Slake (Wet Aggregate Stability)

The slake (wet aggregate stability) test scores how well your soil structure holds together in water.

Healthy soil has good organic matter content and is full of microorganisms that secrete glues which create the aggregated crumb structure we want to see.

When aggregates with a good soil structure held together by organic matter and biological glues are submerged in water they stay together.

When aggregates with a poor soil structure held together by compaction are submerged in water they fall apart.

What to record


  • Score, after 5 minutes submerged in the water
    • 0: Dissolves into single grains – lump collapses completely, as soil is too unstable to isolate aggregates.
    • 0.5: Soil slumps into pyramid – lump collapses into a cone of <2mm grains, water is cloudy.
    • 1: Breaks into angular pieces – the lump breaks up into larger angular pieces, indicating a loose, granular surface layer.
    • 1.5: Stays mainly intact – lump edges crumble slightly but remains largely intact.
    • 2: Lump intact – lump remains completely intact and the water is clear, indicating your soil is resistant to erosion
  • Record photos

Equipment


  • Spade
  • Small bags to carry soil samples / marker pen for labelling bags
  • Sieve (back at office)
  • Bowl of cold water (back at office)
  • Stopwatch (back at office)

How to do the test

1. Choose a day when the soil is not waterlogged and it has not been recently cultivated.

2. Locate your chosen sample site – you can use the GPS mapping feature on the app to do this if you’ve recorded the sample locations previously.

3. Insert your spade to about 20 cm depth into the soil. Dig up the soil and gently break it apart to release pieces of soil.

4. Collect a fist-sized sample of soil put in small labelled bag to take back.

5. Back at the office: select three 1-3mm sized lumps of soil from the bag and leave them to dry overnight. They may need another 2-3 days more drying if very wet – must be completely dry before testing.

6. Once dry, arrange pieces of soil on the sieve and fully immerse into water until the lip of the sieve is reached. A light coloured water bowl (as a background) will make it easier to see how the soil is behaving.

7. Start timer for 5 minutes, & observe the pieces under water. Score the behaviour of the pieces in Soilmentor using the score above.

8. You may like to take another sample in an long-term uncultivated part of the farm, e.g. woodland, for a good comparison to assess the effect of field management on aggregate stability.

Read this article to find out how many times to do the slake test for each field, and watch the video below.

Don’t forget to take photos!

Resources

This methodology is based on the method developed in the following research:

Effect of farm management on topsoil organic carbon and aggregate stability in water: A case study from Southwest England, UK

Sarah M. Collier Sophie M. Green Alex Inman David W. Hopkins Hazel Kendall Molly M. Jahn Jennifer A. J. Dungait

Adapted from USDA ARS On Farm Soil Monitoring

Scoring and protocol updated in collaboration with Nicole Masters and the Integrity Soils team.