properties assure that it sticks to all the particles. Depending on the amount of water, air permeates the soil to a greater or lesser extent. And finally, the fire of the soil is the living creatures, mostly small, highly dominated by fungi and bacteria, but also including a variety of invertebrates, protists, occasional vertebrates and a dynamic population of plant roots. All four of the soils parts are critically important to plants: the solid material is the ultimate source of most nutrients and provides surfaces where significant events occur; water is needed by all plants and also is the medium from which they obtain nutrients; air provides the oxygen that plants and many other living things in the soil require; the living things in the soil carry out a variety of processes significant to plants, in particular, the degradation of large molecules into units that can dissolve in the solution and be available to plants. Each of these four components, earth, water, air and fire, interacts with the others: living things change the gas composition, the gas composition affects living things; water moves and organizes the solid phase; the solid phase controls the water content; the solid components change water chemistry; changes in water chemistry can add or subtract to the solid phase. Soils are a web of interactions, and while components can be listed, the whole is much more than the sum of the pieces