THE REACTIVITY OF CARBONATES IN SELECTED SOILS OF SOUTHERN SPAIN

The properties of many soils of arid and semiarid areas are influenced by the presence of carbonates. This study was conducted to characterize the reactivity of carbonates in the surface horizons of 25 soils (Entisols, Inceptisols, Vertisols and Alfisols) of southern Spain. The samples were analyzed for calcium carbonate equivalent (CCE), "active" calcium carbonate equivalent (ACCE), predominant carbonate mineralogy, Mg content, particle-size distribution (PSD) of carbonates (by sedimentation and by a pH-stat acid-dissolution method), and surface area (SA) of carbonates (by N2 adsorption and by Ca-45 exchange, from the differences between intact and decalcified soil samples, as well as by estimates from the PSD described above). The CCEs ranged from 70 to 778 g kg-1 of soil, and the ACCEs ranged from 48 to 285 g kg-1. Dolomite was present in only eight samples, and in the dolomite-free samples, calcite generally had Mg/(Ca + Mg) ratio < 0.03. The carbonate PSD as determined by sedimentation differed from the PSD calculated from the pH-stat data, with the latter method giving a much higher proportion of clay-size carbonates. The average SAs of carbonates as estimated from the PSD data were 0.49 m2 g-1 soil for the sedimentation method and 3.2 m2 g-1 for the pH-state method. The average SAs determined by the N2 and the Ca-45 methods were 6.0 and 2.3 m2 g-1, respectively. The average specific surface areas (SSAs) of soil carbonate (m2 g-1 CCE) were 1.5, 7.5, 8.8 and 22 for the sedimentation Ca-45 exchange, pH-stat and N2 methods, respectively. The low SA and SSA values from the sedimentation method were a consequence of the occurrence of porous aggregates which have considerably higher equivalent spherical diameters than the particles of which these aggregates are composed. The pH-stat and Ca-45 methods gave SA values of similar magnitude to each other, probably because both methods are dependent on actual reactive surface area instead of aggregate size of the carbonate mass. The higher values obtained by the N2 method might indicate that N2 is physisorbed on carbonate surfaces that are slowly reactive to acid attack or Ca-45 exchange. The pH-stat SA was highly correlated with ACCE, which measures the reactivity of carbonates to oxalate. To obtain a rapid estimate of the reactivity of soil carbonate, the ACCE method. because of its simplicity, may be preferred.