A high throughput approach for measuring soil slaking index.

Authors: Phillips, Claire; Casanova, Joaquin; Emmett, Bryan

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/13/2024
Publication Date: N/A
Citation: N/A

Interpretive Summary: Having a robust set of measurements to describe soil health is important for monitoring how agriculture degrades or improves soils over time. Useful soil health measurements are ones that 1) capture how well soil functions, in terms of storing and providing water and nutrients, 2) capture how well soil stands up to stress, 3) are easy to perform and not expensive, and 4) are sensitive to changes in soil management. Wet aggregate stability is a measure that meets all these criteria, which is why it is commonly used as part of soil health assessments. Wet aggregate stability describes how well soil aggregates—arrangements of mineral particles and organic matter—stand up to disruptive forces from raindrop penetration and submersion. The SLAKES smartphone application, first described in 2016, provides a practical and fast approach for assessing wet aggregate stability on 2 to 3 pea-sized soil aggregates. It involves submerging the aggregates in water and taking pictures as they break apart over 10 minutes, and then analyzing the images to measure how much the aggregates disperse. However, many studies require a lot of soil samples, and this method would be more useful if it could measure more samples simultaneously. Here we described a way to perform the SLAKES test on 20 to 36 aggregates simultaneously, using a custom tray with a compartment for each aggregate, and computer code to analyze the images automatically. We used the method to analyze 160 samples from a long-term agricultural trial in Iowa. We showed that at least 9 to 12 aggregates should be measured for each soil sample to get a reliable average. We also showed the method readily detected improvements in wet aggregate stability when tillage was reduced, and also detected changes in aggregate stability over the growing season. Our findings support the idea that wet aggregate stability is a robust measure of soil health, but that measuring more aggregates is important for accuracy. This method advancement will help researchers and soil testing labs to measure wet aggregate stability more accurately and quickly.

Technical Abstract: Soil wet aggregate stability is widely measured for soil health ratings because it influences a range of processes, is easily measured, and is sensitive to soil management. The SLAKES smartphone application, first described in 2016, provides a practical and fast approach for assessing wet aggregate stability on 2 to 3 pea-sized soil peds. However, techniques are needed to scale the procedure to make it practical for studies with large sample numbers. Here we describe an approach to measure 10-minute slaking index on 20 to 36 peds simultaneously using a custom multi-well plate and automated image-analysis. We used this approach to measure 160 soil samples (2120 peds) from a long-term tillage and cover crop trial in a corn-based agroecosystem in central Iowa. We evaluated the statistical power of slaking index, desired replication, sensitivity to cultural practices, and sensitivity to sample collection date to inform future sampling efforts. We found that the occasional occurrence of highly unstable peds leads to skewed distributions for slaking index, and we therefore recommend measuring at least 9 replicate peds and computing geometric means to characterize soil samples. Slaking index was not sensitive to the initial size of dry aggregates (3 to 10 mm diameter). The field trial showed that aggregate stability was greater under no-till than chisel plow practice, but was not sensitive to cover cropping. Post-hoc power analysis showed four replicate plots provided very high statistical power to detect management impacts. Aggregate stability decreased over the growing season, but could not be definitively linked to biophysical drivers. The development of this high throughput method will help researchers and soil testing laboratories to measure slaking index with greater accuracy.