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SOIL & WATER MANAGEMENT & CONSERVATION

Settling Velocity, Aggregate Stability, and Interrill Erodibility of Soils Varying in Clay Mineralogy

^a Federal Univ. of Santa Maria, 97105-900 Santa Maria, RS Brazil
^b USDA-ARS, National Soil Erosion Research Lab., 275 S. Russell St., Purdue Univ., West Lafayette, IN 47907-1196
^c Federal Univ. of Paraná, Curitiba, PR, Brazil

^* Corresponding author (norton{at}purdue.edu).

Mechanisms of aggregate disruption and the measurement techniques used to quantify them for different aggregate sizes affect the relation of aggregate stability to soil erodibility and to basic soil properties. We evaluated two different techniques of aggregate stability analysis that gave either a settling velocity or stability of aggregates parameter for different sized aggregates which we compared with interrill erodibility for 10 clay soils. We compared the differences in these parameters from slow wetting to reduce slaking to air-dried aggregates and compared these differences to soil properties. Aggregate settling velocity and stability and soil interrill erodibility were strongly affected by clay mineralogy and physical–chemical properties. The mechanism of aggregate disruption was dependent on clay type. Slaking during fast wetting was important in kaolinitic/oxidic soils, whereas highly smectitic clay increased particle dispersion and slaking on swelling, with a consequent reduction in size and speed of settling aggregates. Swelling of clays may have overridden any reduction in slaking by slow capillary prewetting of illite or smectite (with no kaolinite) soils, causing aggregate instability with both slow and fast wetting procedures. Correlation analysis showed that 4.76- to 8-mm aggregates with a high slaking index also demonstrated more slaking under wet sieving and slower fall velocity. Interrill erodibility had greater correlation with the mean weight diameter (MWD) of stable aggregates in the 1- to 2-mm size class, than for the whole soil (aggregates < 8 mm), and no correlation was observed with any of the slaking indexes involving wet sieving or settling in water. Multiple regression analysis indicated that 89% of the variability in erodibility for prewetted soil was explained by MWD of prewetted 1- to 2-mm stable aggregates (MWD_W), available water content, and fall velocity of 1- to 2-mm dry aggregates, while 96% of the variability in erodibility for dry soil was explained by MWD_W for 1- to 2-mm prewetted aggregates, water dispersible clay, and fall velocity for 1- to 2-mm dry aggregates. The interrill erodibility, for dry and wet soil, was greatest for the highly smectitic and least for the high-clay kaolinitic/oxidic, both under annual crops. The higher erodibility and lack of slaking reduction effect on our prewetted soil under simulated rainfall is explained by a confounding effect of high water table, high steady-state runoff and slaking.

Abbreviations: ASI, aggregate stability index • CEC, cation exchange capacity • D, air-dry soil • D for dry sieved • K_i, interrill erodibility • KR, erodibility ratio • MWD, mean weight diameter • SI_AS, slaking index based on aggregate stability • SI_V, slaking index based on settling velocity • V₅₀, fall velocity • W, prewetted soil • W for wet sieved