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Soil Aggregate Stability: What Is It, Why Is It Relevant to Farmers, and How Is It Measured?

This piece was written by Dr. Ryan Stewart, Professor in the School of Plant and Environmental Sciences at Virginia Tech. You can learn more about soil aggregate stability and all things soil health at the SPES Soil Health website.

Soils are formed from a combination of mineral and organic particles. These particles, when separated, are classified based on size as clays, silts, or sands. Over time, many of these particles bind together as a result of various forces, including chemical attraction between particles and organic “glues” produced by plant roots and different microorganisms. This binding process results in the formation of soil aggregates. Soil aggregation combines physical (based on the soil properties), chemical (based on particle attraction) and biological characteristics of soil. Aggregates are constantly forming and changing within the soil, but some aggregates are able to hold themselves together better than others.

Soil aggregate stability refers to how well soil aggregates hold together during a disturbance, for example when planting seeds or using tillage implements. Rainfall and wind also can disturb the soil, and if the aggregates do not have sufficient stability then the soil can become quickly eroded during storms or on windy days. Loss of soil aggregates can also decrease soil structure, which can then negatively affect water infiltration, water storage, and plant growth. Therefore, aggregate stability is a very important factor in the overall productivity of farmlands. Some farming practices tend to disturb soil aggregates, while other practices, such as those associated with regenerative agriculture, can enhance their stability. For this reason, soil aggregate stability is often used as a way to assess soil health.

Many methods exist for measuring soil aggregate stability, from fairly basic approaches that physically separate aggregates using sieves, to more complex approaches using high-tech size analyzers or rainfall simulators. Water is often used during these measurements as a way to mimic the ability of aggregates to hold together when wet, such as during a heavy storm. More recently, scientists have developed a smartphone app called “SLAKES”. Here, the user selects pea-sized aggregates and immerses them in shallow water. The app then uses the smartphone camera to record the aggregates through time. Less stable aggregates tend to fall apart faster and end up with their particles spread farther apart from each other, whereas more stable aggregates tend to remain closely clustered during the analysis. Even though the SLAKES app should be more thoroughly tested for use in Virginia’s soils, it nonetheless holds promise as a low-cost, fast, and easy method for farmers and others to use in their fields.

Photo courtesy of VA NRCS

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