Aggregate stability

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:

You will need:

● 2mm and 0.25 mm (250 micron) sieves (at least 6” (150 mm) diameter are good, to avoid clogging with too much material during wet sieving). 0.25 mm sieves can be made from large diameter plastic pipe and 60-mesh silk screen fabric used in the screen printing industry.
● Water – reasonably clean tap water is fine. Water that is too saline could theoretically lead aggregates to break down more quickly, but we have not tested this, and presumably such water would be undrinkable.
● Small basins for the sieves to fit into, which also allow movement of the sieve into and out of water held in the basins (see photos below)
● Balance to weigh soils and aggregates.
● Squirt bottles with nozzles that allow rinsing of sieves to move and capture soil and aggregates.
● Funnel and squares of fabric (bedsheet-type) or paper filters for catching, visualizing, weighing aggregates. Fabric or filters can be pre-weighed dry and weight written on them to simplify weighing of the aggregate sample.

1. 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. Some aggregates can be picked apart by hand to get them through the sieve.

Passing soil through an 8mm sieve without forcing the aggregates through the mesh.

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 1 cm above sieve.
3. Sprinkle 50g soil (above) onto sieve and wait 5 minutes.

4. 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.

5. 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.
6. After Turning over the sieve into a second basin, remove particles on the front of the upside-down sieve by washing aggressively with the wash bottle from the back. The particles will fall into the basin from the front of the sieve, and you will then follow up with washing from the front side to capture all the stable aggregates and small stones i retained on the 2 mm sieve. fraction)
7. Add some water to this second basin, and decant off any obvious floating organic debris from the 2mm sieve that is now floating in the basin. These are not soil aggregates, so we don’t want to count them as such.
8. Wash contents of this basin, which are the aggregates and stones caught on the 2mm sieve, onto large coffee filter or weighed cloth, dry, and weigh (with preweighed filter, or square/circle of cotton cloth or similar; all preweighed so we can figure out the dry wt. of aggregates from the sieve). This can be air dried in the sun, oven, or other warm place (up to 105 C) to determine dry weight. For quick comparisons or training purposes, the aggregates can also be gathered in a beaker to compare the volume of aggregates from different soils or land uses, without drying.

9. Now turn attention to what went through the sieve, the <2mm fraction of aggregates and soil components. 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.
10. Repeat movement of sieve 50 times in 2 minutes, using the small sieve and water in the basin.
11. Wash content of sieve (aggregates and sand) with wash bottle onto a filter or cloth (i.e. keeping the 250 um fraction), dry, and weigh; or compare volumes of aggregates from different soils.

To calculate Raw % aggregates without accounting for sand:
Large macro-aggregates (LMA): [(Weight of first 2 mm size fraction) – (weight of filter paper or cloth)]/50
Small macro-aggregates (SMA): [(Weight of second 250 um size fraction) – (weight of filter paper or cloth)]/50
Corrected for sand content: this is optional, but may improve the quality of data, especially if we are comparing soils with different stone and sand content. Among soils with fairly fine texture (lots of clay and silt, little sand, especially coarse sand) it probably makes less of a difference to do this.
Sand-corrected stable aggregates: AFTER weighing the total aggregate mass, above, wet-sieve both of the above fractions to completely decompose the aggregates and leave only sand in a larger (that which does not pass the 2mm sieve) and smaller (that which does not pass the 0.25 mm sieve) size fraction. These can be washed back onto the same corresponding filter papers, and then dried (drying sieved sand will be faster than the original sand/soil mixture). When the sand + filter paper weight is subtracted out, the filter paper weights cancel out:
Corrected LMA: [(2.6mm size fraction – 2.6 mm sand)]/50
Corrected SMA: [(250 um size fraction – 250 um sand)]/50

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: