Changes in soil water retention due to soil kneading

Soil water retention curves (WRCs) are required to predict the availability of water to plants and the movement of water through the soil. Soil kneading by hand is a type of extreme structure disturbance which is comparable to mechanical breakdown of soil structure by cattle trampling, tillage and wheeling of heavy machinery, especially when the soil is wet. The objective of this study was to explore how kneading affects soil water retention characteristic for a set of contrasting soils from southern Spain, and identify the soil properties that control the differences in behaviour. WRCs were measured at six soil-water matric potentials in the range of 10-1500 kPa on both undisturbed soil clods and kneaded soil samples in order to quantify differences in water retention characteristics due to sample pre-treatment. Results evidenced that only WRCs of sandy loam soils are not affected by soil pre-treatment; for the rest of the soils kneading increased water retention at low or high potentials or both. Particle size parameters are, for the most part, clearly related to the behaviour observed, but some other explaining variables linked to aggregation and dispersion state of phyllosilicates in the clay and silt fractions could also be involved.