Applicability of the photogrammetry technique to determine the volume and the bulk density of small soil aggregates

Aggregate density (rho) is defined as the relationship between the mass and the volume occupied by an aggregate. Previous studies have characterised rho on large to medium-sized soil aggregates (>4 mm diameter); however, little information is available for smaller aggregates (<4 mm). The objective of this study was to test the viability of the photogrammetry (PHM) technique to determine the volume and subsequent rho of small soil aggregates (1-8mm diameter). The method uses a standard digital camera that photographs a rotating aggregate and reconstructs its three-dimensional surface and the corresponding volume. To validate the method, the volume estimated with PHM on rough stones of different sizes (1-16mm diameter) was compared with the corresponding volume measured by the Archimedes' principle. The method was tested on soil aggregates 1-8mm in diameter, collected from two sites under conventional and conservation tillage treatments. The strong correlation (R-2 > 0.99, P < 0.0001) between the volumes estimated on rough stones with the PHM and Archimedes methods demonstrates that this technique can be satisfactorily used to estimate the volume and, consequently, the rho of small soil aggregates. The results showed an increase in rho with decreasing aggregate size. A general trend of increasing r with the degree of soil disturbance by tillage was also observed.