Evaluation of SLAKES, a smartphone application for quantifying aggregate stability, in soils with contrasting managements
Abstract
The structure of soils, i.e. the macroscopic organization of aggregates and pores, conditions the
storage and transport of water and gas in the soil, and strongly determines the physico-chemical
environment of soil organisms (plants, micro and macro-organisms). The description of the soils
structure dynamics constitutes a major issue in the current context of global change, at the
scientific, environmental and agronomic level. However, few tools are available to monitor this
dynamic non-destructively and in situ. We therefore propose to develop a new method based on
the analysis of acoustic emissions (AE) spontaneously emitted by soils during the evolution of their
structure. A laboratory feasibility study was conducted to explore the links between variations in
soil structure and the AE emitted during soil desiccation.
Two undisturbed soil columns (8 cm in diameter, 5 cm high) were sampled in an agricultural field
(near Chartres in France), in the surface horizon of a Glossic Retisol. These cylinders were air dried
(20°C during 9 days), and the AE produced during drying were monitored using piezoelectric
sensors place at the soil surface. The concomitant soil structure changes were followed through
3D images, acquired by X-ray tomography (CIRE platform, INRAE, Nouzilly) all along the
experiment. These images, with a resolution of 168 μm, were used to characterize the pore
network (porosity, surface density, connectivity, etc.).
The dynamics of the EAs recorded during the drying of the samples is comparable for the two
samples: the AE rates are maximum at the start of the experiment and then reach a plateau.
Changes in soil structure follow the same dynamics, e.g. considering porosity or surface density of
the pores. If we analyze the relationship between the signals recorded by the surface sensors (EA
rate) and the porosity, we observe a linear relationship (R² of 0.79). This relationship, although
encouraging, remains to be consolidated by additional results.
To go further, it is also necessary to define the necessary conditions to perform such a
measurement in situ, and to improve the acoustic signal processing to characterize the EA
produced during soil desiccation. Indeed, a major objective of our work is to differentiate, thanks
to EAs, the various factors responsible for the evolution of soil structure (physical and biological),
by determining their "acoustic signature".