Evaluation of SLAKES, a smartphone application for quantifying aggregate stability, in soils with contrasting managements

The measurement of aggregate stability is an important indicator of soil quality and is widely used for monitoring soil condition. The SLAKES mobile app is an alternative tool to laboratory-based methods to measure soil aggregate stability. It provides aggregate stability measurements through Slaking Index (SI) with SI close to 0 suggesting high stability and values above 7 suggesting minimum stability. As the duration of this low-cost experiment is only 10 minutes, SLAKES is very attractive for scientists and no-scientists. SLAKES was implemented in Australia and has proven its efficiency in several studies.

This study was conducted to determine whether the SLAKES mobile app could be adapted to French soils and then could be an alternative to the Mean Weight Diameter (MWD) method, normalized in France (ISO 10390). More specifically, the three main objectives were: (i) determining whether the aggregate stability measurements depend on the phones used for the experiment, (ii) estimating the number of measurements necessary to get reliable results, (iii) determining whether the app has the ability to detect the effect of contrasting soil managements previously shown using the MWD method.

The study was performed on silty loam soils from EFELE (Effluents d’Elevage et Environnement) experimental site at le Rheu (Brittany, France) which is part of the French “Organic Residues” research observatory (SOERE PRO). The experimental design combines two different tillage practices (conventional tillage and shallow tillage) with two fertilizer treatments (mineral and organic (cattle manure)) randomly replicated three times. Soil samples were collected in March 2017 at both 0-15 cm and 15-25 cm depth from the 12 plots. 

The SI was measured on three aggregates simultaneously and this measurement was repeated 15 times for each sample which provided 45 SI per sample. Outliers above SI=11 were removed before statistical treatments. Four different phones of the same brand and generation were used to measure SI.

An analysis of variance showed that the effect of the smartphone on SI measurements was not significant (p-value = 0.73, 0.88, 0.067 for 3 different samples). The SLAKES results showed comparable significant separation of means (p < 0.0001) between each soil management than the MWD method. According to the results of an analysis of variance, SI was significantly lower in reduced tillage than in conventional tillage condition (p-value = 2.10^-16). These results indicate a higher soil stability in reduced tillage. Yet, the analysis of variance did not underline any effect of fertilization on the SI (p-value = 0.28), as previously found with the MWD method.

This study proved that a relatively simple mobile app can detect the effect of soil management practices on aggregate stability with a similar performance than the MWD method. This conclusion was reinforced by the existing correlation between the SI and the MWD index (p-value = 0.00059, R² = 0.39). We recommend to perform similar experiments on other sampling campaigns or in other pedological and soil management contexts taking at least 15 measures per sample.