A B S T R A C T:Calcium-cadmium and calcium-zinc exchange equilibria were studied at 20~ and constant ionic strength (0.015) on four clay minerals, viz. montmorillonite, bentonite, illite and vermiculite. Vermiculite and, to a lesser extent, illite and bentonite showed the strongest affinity for Cd 2+ or Zn 2+ over Ca 2 § ions, whereas on the Camp Berteau montmorillonite nearly nonpreferential exchange isotherms were observed. With this one exception, selectivity for the heavy metals was greatly enhanced in the trace Cd or Zn regions, suggesting the presence of specific adsorption sites in these clays. Generally, the adsorption increased with the polarizing power of the exchangeable cation. Thermodynamic equilibrium constants and standard free energy changes for the complete exchange of Ca-clay to Cd-or Zn-clay were calculated.

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Des cultures, pures et associées, de trèfle violet et de ray-grass italien ont été menées en phytotron durant 15 semaines, sur un sol limoneux. Trois coupes ont été pratiquées, après quoi le sol a été mélange et conservé à 25 °C pendant 12 semaines. On a suivi l'évolution de la stabilité structurale du sol pendant et après la culture, selon la méthode de Hénin. Le poids et la longueur des racines ont été déterminés à chaque coupe. La vitesse de dégagement de C0 2 par le sol a été mesurée pendant la période d'incubation. Le fait le plus notable pendant la période culturale est que le ray-grass augmente nettement le taux d'agrégats stables, alors que le trèfle est quasi inefficient de ce point de vue. Durant la période post-culturale, on observe dans tous les cas un pic immédiat de stabilité des agrégats, suivi d'une lente décroissance de stabilité. Le taux d'éléments fins diminue de façon monotone pendant la culture et augmente ensuite régulièrement pendant l'incubation. Les 3 traitements culturaux ont des comportements similaires à ce point de vue. On n'a pas trouvé de relation générale (pour les 3 coupes) entre les mesures de stabilité structurale et les paramètres radiculaires. Une relation simple apparaît toutefois entre la longueur radiculaire en fin de culture (3 e coupe) et les paramètres de stabilité. Pendant la période post-culturale, on note un parallélisme entre l'évolution du taux d'agrégats stables et le dégagement de C0 2. Cet effet d'origine microbiologique a été interprété par analogie avec l'addition de matière organique fraîche au sol. Mots clés additionnels : Association végétale, agrégats. SUMMARY Stabilisation of soil structure by the roots of clover and rye-grass. Effects during and after cropping. Red clover and Italian rye-grass (pure and in association) were grown in a phytotron for 15 weeks on a loamy soil (table 1 Three cuts were made ; afterwards, the soil samples were homogenized in each pot and stored at 25 "C for 12 weeks. The structural stabilily of the soil was monitored according to Henin's method, during and after plant growth (fig. 2). Weight and length of roots were measured at each cut (fig. 1). The rate of C0 2 evolution from soil samples was measured during the incubation period (fig. 4). During the plant growth period, the percentage of stable aggregates (particles > 200 h m) was clearly improved by rye-grass whereas clover had no significant effect. During soil incubation, a peak of aggregate stability was immediately observed in all cases, followed by a slow decrease of stability. The percentage of fine particles (< 20 um) decreased steadily during plant growth and increased during incubation. In this respect, the two plants and their association showed similar behaviour. No general relationship (for the 3 cuts) was found between structural stability and root weight or length. However at the third cut (end of growth period) the structural parameters were well related to root length (fig. 3). During incubation, aggregate stability and C0 2 evolution changed in parallel. Th...

fairly good (fig 3). The higher adsorption of Cd in comparison with the noncalcareous soils is due to the reactivity of calcite surfaces (chemisorption and nucleation-precipitation, respectively at low and high rates of Cd) and also to the commonly high pH of the soils studied (7.6 < pH < 8.6). Comparison of the adsorption isotherms measured in chloride (CaCl 2 0,01 mol·l -1 ) and perchlorate (Ca(ClO 4 ) 2 0,01 mol·l -1 ) media (fig 4) points out that adsorption of Cd is mainly determined by the activity of free Cd 2+ in the soil solution.hea vy metal / cadmium / calcareous soil / active CaCO 3 / adsorption / Spain *

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