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

Management of tropical sandy soils for sustainable agriculture : a holistic approach for sustainable development of problem soils in the tropics : proceedings

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

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

Sustainability of continuous Stylosanthes in Northeast Thailand: Soil structure amelioration and accelerated acidification

Paper presented at the Second International Conference on Soil Quality, Environment and Sustainable Agriculture in Tropical and Subtropical Regions, Yingtan, China, 23-28 September 200

Consequences of slotting on the pore characteristics of a sandy soil in northeast Thailand

International audienceIn the sandy soils of northeast Thailand, root development is generally limited to the topsoil (0–20 cm depth) but a simple slotting intervention (20–40 cm) significantly increased the root frequency in the slotted material (Eslot) compared with the undisturbed subsoil (E horizon). The aim of this study was to investigate the consequences of slotting on the soil structure by analysing at different scales the pore characteristics of the original soil profile and of the soil material inside the slot. These characteristics were studied using bulk density measurements, image analysis of thin sections and mercury porosimetry. Our results showed that the total porosity of the E horizon and Eslot material was similar when measured in 100 cm3 cylinders, but that the pore size distribution had been changed by slotting. The unaltered E horizon contained mainly small pores characterized by a narrow distribution related to close packing of the sand grains, associated with some biological macropores probably with poor continuity as they did not contain roots despite their size. On average, pores were larger in the Eslot material, with a broader distribution resulting from looser packing of the sand grains but with fewer biological macropores. Although slotting reduced the number of biological pores, the looser packing appeared to be more favourable to root development than the presence of macropores in the E horizon. Finally, the comparison of the porosity in the different horizons with the porosity of the Eslot material, indicated the significance of the closeness of the sand packing on root development