Aggregate stability

For the entire detailed protocol, see the full downloadable manual

Soil macro-aggregates of a sandy loam soil after wet sieving

This is an indicator of organic matter content in the soil. It is the ability of soil particles to bind together and resisting external force. Soil aggregation and structure in general is closely related to soil texture, so that structure and aggregation should ideally be compared in two soils that are quite similar in texture if we want to assess management impacts on structure. For example, if we compare aggregation in a loamy sand versus a loamy clay, the differences we observe will likely be driven by the clay content and NOT management, so the comparison may not be that interesting.

Watch the video for the steps to fractionate soil aggregates:

Materials you will need:

  • Large size mesh with a hole size of 8 to 12 mm to perform pre-screening of soil aggregates and gently break large soil clods and remove larger stones (see picture below)
Above: Different types of coarse screening with size 8 to 12 mm that can be used to pre-screen soil aggregates for the test.
  • A standard 2mm soil sieve, as well as a 0.25 mm (250 micron) sieve. A sieve diameter of at least 6” (150 mm) is best, to avoid clogging with too much material during wet sieving. A 250-micron sieve can be made by cutting the top of a medium-size paint bucket with a strong snap-on lid, and fastening 250-micron mesh underneath the lid to create the sieve surface (see picture below). This is #60 mesh, also called 24T mesh in european systems, and is used in the silk-screen fabric printing industry. See the manual for more details on sieves and mesh.
Above: at left, a 250-micron sieve made from the top of a plastic paint bucket and plastic mesh used for silk-screen printing. At right, an inexpensive 2mm sieve.
  • Water: clean tap water, captured rainwater, or even clean stream water is acceptable
  • Small basins for the sieves to fit into, which also allow movement of the sieve into and out of water in the basins (see the method below)
  • Balance to weigh soils and aggregates, 0.1 g precision is helpful and 1 g precision is adequate
  • Rinse bottles that allow rinsing of sieves to move and capture soil and aggregates.  These can also be made by punching small holes in the lid of a 500 mL or similar beverage bottle.
  • Cut tube made from a plastic jar, and squares of fabric (bedsheet or t-shirt type) or paper filters for catching, visualizing, weighing aggregates (see image below). Fabric or filters should be pre-weighed dry and weight written on them to simplify weighing of the aggregate sample when it is dry.
Above: a plastic jar cut into a cylinder, with a cloth filter mounted to capture aggregates from water at the end of washing.

Method step by step

  1. Weigh 70 g air dried soil, passed through a large screen (8 mm – 12 mm) to eliminate large aggregates; as shown below. This should be done without aggressively crushing the soil so that the starting aggregate structure is largely preserved. Aggregates should be picked apart by hand to get them through the sieve.
  2. Place on standard 2 mm sieve or similar (e.g. sieve made with 1/8” hardware cloth) in a small basin, an inch or two larger than the sieve, with water level 2 cm above sieve.
  3. Gently pour the 70 g of soil onto the submerged mesh (Fig. 12).
  4. Leave the soil in the water to wet up and slake (break apart) the aggregates naturally for 5 minutes.
  5. Move sieve into and out of water 50 times in 2 minutes. This can be done using a 50 beat per minute metronome app on a cell phone, raising, then lowering the sieve with each beat. Make sure you are not doing this action at twice the pace: you should NOT lower and raise the screen on each beat of the rhythm, which would be 100 times in two minutes. See the video for a demonstration.
Above: washing the aggregates by raising and lowering from a basin of water, 50 times in 2 minutes
Above: Example of a metronome app for a smart phone, set for 50 beats per minute
  1. After this 2 minute plunging action, wash debris and organic matter from the sides of the sieve down through the sieve with a wash bottle; do not directly rinse aggregates with the wash bottle stream as these were stable during the plunging and should not now be destroyed by aggressive washing.
  2. Next, empty the stable 2mm aggregates (particles> 2mm left on the sieve), to a new, dry pan or basin, and then capture them in a cloth or filter. This is easily done by just turning over the 2mm sieve, over a wider pan, and washing all the contents from the back of the sieve out the front into this pan. You can then do a final rinse with a wash bottle to completely empty the sieve, working from the back and the front. As you do this, you can remove any large organic residues such as sticks and long root segments from the sample (these are not aggregates).
  3. When all the 2mm stable aggregate material (and likely some stones) have been moved to the second basin, use the wash bottle to move this material from this basin to the pre-weighed cloth filter mounted on the plastic tube. The cloth can be placed over a cup or other container to catch the water passing through, and leaving the stable aggregates in the cloth. This cloth is set aside for drying while we focus on the smaller 250 micron fraction.
  4. Now turn attention to what went through the sieve, the <2 mm fraction of aggregates and soil. Pour this material gently through a 250 um (0.25mm) sieve into another small basin, creating an even coating of sand as well as aggregated soils that remain in this sieve. Rinse the entire contents from the basin through the sieve, and ensure that there is at least 2 cm of water above the sieve when sitting in the basin as in step 1.
  5. Repeat movement of the sieve 50 times in 2 minutes as above in step 5, using the small sieve and water in the basin.
  6. Rinse the 250 micron sieve gently to wash off clay and silt that are not part of aggregates, without destroying aggregates, as above for the 2mm case.
Above: at left, rinsing the 250 micron aggregate fraction into a filter or filter cloth, at right, examples of different dried aggregate fractions on cloths for weighing, plus stones that were separated from the 2mm fraction.
  1. Wash content of sieve (aggregates and sand) with wash bottle onto a filter or cloth (i.e. keeping the 250 um fraction).
  2. At this point aggregate fractions can be photographed and/or compared visually among different fields or management types to make conclusions for learning purposes, or additionally dried and weighed.
  3. If the fractions are dried, the next step is to hand-pick or sieve the stones > 2mm from the larger size aggregate fraction, and weigh these stones separately (see picture above).  This step is not needed for the smaller aggregate fraction.
  4. Weighing the stones and the two fractions on the cloths, plus the tare weight of the cloths should allow the following calculations (see the full manual method with link above for more details)

Calculations:

To calculate the percentage of stable aggregates, use the following formulas.  The figure of 70g should be changed if a different initial weight of soil was used for the aggregate washing. Note that we subtract the weight of small stones out of the denominator in both cases, as we want to know how much of the stone-free soil is in stable aggregates (stones>2mm are not technically considered soil):

  1. % Large macro-aggregates (LMA) = [(Weight of first 2 mm size fraction without stones) – (weight of filter paper or cloth)]/ [70g – weight of stones > 2mm]
  2. % Small macro-aggregates (SMA) = [(Weight of second 250 um size fraction) – (weight of filter paper or cloth)] / [70g – weight of stones > 2mm)

Additional video on this method with information on mechanisms: see the following excellent video covering this method from Johan Six’ former lab at UC Davis, USA. It explains some of the theoretical reasons why aggregates are stable or unstable in water. We have changed it slightly but the basic process is the same: https://www.youtube.com/watch?v=VOaae2bDDCY