Organisms are a fundamental component of soil. Soil biota are continually pushing aside, ingesting and otherwise contributing to the motions of soil particles due to their physical and metabolic activities. This biomechanical transport and mixing of particles in a soil should lead to particle size-sorting such that within the biologically active section of the soil column, the average particle size decreases with depth. This particle size gradient is a result of two particle fluxes: (1) a downward flux due to advective motions driven by gravity; and (2) and an upward drift flux due to random dispersive motions driven predominantly by the activity of soil biota.Small particles, due to their smaller size, are more likely to fit into the pore spaces between grains than are large particles. Thus, due to gravitational settling and water-borne transport, small particles will have a greater tendency to settle downward than large particles. In contrast, large particles undergoing biomechanically-driven dispersion, even though this is a directionally random process, will have more of a tendency to move upward toward regions of higher porosity than small particles. A simple mathematical model based on probability theory is proposed that predicts these vertical variations in soil particle sizes. Soil samples from sandy soils formed on Pleistocene marine terraces in Northern Florida are used to calibrate and test the model. We find that the predicted pattern of a general fining in particle size with depth within the biologically active section of the soil column corresponds well with field observations. Furthermore, these same principles can be applied in describing mass transport and sorting in unconsolidated sediments, for example in marine and lacustrine environments. As such, the proposed model suggests a simple mechanical explanation for why biological mixing intensities of marine sediments appear to be particle-size sensitive.