Worksheet AS Physical and selected chemical methods of soil analysis,
IHLW, SS2018
Soil Aggregate Stability
Lab method using wet sieving apparatus (method after Kemper)
Sample preparation
•prepare 4.0 g air dry, disturbed soil samples, dry sieved to 1 – 2mm
aggregate size, three replicates per soil sample,
•store soil samples in ceramic cups, weigh cup plus air dry sample (
mPr
),
note cup number
Measurement
•prepare the wet sieving apparatus: fill ca 80mL de-ionised water into each
of the six metal bowls
•put soil samples into the metal sieve cylinders, note cylinder number to the
corresponding line at the form
•insert sieves into the water bowls using the apparatus` cover, ensure that
soil samples are completely below the water table
•start sieving apparatus and timer, samples are now moved vertically in the
water
•stop after 5 min, wash soil samples from the sieves into the corresponding
ceramic cup and dry it in the oven (105°C) for 24 h
•weigh oven dry samples (
mAS
, AS for aggregates + sand), add dispersant
and wait 2 h
•repeat sieving and drying procedure, weigh mass of resisting sample (
mS
,
S for sand)
Results
•calculate the fraction of stable aggregates using Eq. (1)
AS =
mAS −mS
mPr−mS
(1)
Equipment
•wet sieving apparatus after Kemper (1966)
•scale, precision ±0.01 g
•0.4n-sodium pyrophosphate solution as dispersant
Why do we measure?
Aggregate stability is a measure for the
soil`s stability to resist eroding impacts
such as rainfall, surface water flow or
wind.
The result of this measurement is the
fraction of soil aggregates which endure a
defined energy impact. As also sand
particles are resistant but not subject of
this investigation, they need to be
excluded from the analysis by subtraction.
In Practice
Measurements of SAS demonstrated that
soil stability Is higher in conservation
tillage fields than in conventional tillage.
Some background information
Pores are the soil zone where roots grow,
beneficial organisms live, and water is
stored or conducted. Aggregation of soil
particles due to biological activity or
swelling and shrinking extend the pore
system and hence improve soil quality. If
the stability of soil aggregates is low, they
collapse easily and soil is eroded by wind
or water. Furthermore, fine soil particles
seal important pore space.
If the relative air humidity in the lab is
higher than 30%, the gravimetric water
content w of air dry soil samples should
be determined on an additional
subsample. The mass of the air dry
sample needs to be corrected for the
water content.