Soil Health: Aggregate Stability

Improving aggregate stability is the first step in improving soil health. Many of you have likely seen a slake test where two soil samples are placed in clear tubes that are filled with water to show how quickly the samples break apart.  This test shows how well the soil particles are bound together, or their aggregate stability.  This property is the foundation upon which healthy soils are built.  Unlike most foundations, aggregate stability starts at the soil surface and goes down.


Soils with higher aggregate stability are less susceptible to erosion, allow more infiltration, have higher water holding capacity, and allow for greater root development. Two of the biggest factors affecting aggregate stability are tillage and soil biology.  Tillage physically breaks the aggregates apart.  It also decreases the soil biology by stirring air into the soil, which increases the rate of organic matter decomposition. Aggregates are held together by organic “glues” that are produced by plant roots and soil microbes.  When these glues decompose, the aggregate stability decreases.  There are also many living creatures in soil that help to increase poor space (such as earthworms) and produce soil glues.  Tillage destroys their habitat, their food webs, and in many cases can kill these creatures.

Unstable soil particles affect the soil layers below them by filling in the pore space as water carries the particles lower in the soil profile. This can create an impervious layer much in the same way that heavy machinery can.   This impervious layer reduces infiltration and restricts root growth.

Adopting as many of the 5 Basic Soil Health Principles as possible will improve aggregate stability.

  1. Manage soils more by disturbing them less
  2. Keep the soil covered as much as possible
  3. Keep plants growing throughout the year to feed the soil
  4. Use plant diversity to increase diversity in the soil
  5. Introduce livestock and/or manure.

One nice aspect of aggregate stability is that it can show improvement in a (relatively) short time frame. Organic matter takes years to improve, but aggregate stability can show some improvement after the first year of management changes.  While it may be difficult to directly measure and compare aggregate stability between fields, there are other variables that can be compared.  The NRCS office has an infiltration ring which can show how long it takes for water to infiltrate into the soil as well as penetrometers to measure the compaction level of your soil.  Solvita test kits can be used to measure soil respiration, which indicates the amount of soil biology present in the soil.  If you are interested in finding out how to measure different aspects of soil health, please stop by your local NRCS office.  Staff would be happy to visit your field(s) this spring to do some quick field tests and discuss ways to improve soil health.