Assessing soil erodibility and mobilization of phosphorus from Swedish clay soils - Comparison of two simple soil dispersion methods

Erodibility of Swedish clay soils estimated according to the existing methods is usually low, but high levels of suspended solids and attached unreactive phosphorus have been recorded in drainage water from fields and catchments dominated by clay soils. Inherent susceptibility to soil erosion is usually assessed through aggregate stability studies or dispersion tests. The latter are simple to perform and produce good results when compared against runoff lysimeter experiments. The environmental soil test to determine the potential for sediment and phosphorus transfer in runoff from agricultural land (DESPRAL) and soil suspension turbidity (SST) dispersion tests, which differ in soil-liquid ratio and shaking and settling times, were compared here for their ability to indicate the erodibility of 10 Swedish clay soils. The tests proved to be significantly correlated (r = 0.78), but DESPRAL showed higher repeatability (r(i) = 0.995) than SST (r(i) = 0.824). Variation in soil dispersion was explained by clay, sand and organic matter content in DESPRAL and by clay and sand content in SST. An additional study on the effect of soil storage duration on dispersion (DESPRAL test) in 15 soil samples showed that storage had no effect on some soils, but significantly decreased dispersion in others after only 8 weeks. Therefore, soil dispersion tests should be performed as soon as possible after sample drying. The DESPRAL and SST tests proved to be a good option for estimating the erodibility factor K in the Revised Universal Soil Loss equation under Swedish conditions and were able to differentiate the susceptibility to sediment losses for different clay soils. They provided an indirect measure of the amounts of sediment and P mobilized, but further work is needed to calibrate them against measured values at field and catchment scale.