A high throughput approach for measuring soil slaking index

Abstract

Soil wet aggregate stability is widely measured for soil health ratings, and two mobile applications have been recently developed that provide simple and fast measurements of wet aggregate stability on three pea-sized soil aggregates. However, techniques are needed to scale the test to make it practical for studies with large sample numbers. Here, we describe an approach to measure 10-min slaking index on 20–36 aggregates simultaneously using a multi-well plate and automated image analysis. We used this approach to measure 160 soil samples (2120 aggregates) from a long-term tillage and cover crop trial in a corn (Zea mays L.)-based agroecosystem in central Iowa. We evaluated the statistical power of slaking index, needed replication, sensitivity to cultural practices, and sensitivity to sample collection date to inform future sampling efforts. We found that small numbers of highly unstable aggregates lead to skewed distributions for slaking index. We concluded that at least 20 aggregates per sample were useful to provide confidence in measurement precision, although the experiment had high statistical power with only 10–12 replicates per sample. Slaking index was not sensitive to the initial size of dry aggregates (3- to 10-mm diameter); therefore, pre-sieving soils was not necessary. Consistent with prior studies, the field trial showed greater aggregate stability under no-till than chisel plow practice and changing stability over a growing season. This multi-well method will help researchers measure a useful soil health indicator with greater precision, and more efficiently sample spatial and temporal variation.