A soil is acidic if the pH value of the soil solution is less than 7.0. This condition is met in many soils where rainfall exceeds evapotranspiration, including Alfisols, Histosols, Inceptisols, Oxisols, Spodosols, and Ultisols—almost half of the ice-free land area worldwide. Soils of the humid tropics offer examples of acidic soils (Ultisols and Oxisols), as do soils of forested regions in the temperate zones of Earth (Alfisols, Histosols, Inceptisols, and Spodosols). Soils in peat-producing wetlands and those influenced strongly by oxidation reactions, such as rice-producing uplands, can be mentioned as examples in which the biota play a direct role in acidification. The phenomena that produce a given proton concentration in the soil solution to render it acidic are complex and interrelated. Those pertaining to sources and sinks for protons are shown in Fig. 11.1, which is a special case of Fig. 1.4 with “free cation or anion” in the center of the latter figure now interpreted as H+. In addition to the biogeochemical determinants of soil acidity, the field-scale transport processes wetfall (rain, snow, throughfall), dryfall (deposited solid particles), and interflow (lateral movement of soil water beneath the land surface down hill slopes) carry protons into a soil solution from external sources. Their existence and that of proton-exporting processes, such as volatilization and erosion, underscore the fact that the soil solution is an open natural water system subject to anthropogenic inputs that may dominate the development of soil acidity. Industrial effluents, such as sulfur and nitrogen oxide gases or mining waste waters, that produce acidic deposition or infiltration, and nitrog-enous fertilizers, the transformation and transport of which produce acidic soil conditions, are examples of anthropogenic inputs. Despite all this complexity, proton cycling in acidic soils at field scales has been quantified well enough to allow some general conclusions to be drawn. Acidic deposition, production of CO2(g) and humus, plus proton biocycling, all serve to increase soil solution acidity, whereas proton adsorption and mineral weathering serve to decrease it.
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