How is soil aggregate stability tested?

Aggregate stability testing measures the resistance of soil aggregates to breakdown or degradation. Aggregates are clumps of soil composed of mineral material held together by mycorrhizae, fine plant roots, earthworm slime, and residues of dead soil organisms. Besides serving as glue to hold aggregates together, these materials serve as food and habitat for soil organisms. Aggregates provide many soil health benefits: Since they are irregularly shaped, they enhance porosity, increasing water infiltration, aeration, and the movement of water and nutrients through soil. Plant root growth is also more abundant in porous soils, allowing for better plant stabilization and greater access to water and nutrients. Chemically, organic matter coatings on aggregates provide abundant cation exchange sites, enhancing nutrient availability to plants and decreasing the potential for nutrient losses due to leaching and runoff. The hydrophilic organic matter coatings on aggregates also enhance soil water absorption and water-holding capacity.

Test Procedure and Limitations
The oldest and most commonly used method for testing aggregate stability involves placing a soil sample on a nest of soil sieves with screen sizes typically ranging from 1 mm to 45 microns and then moving this nest of sieves up and down in a bucket of water. The more stable aggregates will stay on the top sieve, while less stable aggregates will move through the larger sieves to the finer sieves. The Cornell Soil Health Assessment uses an alternative test where a set of irrigation spigots on horizontal pipes “rain” down on the soil sample, which is on a screen that is placed below the spigots. The percentage of stable aggregates is then calculated based on the weight of soil remaining on the screen in relation to the amount of soil that passes through the screen. Field-based tests involve placing a soil sample on a kitchen sieve that is moved up and down through water.

A simplified “quick and dirty” test of aggregate stability, often used in soil health demonstrations, is the slake test. A handful of soil is placed in a clear jar or beaker, either directly in the water or in a small mesh bag placed in the water. Typically, aggregated soil from a field under conservation tillage is placed into the water in one jar, and nonaggregated soil from a field under conventional tillage is placed in another jar. The aggregated soil typically holds together in the water while the non-aggregated soil turns to mush.

Aggregate stability is widely accepted among soil scientists as a critical indicator of soil health, can be observed in the field without special equipment, and is an excellent indirect indicator of biological activity, water-holding capacity, and the presence of mycorrhizae. Methods for analyzing soil aggregate stability are not consistent across laboratories, however, making precise measurement and comparisons difficult. Good aggregate stability may also have more to do with soil texture than with management practices. For example, soils high in clay usually exhibit a higher aggregate stability than silty or sandy soils, since chemical reactions on the surface of clay minerals allow for effective bonding with organic matter, forming aggregates.

How to Increase Soil Aggregate Stability
Aggregate stability is enhanced by all the practices that enhance the growth of mycorrhizae, minimize soil disturbance and compaction, and enhance plant diversity: practices such as reduced or no-till land preparation and seeding, crop rotations, planting diverse cover crops, and rotational or managed grazing.

The new ATTRA publication Soil Health Indicators and Tests is a great resource to learn more about soil testing. It describes several of the most common soil health assessment methods, including total soil organic matter, active organic matter, soil respiration, aggregate stability, and the Haney soil test. It identifies benefits and limitations for each method and provides suggestions and resources for conducting these analyses on your farm or ranch.

And don’t miss the new ATTRA podcast Demystifying Soil Health Indicators and Tests , which features a robust conversation between Dr. Barbara Bellows and NCAT’s Mike Morris about the new publication.