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P>Latosols (Oxisols, Ferralsols) are characterized by a poor soil horizon differentiation, with a strong microgranular structure formed by microaggregates 50 to 300 mu m in size. This microgranular structure is generally considered to be very stable and the shrinkage properties of microaggregates as poor or absent. The objective of our study was to analyse the shrinkage properties of microaggregates in diagnostic Bw horizons collected in Latosols with various particle size distributions and mineralogical compositions depending on their location in a regional toposequence across the Brazilian central plateau. We measured the water retention properties at -300 and -1500 kPa with a centrifugation method. The morphology and size of elementary particles were examined by transmission electron microscopy; specific surface area (SSA) was determined by nitrogen adsorption. We also determined the pore size distribution of microaggregates by the combined use of mercury intrusion and the nitrogen desorption isotherm. Our results show that microaggregates of the Latosols studied were not rigid and shrank during drying. The pore volume of the -300 and -1500 kPa-treated samples, that of the dried microaggregates and the SSA were closely related to the clay content. The mineralogy of particles < 2 mu m played a secondary role in the properties studied. Our results show that microaggregate shrinkage at -300 kPa and -1500 kPa varied with the location of the Latosols in the landscape. Thus, microaggregates of Latosols located on the upper highland surface of Late Tertiary shrank less than those of the lowland surface of Late Quaternary, and this difference was attributed to their different hydric stress histories. Proprietes de retrait des microagregats de Latosols bresiliens: importance de la teneur en argile, de la composition mineralogique et de l'histoire hydrique. Resume Les Latosols (Oxisols, Ferralsols) sont caracterises par une faible differenciation de leurs horizons et une structure microgranulaire representee par des microagregats de 50 300 mu m de diametre. Ces microagregats sont generalement consideres comme etant tres stables et leurs proprietes de retrait comme etant tres limitees, voire absentes. L'objectif de cette etude est d'analyser les proprietes de retrait des microagregats dans l'horizon diagnostic Bw de Latosols de composition granulometrique et mineralogique variable en fonction de leur position le long d'une toposequence regionale selectionnee pour l'etude dans le Plateau Central bresilien. Nous avons mesure les quantites d'eau retenues -300 et -1500 kPa en utilisant la methode par centrifugation. La morphologie et la taille des particules elementaires ont ete etudiees en microscopie electronique transmission et l'aide de mesure de surface BET. Nous avons aussi determine la distribution de taille des pores dans les agregats deshydrates en combinant les donnees de la porosimetrie au mercure et celles issues de l'isotherme de desorption d'azote. Nos resultats montrent que les microagregats des Latosols etudies ne sont pas rigides et diminuent de volume lors de la dessiccation. Le volume poral des microagregats -300 et -1500 kPa ainsi que leur volume poral apres deshydratation sont etroitement lies la teneur en argile. En revanche, la composition mineralogique de la fraction < 2 mu m ne jouerait au plus qu'un role secondaire dans les variations du volume poral. Nos resultats ont montre que les proprietes de retrait des microagregats varient en fonction de la position des Latosols dans le paysage. Ainsi, les microagregats des Latosols situes sur les hauts plateaux de la fin du Tertiaire diminuent moins de volume lors de la dessiccation que les agregats situes sur les surfaces du Quaternaire recent et situees en contrebas dans le paysage. Une telle difference de comportement serait liee l'histoire hydrique des Latosols, ceux situes sur les surfaces les plus anciennes ayant vraisemblablement subi des contraintes hydriques plus elevees que ceux situes sur les surfaces les plus recentes.

International audience; In working with soil physics, getting new answers to the same questions is a challenge. As soil physicists, we are always hoping to find new ways of understanding such a complex soil science area. In this chapter, we will discuss some of the ways to assess soil physical quality for crop growth, using ascending complexity classification, from the simplest to the more complex soil physical indicators for crop growth.

International audience; Brazilian Latosols are characterized by low activity clay, little horizon differentiation, a weak macrostructure and a strong microgranular structure. The development of the latter that was reported as being possibly related to the fine material mineralogy, location in the landscape, parental material and biological activity is still under discussion. The aim of this study is to discuss the origin of the microgranular structure of Latosols located in the Brazilian Central Plateau. Ten Latosols (L) developed in different parent materials were selected along an approximately 350-km long regional toposequence across the South American Surface (SAS) (L1 to L4) and Velhas Surface (VS) (L5 to L10). The structure of the Latosols was studied in the field and samples of the diagnostic Bw horizons were collected for laboratory analyses. Basic soil characterization was performed on the

International audience; This study aims at comparing the centrifuge and pressure plate methods when appropriate duration of run is adopted. Samples collected in tropical soils located in Brazil along a 10-km local hydrosequence across the Cerrado-Amazonia transition (set 1) and along a 350-km regional toposequence across the Cerrado region (set 2) were selected to compare and discuss statistically the similarity of the soil-water retention recorded by using the pressure plate and centrifuge methods. The results showed good agreement (R2 = 0.99) for the 1:1 comparison of measured point-wise soil-water content values (set 1) as well as for the fitted soil-water content curves by van Genuchten model using data points obtained with the two methods (set 2). Thus, the centrifuge method should be considered as an appropriate method for determining soil-water retention properties not only for the similar results with the pressure plate method but also for being much less time consuming.

The physical properties of Brazilian Oxisols are closely related to the development of their microstructure, which typically consists of stable microaggregates >1 mm. There is no model available to predict changes in microstructure in Oxisols. The objective of this work was to relate the proportion of microaggregates to the bulk density (D b ) in the soil studied. Five sites of a typic Haplustox under native vegetation (two sites) and pasture (three sites) were sampled. Soil bulk density, sand, silt, and clay content and aggregate-size distribution were measured from the surface to 1.6 m deep in increments of 0.1 m. Thin sections were prepared from undisturbed samples collected in duplicate from 0- to 0.1-, 0.3- to 0.4-, 0.8- to 0.9-, and 1.5- to 1.6-m depths, and backscattered electron scanning images (BESI) were generated. Clay content ranged from 672 to 798 g kg - 1 and bulk density between 0.87 and 1.18 g cm among the 80 samples studied. Bulk density was poorly correlated with clay content (R 2 = 0.358) and at any depth was not significantly smaller under native vegetation than under pasture. Visual assessment of BESI revealed that soil material corresponded to either microaggregates (