|
1. Definition
| Name |
SOIL
STABILITY INDEX |
| Brief definition |
This index
describes the relative stability of aggregated soil material
when a) it is subjected to a test that usually involves either
rapid immersion in water, raindrop impact or disruption with
ultra-sound or b) a test that reveals a decrease in permeability
or a change in the soil pore volume. The index describes the
sensitivity of soils to the processes of dispersion, slaking
and swelling. |
| Unit of measure |
Dimensionless
ratio |
| Spatial scale |
|
| Temporal scale |
|
2. Position
within the logical framework DPSIR
| Type of Indicator |
Usually a state
indicator but the index responds rapidly to change so that it
can be used to measure impact and response. |
3. Target and
political pertinence
| Objective |
The
soil stability index can be seen as a headline indicator of
soil quality, and it is being promoted as such. Previously the
main objective was to provide an index that could be used to
evaluate and assess soil erodibility. However, soil stability
is strongly positively influenced by functioning organisms,
water-soluble salts in the soil and the dynamics of organic
matter. It provides an integrated measure of the way in which
the soil is performing its regulation and production functions.
The objective is to use this index as an easily measured parameter
that integrates the effects of a range of processes that would
otherwise be more complicated and expensive to measure. |
| Importance
with respect to desertification |
The soil stability
index is important for desertification because it is related
to both water availability and the susceptibility of the soil
to different erosion processes. It can also provide information
on early warning and be used to evaluate the positive effects
of mitigation actions. Patterns in stability play a role in
creating source and sink areas of water. The tests require only
simple or limited equipment and can be taught and demonstrated
to stakeholders. It is even possible to apply this approach
using photos of soil structure. |
| International
Conventions and agreements |
The UNCCD emphasizes
the fact that combating desertification must be tackled within
the general framework of actions to promote sustainable development.
This index can be used by people to monitor their own soil conditions.
Within Agenda 21 soil stability is relevant to Chapter 12 -
Management of fragile ecosystems: combating desertification
and drought. |
| Secondary
objectives of the indicator |
As
already mentioned, this indicator is not only relevant for desertification,
it can also be used to evaluate soil quality. It responds rapidly
to climate and land use changes so it is a useful index for
demonstrating the relative impact of different management practices
on the soil. |
4. Methodological
description and basic definitions
| Definitions
and basic concepts |
The indicator describes
how a soil retains its integrity under the influence of different
disruptive processes. Along with infiltration capacity and
soil surface stability, it is part of a system designed to
identify, characterise in detail, and to classify source areas
(areas that become sources of sediment and surface flow under
rainfall of varying intensity).
Dispersion. Here
the actual amount of water dispersible silt and clay is compared
with the water dispersible amount of silt and clay when the
soil is treated according to a certain procedure. A more simple
approach (Emerson 1966) is to compare how soil aggregates
behave when immersed in water. Loveday and Pyle showed how
this could be used to calculate a dispersion index. This works
very well for dispersive soils.
Falling water drops.
Another approach is to determine aggregate stability using
falling water drops. The number of drops needed to break down
pre-wetted soil aggregates for them to be able to pass though
a sieve is counted.
Wet seiving. Essentially
this is the basis of the Jornada soil stability kit that is
now being recommended in the USA as a key indicator of soil
quality.
|
| Benchmarks
Indication of the values/ranges of value |
Threshold
values are test dependent. There is usually a clear threshold
between soils that is easy to identify. Some tests have indices
that range from 1 to 16 and others from 1 to 100. There are
many different methods, all of which work. For comparative purposes
it is possible to scale the different tests so that they fall
within either of these two. Two procedures, that use either
the impact of falling water drops to disrupt the soil, or ultrasound,
can be described in units of energy or power. Another procedure
involves studying the size distribution of soil aggregates that
can be observed also from photographs. |
| Methods
of measurement |
It
is recommended that you construct a soil stability test kit
according to USDA guidelines. The severity of the test should
be adjusted to meet the conditions found at the site of interest.
The Loveday and Pyle dispersion test is also recommended. This
is also useful for soils affected by salinity and irrigation.
The test involves immersing soil aggregates in water and observing
and scoring the responses after different lengths of time. The
water drop test involves building a very simple water-drop former
and allowing water drops to impact upon pre-wetted or dry soil
aggregates that are of 4-5 mm in size. Procedures can be found
for example in Imeson and Vis (1978) |
| Limits
of the indicator |
Limits
are mainly of an operational nature due to: the high cost of
surveys, both in of time and personnel; the difficulty of identifying
a number of sufficiently representative sites within the same
area; and the difficulty of finding comparable plots within
the same site, (i.e. with characteristics that are not likely
to bias the subsequent statistical data analysis). |
| Linkages
with other indicators |
Infiltration
capacity, Soil surface
stability. |
5. Evaluation
of data needs and availability
| Data
required to calculate the indicator |
These
tests should all be done in the field as changes take place
within soil samples during transport. |
| Data sources |
Based on individual
observations. |
| Availability
of data from national and international sources |
If the tests
are incorporated into soil evaluation score-cards, then a national
data base could be established as part of a soil quality data
base. |
6. Institutions
that have participated in developing the indicator
| Main institutions
responsible |
University
of Amsterdam, University of Lisbon. University of Valencia
|
| Other contributing
organizations |
3D-EC, Netherlands |
7. Additional
information
| Bibliography
|
Emerson, W.W., 1967: A
classification of soil aggregates based on their coherence
in water. Australian Journal of Soil Research 5, 47-57.
Loveday, J. and Pyle,
J., 1973: The Emerson dispersion test and its relationship
to hydraulic conductivity. CSIRO Australia, Division of Soils
Technical Paper No. 15.
A description of the soil
stability kit and its use can be found amongst other places
in this manual on page 28. http://usda-ars.nmsu.edu/JER/Monit_Assess/monitoring_main.html
The water drop test can
be found in: Imeson A.C. and Vis, M. Assessing soil aggregate
stability by water drom impact and ultrasonic dispersion in
Geoderma 34 185- 200
|
| Other references |
This reference by C.C
Boucher describes the history of the approach and provides
excellent illustrations of the process: http://www.dpi.vic.gov.au/dpi/vro/vrosite.nsf/pages/soil_mgmt_slaking?OpenDocument
Oades, J.M. and Waters,
A.G. 1991: Aggregate hierarchy in soils. Australian Journal
of Soil Research 29, 815-828.
|
| Contacts Name
and address |
A.C.Imeson,
Foundation for Sustainable Development (3D-EC), The Netherlands.
Tel: (31) 20 525 7457
Fax: (31) 20 525 7431
Email: 3de@hetnet.nl3de@hetnet.nl
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