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

Abstract

Aggregate density (ρ) is defined as the relationship between the mass and the volume occupied by an aggregate. Previous studies have characterised ρ 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 ρ of small soil aggregates (1–8 mm 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 (1–16 mm diameter) was compared with the corresponding volume measured by the Archimedes’ principle. The method was tested on soil aggregates 1–8 mm in diameter, collected from two sites under conventional and conservation tillage treatments. The strong correlation (R2 > 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 ρ of small soil aggregates. The results showed an increase in ρ with decreasing aggregate size. A general trend of increasing ρ with the degree of soil disturbance by tillage was also observed.