Aggregate strength
What is it?
In healthy topsoil, minerals combine with organic
matter, soil life and plant roots to form small
discrete pieces of soil. These small pieces of soil
are known as aggregates and they are the small
building blocks of soil structure. They are also
referred to as crumbs or peds.
Aggregate strength refers to the ability of soil
aggregates to keep their structure under stress.
Soil aggregates that hold together indicate stable
soil structure in good condition.
When placed in water, unstable soil aggregates
can either:
1.
remain intact;
2. slake – where the soil falls apart into very
small crumbs; or
3. disperse – where the individual clay
particles in soil aggregate disperse into a
fine suspension in water. The water then
becomes cloudy.
Clay content, organic matter, soil organisms and
fine plant roots give soil good structure and
reduce slaking and dispersion. Dispersion is also
a potential indicator of soil sodicity. This is where
there is excessive exchangeable sodium in the
clay minerals of a soil.
Why it is important:
Good structure is important for all soil functions.
It ensures vigorous plant root growth, good soil
aeration, and an effective water cycle. It also
provides habitat for soil organisms critical for
nutrient cycling. Sodic soil (with a relatively high
sodium content in its clay) compromises the
growth of many plants. Sodic soil is also highly
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prone to rill and gully erosion, as the clay in it is
unstable. This is a problem in some land types
where this soil property is present.
How to assess it:
Aggregate strength is measured using the
Aggregate Stability in Water Test (ASWAT). A
small dry aggregate of soil (3-5mm) is placed into
a shallow dish of distilled water, or rain water and
is monitored for slaking and dispersion during the
10-minute test. Score the soil sample using the
Aggregate Strength Score system, and complete
the results on the Bullseye Score Card. If possible,
select a dry sample, or air dry the soil for 24-48
hours before conducting the test.
Measuring method:
At a minimum, assess aggregate strength at one
point along the transect. Walk 15 paces along the
transect, then use a spade to dig out a sample of
soil. Try to keep it as an intact cube and try to get
to 10cm deep if possible. Use this sample for the
aggregate strength test. If the monitoring point
consists of grasses or shrubs, get a sample that
includes a plant, or is directly adjacent to one
if possible. Take soil aggregates from between
5-10cm deep. Test three soil aggregates and get
an average result. For more accurate monitoring,
conduct the test three times along the transect
and calculate an average. Walk along the transect
and stop every 15 paces to dig a sample and do
the test.
Equipment:
Drinking water bottle with distilled water, small
plastic containers or small paint pallet, stopwatch
or smartphone.