Colloidal suspensions of calcium carbonate in soils and their likely significance in the formation of calcic horizons

Abstract

Evidence is presented to demonstrate that colloidal suspensions of calcium carbonate can be produced in the laboratory from naturally occurring CaCO 3 (Upper Chalk) and from a range of calcic horizons (Typic Hapludalf, southern England; Calcixerollic Xerochrept, southern Iran; and Xerollic Calciorthid, lower Egypt). Calcium carbonate in these calcic horizons and associated k-fabrics is in the form of colloidal-sized platelets. Particle size and ability to remain in colloidal suspension were determined for unground and for “TEMA” ground samples. Particle composition was determined using X-ray diffraction, differential thermal analysis and from X-ray fluorescence spectra (XRF). Particle morphology was obtained using the scanning electron microscope (SEM). The spatial morphological patterns observed in the calcic horizons of the three soils consisted of silt-sized, calcite platelets linked together by discontinuous coatings of both single and aggregated clay-sized, calcite platelets. Thus, as calcium carbonate in the soils was shown to be in colloidal form, the spatial and morphological patterns observed at the submicroscopic scale are attributed to physical transport of colloidal complexes rich in calcium carbonate. On mobilization, these colloidal complexes will be transported through inter-, intra- and transpedal pores by movement of soil water and deposited on ped surfaces and as linings to pore walls. Transport of colloidal calcium carbonate and deposition from suspension onto ped and pore surfaces is considered to be enhanced by irrigation practices and by lateral throughflow of water, that is by periodic and discrete soil/water/gravity events.