Conditions for the occurrence of slaking and other disaggregation processes under rainfall

Aggregation is a major feature of soil structure. It affects transfer of water and gas through the soil and at the soil surface, plant growth, etc. Under rainfall, seedbed structure evolves, leading to crust development, which affects overland flow and erosion. This evolution depends on both rainfall properties (intensity, drop size distribution, initial wetness) and soil properties (aggregate size distribution, stability). Four major disaggregation processes have been identified : (1) slaking due to air entrapment, (2) micro-cracking due to differential clay swelling, (3) mechanical breakdown due to kinetic energy and (4) clay dispersion. Clay dispersion occurs under sodic conditions. It was previously shown that mechanical breakdown and micro-cracking can actually take place under rainfall. However, slaking has only been demonstrated for aggregate stability tests. To test the occurrence of slaking under rainfall, a experiment was conducted under controlled conditions. It used the rainfall simulation facility of the soil science laboratory (INRA, Orléans. France). Two soils with different textures (a clay loam and a silt loam) were used because of their known difference in sensitivity to slaking, as shown by aggregate stability tests. Initially dry aggregates (1-3 mm and 3-5 mm size ranges) were placed under rainfall (deionized water) for various durations, allowing the measurement of disaggregation through time using a laser diffraction sizer. Four rains with different kinetic energy and rainfall intensity were used. Results allow to distinguish between disaggregation processes and to specifically identify slaking. Using complementary studies from the literature, their conditions of occurrence under rainfall are outlined.