Field shrinkage curves of a swelling clay soil: analysis of multiple structural swelling and shrinkage phases in the prisms of a Vertisol

Abstract

The variations in the layer thickness of a Vertisol cropped with sugarcane were recorded under natural conditions for several months. Drainage pipes were installed at 1.10 m depth to drain water quickly from macro-cracks. The soil gravimetric water content was measured from the surface to 1.10 m depth at weekly intervals. The relation between soil water content and soil bulk specific volume (shrinkage curve) is not unique. A significant structural swelling or shrinkage can occur at any water content of the clay matrix while the macro-cracks remain air-filled. The variations in structural water content account for >60% and 80% of the total water content variations, respectively, in topsoil and subsoil. In the latter, the intra-prism structural porosity is filled and then emptied prior to matric porosity. This explains the disappearance of the water stress in such dry soil after any rainfall event. A more detailed description of the intra-prism structural porosity is required for a better understanding of the processes. Our work suggests that this porosity is mainly formed by planar inter-aggregate voids in the tilled layers, whereas its shape is tubular in the deeper layers. The results already provide guidelines concerning the modelling of water flow in such Vertisol, i.e. water flow in structural pores is quite independent of matric and crack flows.