[Contribution of Clay and Organic-matter To the Cation-exchange Capacity of Southeastern Belgium Soils]

The cation exchange capacity (CEC) is the most important soil physicochemical property with respect to mineral nutrients retention and bioavailability. Its use spreads, as a reference in qualifying soil fertility standards. It is well established that soil CEC originates from two major components: clay minerals and organic matter (OM). A multiple regression analysis was performed in order to estimate the mean contribution of clay and OM to the CEC of surface horizon of soils from three Belgian natural regions: Famenne,Ardenne and the Jurassic Region. The main characteristics of these soils are listed in table 1. The clay-CEC statistical estimates are then compared with the results of a direct method described by Addiscott (1970); the analytical procedure is as follows: the exchange sites are saturated with calcium, which is desorbed from non-organic sites by potassium fluoride; organically-bound calcium is not desorbed due to high affinity of organic sites for that ion. Clay and OM-CEC estimates provide good approximations of CEC for the Jurassic Region and Famenne soils (Table 2). The best predictor of Ardenne soils CEC is OM alone. Mean OM-CEC is estimated about at 300 meq/100g of oxidable carbon, and does not differ from one region to another. Mean clay-CEC estimates are 38, 33 and 43 meq/100g of clay for Famenne, Ardenne and the Jurassic Region, respectively (Table 2). The significant difference between Ardenne and the Jurassic Region can be explained on the basis of mineralogical data: high CEC smectites are found in most of the Jurassic Region soils, whereas they are not in Ardenne. Addiscott CEC measurements resulted in mean clay-CEC of 39, 39 and 51 meq/100g of clay for Famenne, Ardenne and the Jurassic Region, respectively (Table 3). These values do not conflict with statistical estimates.