Comparing Aggregate Stability Methods

A person holds soil aggregates collected from a tilled field.

Soil structure is the backbone of a soil’s ability to support processes vital to the health and productivity of plants, animals, and ecosystems. Soil structural units, or aggregates, form when fresh organic matter is decomposed and transformed by soil microbes into binding agents among mineral soil particles. Because soil structure is difficult to measure, aggregate stability is often used as an indicator. Wet aggregate stability is a measure of a dry soil aggregate’s ability to resist dispersion when rewetted. Increases in aggregate stability are linked to improved water infiltration and reduced erodibility.

The popularity of the measurement in soil health over the past few decades has spurred the creation of numerous methods designed to assess aggregate stability. While most methods appear responsive to management practices, few studies have directly compared the methods. Those studies that have been conducted found that different aggregate stability methods responded differently to management, making comparison among the indicators difficult. This has left stakeholders wondering, “Is any specific aggregate stability indicator advantageous to use over another?”

To address this and other questions, Soil Health Institute scientists and partners conducted a study to evaluate over 30 soil health measurements at 124 long-term agricultural research sites across the U.S., Canada, and Mexico. We compared four common methods of measuring aggregate stability: water stable aggregates using the Cornell Rainfall Simulator, wet sieved water stable aggregates (Yoder method), slaking captured and adapted from SLAKES smart-phone image recognition software, and the mean weight diameter of water stable aggregates. We evaluated each indicator’s sensitivity to climate and inherent soil properties. Additionally, we explored each indicator’s sensitivity to long-term adoption of soil health management practices, including rotation diversity, number of cash crops, residue management, organic nutrient amendments, cover crops, and tillage. Finally, we examined the cost and availability of each indicator to determine its scalability.

All four indicators were sensitive to climate and inherent soil properties to a certain extent. This means that the indicators should only be compared among similar soils within similar climates. None of the indicators detected differences between diverse and monoculture rotations or increases in the number of different cash crops included in the rotation. All four methods, to a certain degree, were sensitive to organic nutrient amendments, cover crops, and tillage. Considering these results, no single method was clearly superior and all four are viable options for measuring aggregate stability. Although all measurements responded to management, SLAKES was the most practical in terms of time and cost. Therefore, we concluded that slaking via image recognition measured on intact 4-10 mm diameter aggregates was the most suitable method for scaling up assessments of aggregate stability as a soil health indicator.