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1. Definition
| Name |
SOIL
STRUCTURE |
| Brief
definition |
Soil
compaction, tillage, shrinking and swelling and biological processes
in the soil, as well as climate and transport processes all
lead to the development of soil structure. When disturbed or
observed in pits the soil is not a compact mass and it naturally
breaks up into structural elements that may be characteristically
granular, crumblike, blocky, sub-angular blocky, platey, collumnar
or prismatic. |
| Unit
of measure |
Soil
structure is classified according to class (e.g. prismatic),
size and degree. This can be done using the FAO Guidelines for
Soil Description. The terms fine, medium and coarse and strong
medium and weak are used and are exactly defined in mm. |
| Spatial scale |
|
| Temporal scale |
|
2. Position
within the logical framework DPSIR
| Type of Indicator |
State, impact
and response. |
3. Target and
political pertinence
| Objective |
Changes
in soil structure provide insight into the impact that degradation
and regenerative processes are having on the soil. Soil structure
is also an indicator of sensitivity to erosion. |
| Importance
with respect to desertification |
A natural development
of soil structure occurs over time. In the top soil, biological
and climatologic processes result in the transformation of
homogenous material with a high bulk density into a material
that is composed of agglomerated particles separated by voids.
Structure gets finer in time.
Granular and crumb structure
usually results in favourable conditions for water retention,
provided the agglomerated elements are water stable. Slaking
(spotaneous breakdown of soil agglomerations in water) leads
to the loss of structure and the formation of a surface crust
that inhibits infiltration. Soil structure is strongly influenced
by small amounts of water soluble salts that promote shrinking,
swelling and dispersion. This results in soils that are very
hard when dry but soft and weak when wet. They have a high
bulk density, and a prismatic or columnar structure. Such
soils are prone to piping, rill and gully erosion. Soil structure
is strongly related to soil age as it is affected by mineral
transformation and weathering. It is also affected by climate.
In relation to desertification, the impact of organic matter
and salt are important. The development of soil structure
is essential as it increases the capacity of the soil to retain
water. High temperatures, trampling and tillage will lead
to a decline in structure. On the other hand shade, moisture
and nutrients promoting plant growth will lead to its development.
For the expert, soil structure is a sensitive indicator of
soil health and of how the soil is being affected by desertification.
In areas with a patchy vegetation, there is a great heterogeneity
in the structure of the surface soil with a good structure
being found where the soil is shaded. Stones promote good
structure because the fine soil between stones is usually
relatively moist.
|
| International
Conventions and agreements |
Soil structure
is an indicator that can be used by stakeholders to monitor
state impact and response of desertification. |
| Secondary objectives
of the indicator |
Descriptions
of soil structure form part of standard soil descriptions. The
user can relate his soil to existing knowledge. |
4. Methodological
description and basic definitions
| Definitions
and basic concepts |
Soil
structure is considered as a consequence of soil forming processes.
Plant growth, soil erosion and ecosystem resilience are all
related to it. When the soil is exposed on or near the soil
surface, its structure rapidly responds to environmental factors
such as climate (including water and temperature effects), and
macro and micro-organisms. Different types of structure have
specific physical, chemical, and morphological properties closely
related to the climate and parent material. |
| Benchmarks
Indication of the values/ranges of value |
|
| Methods of
measurement |
Use standard
FAO soil descriptions |
| Limits of the
indicator |
Requires demonstration
and training. |
| Linkages
with other indicators |
Soil
depth, Soil texture, Slope
gradient, Vegetation cover,
Rock fragments |
5. Evaluation
of data needs and availability
| Data required
to calculate the indicator |
Identification
of the type of parent material of the specific area. |
| Data sources |
Necessary data
are usually available and accessible and the cost/benefit ratio
is reasonable. |
| Availability
of data from national and international sources |
Data
can be obtained from various regional, national or international
institutions involved in the collection and the analysis of
soils data. |
6. Institutions
that have participated in developing the indicator
| Main
institutions responsible |
FAO
Rome, Agricultural University of Athens Greece |
| Other contributing
organizations |
Universities
of Lisbon, Murcia, Basilicata, Amsterdam, Leeds |
7. Additional
information
| Bibliography
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| Other references |
|
| Contacts
Name and address |
Anton Imeson
University of Amsterdam
aimeson@science.uva.nl
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