Chapter 9 : Utilizing mineralogical and chemical information in PTFs.

Soil structure is known to reflect mineralogical composition of clay fraction and soil chemical composition. Because soil hydraulic properties are likely to depend on soil structure, chemical and mineralogical composition have long been expected to be important predictors of soil ability to retain and transmit water (Rawls et al., 1991). Nevertheless, the quantitative, pedotransfer-type information about the effect of clay mineralogy and chemistry of soil solution on soil hydraulic properties remains relatively scarce. This chapter summarizes available data and ideas that may show future research directions in this field

Concentration and mobility of lithogenic trace metals in soils : significance of anthropogenic lateral redistributions.

One of the main current concerns regarding soil quality is the presence of trace metals that constitutes a risk of toxicity for plants and animals [1, 5, 7 and 11]. Trace metals in soils can have either a lithogenic origin when they inherit from the lithosphere or an anthropogenic origin when they result from man's activities (mining, metallurgy of non-ferrous metals, combustion of fossil energy sources, solid waste or sewage sludge) [5]. Recent studies showed that concentration of lithogenic trace metals in soils could not be explained without taking man's activities into account

Toward conditions favourable to mobility of trace elements in soils.

The mobility of trace elements in soils is often discussed because of consequences for their bioavailability and thus their potential ecological toxicity. Numerous studies showed that oxides and oxi-hydroxides could be an important sink for trace elements [1,2]. In many soils, trace elements tend to accumulate in oxides and oxi-hydroxides that act as scavengers [7,9,10]. Indeed, as a recent example, extraction for As speciation in contaminated soils showed that As originating from irrigation was partly retained by Feoxi- hydroxides [3]. Other experiments also showed that adding Fe-bearing compounds to contaminated soils reduces As mobility, but increases Pb and Cd mobility [8], thus indicating that further investigations are required to elucidate trace-element mobility in soils. However, soil oxides and oxi-hydroxides play a significant role in controlling trace element mobility and new conditions leading to dissolution of oxides and oxi-hydroxides should be carefully considered

Elaboration et utilisation de fonctions de pédotransfert pour la prédiction des propriétés hydrauliques des sols.

La prédiction des propriétés hydrauliques (propriétés de rétention en eau et conductivité hydraulique) des sols a fortement mobilisé la communauté des physiciens du sol au cours des trois dernières décennies. La raison en est simple : la connaissance de ces propriétés est nécessaire à l'appréciation des potentialités d'un sol à supporter telle ou telle culture. De très nombreux travaux ont par conséquent été conduits dans ce domaine. On doit cependant reconnaître qu'il reste beaucoup à faire tant la « demande » se fait de plus en plus exigeante. S'il ne s'agissait dans les années 70 que d'estimer la « réserve utile » d'un sol, c'est l'ensemble de la courbe représentant la quantité d'eau présente dans un sol en fonction de l'énergie avec laquelle elle est retenue par ce sol ainsi que la courbe représentant la conductivité hydraulique en fonction de la teneur en eau, qu'il est nécessaire aujourd'hui de connaître

Creep and fluidity of a real granular packing near jamming

We study the internal dynamical processes taking place in a granular packing below yield stress. At all packing fractions and down to vanishingly low applied shear, a logarithmic creep is evidenced. The experiments are analyzed under the scope of a visco-elastic model introducing an internal "fluidity" variable. For all experiments, the creep dynamics can be rescaled onto a unique curve which displays jamming at the random-close-packing limit. At each packing fraction, a stress value is evidenced, corresponding to the onset of internal granular reorganisation leading to a slowing down the creep dynamics before the final yield

Water retention properties of the clay in soils developed on clayey sediments: significance of parent material and soil history

International audienceWe have investigated the water retention properties of clayey subsoil horizons according to the variation of clay characteristics. The horizons studied developed on a large range of age and facies of calcareous or calcium-saturated clayey sediments. The water retention properties were studied from −10 hPa to −15 000 hPa water potential using small clods collected in winter when swelling is at a maximum and water content close to field capacity. The specific water content and volume of the clods at field conditions, their specific volume at −15 000 hPa water potential, the clay content, the organic carbon content, the cation exchange capacity, the N2-BET surface area and calcareous content were measured. The clay fabric, which is the spatial distribution of the elementary clay particles, was quantified when the soil was close to field capacity and we could attribute the whole pore volume to the porosity of the clay fabric. Our results show that the water retention properties of the clay vary greatly from one soil to another with respect to the clay fabric. The variation of the latter depends on the cation exchange capacity, the size of elementary particles and hydric stress history of the clay. We show that the water retention properties of the studied clayey soils vary according to the clay content and fabric, the latter being related either to parent material fabric or to both the hydric history of the soil and size of the elementary clay particles

La matière organique des sols (MOS) : un héritage difficile mais fructueux

Interface entre géosphère et biosphère, les sols doivent, de manière immédiatement perceptible, la MO qu'ils renferment aux végétaux, voire aux animaux qu'ils supportent et/ou abritent. La présence de MO au sein des sols et son importance sont connues de longue date. Cette importance a même été très exagérée avec la théorie de l'humus qui, vers la fin du 18edébut du 19e siècle, supposait que les végétaux tiraient de ce substrat toutes leurs substances nutritives, carbone compris. En 1840, dans son traité « la chimie organique et son application en agriculture et en physiologie », Liebig rejette cette théorie et promeut des concepts plus proches de la réalité. Ramenée à une place plus modeste, la MOS est cependant bien prise en compte lors de l'invention de la pédologie moderne, attribuée au géologue russe Dokouchaev, à la fin du 19e siècle. Dépêché en Ukraine en 1877, pour en étudier les sols, Dokouchaev a fondé la reconnaissance, dans les profils qu'il y a étudiés (des chernozems), de trois horizons superposés se distinguant avant tout de bas en haut, par leur richesse en matière organique

Physical properties of tropical sandy soils: A large range of behaviours

Sandy soils are often considered as soils with physical properties that are easily defined, however they are far from being simple. This is particularly the case for sandy soils in the tropics where they are subjected to a cycle of wetting and drying associated with seasonality. In this respect small changes in composition lead to significant differences of physical properties. One of the major soil characteristics to be taken into account is the size distribution of the sand grains. If fine sand induces greater porosity, water retention and resistance to penetration than coarse sand, they exhibit lower permeability. Porosity decreases when the heterogeneity of the sand grain distribution increases leading to an increase in resistance to penetration and decreases in permeability. The presence of silt particles leads to similar consequences. Thus, silty sands are more compact than sandy soils, most silt particles occupying the voids between sand grains thereby reducing porosity and consequently permeability. Size distribution and mineralogy of silt and clay sized particles that are associated with sand grains are also responsible for variations in physical properties of tropical sandy soils. Under tropical environments, sandy soils undergo significant weathering to depth thus resulting in a mineralogy where quartz the dominant mineral in the sand and silt fraction and forms a significant proportion of the clay sized fraction. On the other hand, sandy soils can be present in the lower part of the landscape where clays or salts form during the dry season. As a consequence, sandy soils with similar particle size distribution but due to differences in mineralogy of the clay sized fraction that represents not more than a few percent of the soil mass, show very different physical properties. Finally, in sandy soils unlike other soils, the elementary fabric is easily affected by tillage practices. If greater porosity can be produced through tillage operations, the stability of these systems is very weak and compaction by wheels or other actions can in return produce a dense structure. Thus, compaction results from a variation of the structure at all scales, i.e. from the macroscopic to microscopic scales

Estimation des propriétés de rétention en eau des sols à partir de la base de données SOLHYDRO : Une première proposition combianant le type d'horizon, sa texture et sa densité apparente.: Une première proposition combinant le type d'horizon, sa texture et sa densité apparente

Mis en ligne avec l'aimable autorisation de l'éditeurLa base de donnée SOLHYDRO regroupe un ensemble de mesures des propriétés de rétention en eau acquis en France sur des horizons de sol en fonction de leurs caractéristiques. Dans un premier temps, des valeurs de teneur en eau volumique à 7 potentiels de l'eau variant de -10 (01,0) à -1500 (O4,2) hPa sont proposées en utilisant le type d'horizon (horizons A d'une part, horizons E, B et C d'autre part) et sa texture. Dans un second temps, pour accroître la précision et réduire le biais lors de l'estimation, des valeurs de teneur en eau volumique sont proposées en utilisant cette fois le type d'horizon, sa texture et sa densité apparente. Une estimation par excès et par défaut de la réserve en eau utile est aussi proposée dans chaque cas. Ces différents jeux de valeurs constituent une première proposition qui a vocation à être précisée et élargie lorsque la base SOLHYDRO disposera d'un nombre de données supplémentaires suffisant