Abstract
Abstract. Soil aggregate stability is a useful indicator of soil physical health and can be used to monitor condition through time. A novel method of quantifying soil aggregate stability, based on the relative increase in the footprint area of aggregates as they disintegrate when immersed in water, has been developed and can be performed using a smartphone application – SLAKES. In this study the SLAKES application was used to obtain slaking index (SI) values of topsoil samples (0 to 10 cm) at 158 sites to assess aggregate stability in a mixed agricultural landscape. A large range in SI values of 0 to 7.3 was observed. Soil properties and land use were found to be correlated with observed SI values. Soils with clay content >25 % and cation exchange capacity (CEC) : clay ratio >0.5 had the highest observed SI values. Variation in SI for these soils was driven by organic carbon (OC) content which fit a segmented exponential decay function. An OC threshold of 1.1 % was observed, below which the most extreme SI values were observed. Soils under dryland and irrigated cropping had lower OC content and higher observed SI values compared to soils under perennial cover. These results suggest that farm managers can mitigate the effects of extreme slaking by implementing management practices to increase OC content, such as minimum tillage or cover cropping. A regression-kriging method utilising a Cubist model with a suite of spatial covariates was used to map SI across the study area. Accurate predictions were produced with leave-one-out cross-validation, giving a Lin's concordance correlation coefficient (LCCC) of 0.85 and a root-mean-square error (RMSE) of 1.1. Similar validation metrics were observed in an independent test set of samples consisting of 50 observations (LCCC = 0.82; RMSE = 1.1). The potential impact of implementing management practices that promote soil OC sequestration on SI values in the study area was explored by simulating how a 0.5 and 1.0 % increase in OC would impact SI values at observation points and then mapping this across the study area. Overall, the maps produced in this study have the potential to guide management decisions by identifying areas that currently experience extreme slaking and highlighting areas that are expected to have a significant reduction in slaking by increasing OC content.
Highlights
Objective and quantitative metrics are required to assess soil health and monitor soil condition through time
The current study investigated the use of the SLAKES application and digital soil mapping (DSM) techniques to assess variation in slaking index (SI) across a landscape with different agricultural and natural land uses
This indicates an extreme level of aggregate disintegration, it remains below the maximum theoretical SI of 7.8 suggested by Fajardo et al (2016)
Summary
Objective and quantitative metrics are required to assess soil health and monitor soil condition through time. Slaking is the disintegration of soil aggregates as a result of rapid wetting (Yoder, 1936; Oades and Waters, 1991). Most cultivated soils in Australia are prone to some degree of slaking. The degree of slaking determines if the process produces a favourable or unfavourable environment for cultivation and plant growth, and it has implications for soil conservation. A small degree of slaking can be beneficial and is associated with selfmulching – an ability to recover from disturbance by reforming small (< 5 mm) aggregates at the soil surface following wetting–drying cycles (Grant and Blackmore, 1991) – and mellowing: a partial disintegration of soil aggregates on wetting that results in increased friability (Barzegar et al, 1996). Soil strength increases as the soil dries, producing difficulty in cultivation until the soil is rewetted, and shoot emergence and root growth may be restricted (Mullins et al, 1990)
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