A A HORIZON See Horizon, Profile, Horizon Designations. ABIOTIC Describes soil constituents (for example quartz, kaolinite), pro- cesses (hydrolysis, redox reactions), or factors (temperature, relative humidity, salinity for example), that are inorganic in nature, and that are capable of forming or acting in the absence of life. However, since even the most rudimentary of soils con- tains organisms, organic constituents and processes inevitably interact with and impact upon the inorganic ones. In 1840, Justus von Leibig suggested that a biological population is lim- ited by whichever extensive factor (particularly abiotic ones such as inorganic nutrient concentrations) is in shortest supply (Lomolino et al., 2006, p.79). Bibliography Lomolino, M.V., Riddle, B.R., and Brown, T.H., 2006. Biogeography , 3rd edn. Sunderland, MA: Sinauer Associates, 845 pp. Cross-reference Law of the Minimum ABRASION The wearing away by surface friction of solid bodies (minerals and rocks for example) when brought into contact with each other by such agents of erosion as wind, water, ice or animals (including Homo sapiens). The reaction of a mineral with water at an abraded surface yields a characteristic pH. A typical range of abrasion pH is 6–7 for clay minerals and quartz, 7–8 for micas, 8 for calcite, 8–11 for silicates other than micas (Porta et al., p 442). Bibliography Porta, J., Lopez, M., and Roquero, C., 1994. Edafologia para la agricul- tura y el medio ambiente. Madrid: Editions Mundi‐Prensa, 807 pp. ABRUPT TEXTURAL CHANGE A phrase used in the WRB Classification to mean either a dou- bling of the clay content within a vertical distance of 7.5 cm if the overlying horizon has less than 20% clay, or an absolute increase in clay of 20% within 7.5 cm if the overlying horizon has 20% or more clay. In this case some part of the lower hor- izon should have at least twice the clay content of the upper horizon (FAO, 2001, Annex 2). Bibliography FAO, 2001. Lecture notes on the major soils of the world. World Soil Resources Reports, 94. Rome: Food and Agriculture Organization of the United Nations, 334 pp. ABSORPTION The assimilation of one substance by another, or by an orga- nism (water or aqueous solution by plants for example). Absorption of chemical elements by roots may modify the chemistry (pH and redox potential for example) in the immediate