Ž . Geoderma 100 2001 321–353 www.elsevier.nlrlocatergeoderma The chemistry of pedogenic thresholds Oliver A. Chadwick a, ) , Jon Chorover b a Department of Geography, UniÕersity of California, Santa Barbara, CA 93106, USA b Soil Science Program, Department of Agronomy, PennsylÕania State UniÕersity, UniÕersity Park, PA 16802-3504, USA Received 10 January 2001; received in revised form 23 February 2001; accepted 1 March 2001 Abstract Pedogenesis can be slow or fast depending on the internal chemical response to environmental forcing factors. When a shift in the external environment does not produce any pedogenic change even though one is expected, the soil is said to be in a state of pedogenic inertia. In contrast, soil properties sometimes change suddenly and irreversibly in a threshold response to external stimuli or internal change in soil processes. Significant progress has been made in understanding the thermodynamics and kinetics of soil-property change. Even in the open soil system, the direction of change can be determined from measures of disequilibrium. Favorable reactions may proceed in parallel, but the most prevalent and rapid ones have the greatest impact on product formation. Simultaneous acid–base, ion exchange, redox and mineral-transformation reactions interact to determine the direction and rate of change. The nature of the governing reactions is such that soils are well buffered to pH change in the alkaline and strongly acid regions but far less so in the neutral to slightly acid zones. Organic matter inputs may drive oxidation–reduction processes Ž . through a stepwise consumption of electron acceptors thereby producing thresholds but disequi- librium among redox couples and regeneration of redox buffer capacity may attenuate this response. Synthesis of secondary minerals, ranging from carbonates and smectites to kaolinite and oxides, forms a basis for many of the reported cases of pedogenic inertia and thresholds. Mineralogical change tends to occur in a serial, irreversible fashion that, under favorable environmental conditions, can lead to large accumulations of specific minerals whose crystallinity evolves over time. These accumulations and associated AripeningB processes can channel soil processes along existing pathways or they can force thresholds by causing changes in water flux and kinetic pathways. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Pedogenesis; Inertia; Threshold ) Corresponding author. Tel.: q 1-805-893-4223; fax: q 1-805-893-8686. Ž . Ž . E-mail addresses: oac@geog.ucsb.edu O.A. Chadwick , jdc7@psu.edu J. Chorover . 0016-7061r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. Ž . PII: S0016-7061 01 00027-1