The Composition of Soils

Abstract Soils are porous media created at the land surface through weathering processes mediated by biological, geological, and hydrological phenomena. From the point of view of chemistry, soils are open biogeochemical systems containing reactive solids, liquids, and gases. That they are open systems means they exchange both matter and energy with the surrounding atmosphere, biosphere, and hydrosphere. That they are biogeochemical systems means their development over time is a result of chemical transformations of earth materials linked to the life cycles of the soil biota and plant roots. Soils are the central feature of the life-supporting Critical Zone, which extends from the top of the vegetation canopy to the bottom of the groundwater aquifer in a terrestrial ecosystem. The Critical Zone provides essential ecosystem services (outputs of food, fiber, fuel, and water, including their quality) that sustain the biosphere. Other earth materials than soil may occur in the Critical Zone (for example, weathered rock [saprolite]), but soils are unique in showing a distinctive vertical stratification, the soil profile (Fig. 1.1), created by percolating water under the combined influence of parent material, topography, climate, living organisms, and pedogenic time—the five factors of soil formation. Analogous to biomes, which classify terrestrial ecosystems according to similar climate and vegetative cover, orders classify soils according to similar climate, parent material, or pedogenic time. With respect to climate, for example, Oxisols reflect tropical conditions, whereas Mollisols reflect temperate conditions. Spodosols and Gelisols reflect mainly boreal conditions (Table 1.1). Andisols, Histosols, and Vertisols, on the other hand, are not defined by climatic region, but instead by parent material (volcanic ash, organic litter, or swelling clay, respectively), whereas Entisols and Inceptisols reflect pedogenic time being insufficiently long for significant A or B horizon development, respectively. Biomes are basic classification units of the aboveground biosphere useful for characterizing its ecosystem services, whereas orders are basic classification units of the pedosphere useful for the same purpose. The natural capitalof soils is the set of assets that allows them to function beneficially as providers of ecosystem services.