The effect of wetting pre-treatment on the desorption moisture characteristic of vertisols

Abstract

Four experiments were conducted by using individual aggregates and ground samples of different soils and a range of wetting pre-treatments to investigate the effect of wetting on the desorption moisture characteristic of Vertisols. Wetting rate was varied by manipulating the energy of the water used to wet up the soil. Fast wetting was found to increase moisture uptake in Vertisols over a wide range of matric potentials (to -38 MPa in one soil). At high matric potentials, additional water uptake ranged up to 0.26 g g-l. These results were attributed to the creation of planar voids within soils between small water-stable aggregates on fast wetting. These voids retained water at matric potentials as low as -90 kPa. At this potential, 40% of additional water taken up on fast wetting was still retained in one soil. At -1500 kPa, fast wet soils retained up to 1.2 times more moisture relative to slow wet soils. The effect of wetting at potentials of -1500 kPa and less appeared to be related to the smectite content of the soil clay fraction and the degree of order of clay quasi-crystals. The soil which showed the greatest effect of fast wetting at these potentials had a less well ordered clay micro-structure. These findings have relevance to field studies, as variations in maximum field soil moisture content, with depth, could be reproduced by tension wetting pre-treatments. This result indicates that reduced rate of subsoil wetting may be responsible for the decrease in maximum field water contents with increasing soil depth, which has been observed in Vertisols. Such profiles have previously been attributed to the effect of overburden and stress potentials alone.