A considerable amount of progress in the understanding of the factors that control the genesis of calcic soils has been made by integration of field and laboratory studies supplemented by numerical modeling. Modeling of pedogenic carbonate accumulation using a compartment model strategy has produced simulated depth functions similar to those observed in field investigations and emphasizes the significance of varying eolian dust flux and soil pCO2 on the magnitude and depth of carbonate accumulation. Recent studies have provided significant new information concerning factors that influence carbonate accumulation in soils, which should significantly enhance prospects for numerical simulation of calcic soils in complex circumstances (e.g., soils subjected to glacial-to-interglacial climatic change). Such studies include carbonate dissolution rates, seasonal variation and depth variation of soil pCO2, soil-available water-holding capacity and evapotranspiration in arid climates, and isotopic composition of pedogenic carbonate. Isotopic studies also show that carbonate dissolution and precipitation occur in a chemically open system, demonstrating that calculations of carbonate solubility using models assuming a chemically open soil system are reasonable. Additional data from new studies regarding influences of presence of gypsum and other soluble salts on carbonate dissolution.and accumulation enable evaluation of the coprecipitation of such materials on calcic soil genesis. Numerical modeling encourages integration of results of diverse studies of soils and emphasizes the necessity of using multidisciplinary strategies in the solution of important problems in pedology.
During the past few decades, scientists from several disciplines have focused a large amount of research on the rates and processes of development of calcic soils1. The reasons for this are many, including the paleoclimatic and paleoenvironmental significance of calcic soils implied by the character of the pedocal-pedalfer boundary and carbonate depth functions of pre-Holocene soils (Birkeland, 1984) and the relation of evolution of calcic soils to the global carbon cycle (Schlesinger, 1982). Also, horizons of carbonate accumulation are relatively easy to identify in the field, and carbonate is readily measured in the laboratory.
The benchmark paper by Gile and his colleagues (Gile et al., 1966) profoundly influenced the manner in which a generation of soil scientists, geologists, and geographers think about the genesis of arid soils. Although, prior to Gile's research, others had noted the potential role of dust in the genesis of calcic soils (Brown, 1956), it was Gile’s paper that not only focused the attention of scientists on the impact of dust in the genesis of arid soils, but clearly demonstrated the systematic and time-dependent character of calcic horizon development. The research of Gile and his colleagues elegantly demonstrated the utility of the factorial approach emphasized by Jenny (1941) as a framework for studying rates and, indirectly, processes of soil genesis.
Studies of calcic soils during the 1960s and 1970s also indicated that calcic horizon morphology and carbonate distribution might record past climates (Birkeland, 1984). However, at least among geoscientists, the significance of the research of Arkley (1963) and Arkley and Ulrich (1962) with regard to soil-water infiltration and the genesis of calcic soils was much less recognized compared to that of Gile. Nonetheless, Arkley's research has played a major role in demonstrating the strong relation between climate and calcic soil genesis. Arkley's research, integrated with research of Gile and his colleagues, established the framework that was utilized in the initial attempt to simulate calcic horizon development through the use of numerical models (McFadden, 1982). Machette (1985) later published a model that demonstrated the significance of changes in the rate of dust influx relative to changes in precipitation on soil development, demonstrating the potentially complex genesis of calcic soils. In general, the results of modeling were significant in that (i) model simulations in diverse climates approximated carbonate depth functions observed in field studies, and (ii) model results provided support for hypotheses concerning polygenetic calcic soil development and Quaternary climatic changes that had been asserted in numerous studies. During the last few years, several studies have provided new data that yield important insights into calcic soil development. In this chapter, we review some of the results and implications of these studies and discuss how the results of these studies may be integrated in a new generation of numerical models that can be used to further the understanding of calcic soil genesis in arid regions.