1 Introduction Some evidence in the literature suggests that silt and sand fractions can contribute significantly to cation exchange capacity (CEC) of soil (Joffe and Kunin, 1943; Karim and Islam, 1956; Mc Aleese and McConaghy, 1957; Sµnchez, 1969; Martini, 1970; Curtin et al., 1987; Thompson et al., 1989; Asadu et al., 1997 and others). However, Morrµs (1995) and Turpault et al. (1996) provided evidence that clay coatings on coarser particles and partial transforma- tions of silt and sand were the reasons for CEC in these fractions. Weak correlation between clay contents and CEC values can be explained by the blocking of negative charges at clay particles by organic matter that reduces the effective CEC of clays (Wilding and Rutledge, 1966; Syers et al., 1970). Curtin and Smillie (1976) also pointed to the inadequacy of using clay content as an estimate of the mineral component of CEC in soils due to variations in clay mineralogy and the possible occurrence of phyllosilicates in the coarse soil fractions. Recently, Morrµs (1995) studied the mineral composition and CEC in two loess soils of the southern Chaco region, indicating that the silt fraction of these soils was responsible for between 1/6 and 1/3 of the total soil CEC. Curtin and Rostad (1997) attributed 15 % of the CEC of fine silt to the occurrence of clay minerals. These findings led us to formulate following questions: 1. Are clay minerals always present in the coarser particle- size fractions of soils from different origin? 2. Is the effect of organic matter on the CEC of particle- size fractions similar for the different clay and silt-size fractions of soils? 2 Materials and methods Five samples of surface soil horizons (0±20 cm) from four Mollisols and one Ultisol in different regions of Argentina were collected in order to cover a broad spectrum of parent materials and mineral composition (Fig. 1). ± Sample I: EEA-INTA Cerro Azul (Dto. Leandro Alem, Misiones) (27 o 39¢S;55 o 20¢W) (20.3 o C; 780 mm); Humic Nitosol on basalt (ortoxic Palehumult). ± Sample II: annexed field `San Isidro' INTA (Dto. Capital, Santiago del Estero) (27 o 46¢S; 64 o 18¢W) 21 o C; 550 mm); Calcic Kastanozem on loess (entic Haplustoll). ± Sample III: EEA-INTA Balcarce, Buenos Aires) (37 o 51¢S; 58 o 15¢W) 13.7 o C; 850 mm); Luvic Phaeozem on loess (typic Argiudoll). ± Sample IV: Est. Mahuida Có (Sierra de la Ventana, Buenos Aires) (38 o 00¢S; 62 o 06¢W) (14.5 o C; 780 mm); Haplic Chernozem on colluvial material (typic Hapludoll). J. Plant Nutr. Soil Sci. (2000), 163, 47±52 (2000)  WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000 1436-8730/00/0102-0047 $17.50+.50/0 Effect of Clay Minerals and Organic Matter on the Cation Exchange Capacity of Silt Fractions Norman Peinemann * , Nilda Mabel Amiotti, Pablo Zalba and Maria Bonita Villamil Departamento de Agronomía, Universidad Nacional del Sur. 8000 Bahía Blanca, Argentina Accepted 16 November 1999 Summary ± Zusammenfassung The cation exchange capacity (CEC) and specific surface properties were investigated in four particle-size fractions < 50 mm from three loess (one Kastanozem and two Phaeozems), a holocene (Fluvisol) and a basalt soil (Nitisol) before and after destruction of organic matter. Particle-size fractions were separated by sedimenta- tion after chemical and physical dispersion of the soil samples. Illite, amorphous minerals, mixed layers, smectite and kaolinite were the predominant clay minerals. They were detected in all size fractions. The CEC increased with increasing organic matter contents and this effect was more pronounced in coarser fractions. The organic matter content per unit surface area was two or three times larger in coarse silt than in clay, irrespective of the soil type. Key words: cation exchange capacity / specific surface areas / particle-size fractions / organic matter Einfluss von Tonmineralen und der organischen Substanz auf die Kationenaustauschkapazität von Schlufffraktionen Die Kationenaustauschkapazität (KAK) und spezifische Ober- flächeneigenschaften von vier Korngröûenfraktionen < 50 mm wurden in drei Löûböden (ein Kastanozem und zwei Phaeozeme) sowie je einem Boden aus holozänem Sediment (Fluvisol) und aus Basalt (Nitisol) vor und nach Zerstörung der organischen Substanz untersucht. Die Partikelgröûenfraktionen wurden durch Sedimenta- tion nach erfolgter chemischer und physikalischer Dispergierung getrennt. Die überwiegenden Tonminerale waren: Illit, amorphe Minerale, Wechsellagerungsminerale, Smectit und Kaolinit. Sie waren in allen untersuchten Partikelgröûenfraktionen vorhanden. Die KAK-Werte nahmen mit steigenden Gehalten an organischer Substanz zu, insbesondere in den gröberen Fraktionen. Der Gehalt an organischer Substanz je Oberflächeneinheit war im Grobschluff zwei- bis dreimal gröûer als in der Tonfraktion. * Correspondence: Prof. Dr. N. Peinemann; E-mail: npeinema@criba. edu.ar 47