On Determining Soil Aggregate Bulk Density by Displacement in Two Immiscible Liquids

Volume of soil aggregates can be determined with a combination of two immiscible liquids by saturating aggregates with one of the liquids and measuring buoyancy while submerged in the second one. The saturating liquid may get displaced from pores resulting in volume measurements that do not remain constant in time. Theory predicts a faster rate of displacement when the saturating liquid has a smaller viscosity than the displacing liquid. Our objective was to test the effect of two liquid combinations on the dynamics of liquid displacement from soil pores. Kerosene was used alternatively as the saturating (C-1) and the displacing (C-2) liquid and a glycerin-ethanol mix (1:1, v/v) as the complementary immiscible liquid. For each combination, nine aggregates from an unplowed native prairie (NP) and eight from an agricultural field (AG) ranging in mass from 0.1 to 15 g were saturated in the saturating liquid and submerged for 15 min in the displacing liquid. Aggregate volume change was about nine times faster in liquid combination C-1 than in C-2 (P < 0.001), validating theoretical predictions. Also, aggregates of mass < 1 g from both soils showed a greater change in volume with combination C-1 than larger aggregates (P <= 0.003). When selecting a combination of two immiscible liquids it is important that the viscosity of the saturating liquid be greater than that of the displacing liquid to assure constant values of aggregate volume that are independent of aggregate size.