Soil aggregate stability: the slaking and dispersion test · Concept Studio

The idea

Soil crumbs that fall apart in water (slaking) or cloud it with clay (dispersion) have poor structural stability, which leads to surface crusting, slow infiltration and erosion.

What you need

air-dry soil aggregates 3–5 mm across, from topsoil and subsoil and from a cultivated paddock and a fence line
rainwater or deionised water
clear jars or Petri dishes, one per sample, on a white background
gypsum (calcium sulfate) for the extension, 2 g per litre of water
timer, camera, labels

How to do it

Break larger clods to expose aggregates 3 to 5 mm across and let wet samples air-dry.
Fill each jar with enough rainwater to cover the aggregates, then gently place a few aggregates in each jar; do not stir or move the jars.
Watch the first few minutes: aggregates that break into smaller pieces while the water stays clear have slaked (air bubbles are often seen).
Score dispersion (milkiness around the aggregates) at 2 hours and at 20 hours, using the photo scale on the WA Soil Knowledge Base page.
For samples that did not disperse, knead a little moistened soil for 1 minute, mould 5 mm cubes, place them in rainwater and score again at 2 and 20 hours.
Calculate the dispersion index: 2 hour score plus 20 hour score, plus 8 if the original aggregates dispersed.
Extension: repeat the dispersive samples in gypsum water and compare the cloudiness.

What you should see

Slaking, where it happens, usually takes less than a minute. Highly dispersive aggregates break down and cloud the water within about 20 minutes; others disperse over several hours; aggregates still intact after 20 hours are stable. Dispersion index values fall between 9 and 16 for spontaneously dispersive samples and 0 to 8 for remoulded ones; the higher the index, the more likely the soil is to respond to gypsum, and dispersive samples cloud the gypsum water less than the rainwater. Fence-line soil that has not been cultivated or trampled is expected to be more stable than the cultivated paddock. The learner knows it worked when repeat jars of one sample give the same scores.

What changes

What you change: soil sample (topsoil or subsoil; cultivated paddock or fence line)
What you measure: slaking (yes or no), dispersion scores at 2 and 20 hours, dispersion index
What you keep the same: aggregate size 3–5 mm
air-dry aggregates
rainwater or deionised water
jars left undisturbed

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Muddy runoff after rain is only loose dirt, not a sign of an unstable soil.
Slaking and dispersion are the same process.
Every clay soil falls apart in water.

Safety card

Low riskLearners carry it out

Hazards

soil contaminants
gypsum dust irritates eyes

Controls

gloves for soil
eye protection when weighing gypsum
wash hands afterwards

Note

NSW Department of Education, Chemical Safety in Schools (CSIS) 2021 Technical Update: Section 1 (general information for all staff) and Volume 2 Appendix D (generic risk assessment advice for each chemical). Complete a CSIS or RiskAssess site-specific risk assessment before the lesson.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Agricultural Technology Years 7–10 Syllabus (2019)AG5-8 AG5-11 AG5-12
Agriculture Technology 7–10 Syllabus (2024), new syllabus, implementation from 2027AGT5-ENV-01
Australian Curriculum v9AC9TDE10K04AC9S10I03

Sources

The pages the author read to write this activity.