SOIL DEGRADATION
WITHIN THE
SOILCRITZONE
CONCEPT
by
Winfried E.H. Blum and Svetla Rousseva
SOIL DEGRADATION = SOIL LOSS
- as solid material = erosion by
water and wind (on erosion, there
exists an enormous amount of
literature)
- as soluble matter = leaching
EROSION THROUGH WATER AND
WIND ARE DIFFERENT PROCESSES
IN PRINCIPLE, THREE STEPS CAN BE
DISTINGUISHED:
- MOBILISATION
- TRANSPORT
- DEPOSITION
LEACHING OCCURS THROUGH
DIFFERENT PHYSICO-CHEMICAL AND
BIOLOGICAL PROCESSES:
- HYDRATATION
- HYDROLYSIS = PROTOLYSIS
- OXIDATION
- COMPLEXATION
IN THREE STEPS:
- SOLUBILISATION
-TRANSPORT
- PRECIPITATION AND FIXATION
WATER EROSION:
USLE = Universal Soil Loss Equation
A = R.K. L.S .C .P
A = soil loss ( g.m-2; t.ha-1)
R = rainfall erosivity (kinetic energy of rain)
K = soil erodibility (texture, structure etc.)
L = length of the slope
S = slope degree (inclination)
C = influence of land use (type of agriculture,
forestry etc.
P = measures against erosion (e.g. terracing,
countour stripping etc.
FORMS OF EROSION:
- sheet erosion
- rill erosion
- gully erosion
- tunnel erosion
- harvest erosion
(see pictures)
MEASUREMENT OF SOIL DEGRADATION
- Recent (actual) erosion
 Measurement on erosion plots (on site)
 Hydrological measurements in small
watersheds by gauges
Problems:
- not all eroded soil material reaches the waterways,
but is deposited close to the erosion site;
- extrapolation of results from some m² to large areas
is difficult or impossible;
- because soil formation is a process of thousands of
years, as well as soil degradation, actual measurements
are not very indicative for long time periods
- HISTORICAL (PAST) EROSION
might have been quite different to the actual one,
because of different climate conditions (climate variations
in historical times):
- topomorphic approaches: measurement of lake
sediments or sediments deposited downhill in lower
slope positions or in alluvial plains;
- measurement of isotopes;
- luminescence measurements;
- time markers, such as volcanic depositions of known
age in sediment layers;
Exact results are in general difficult to obtain.
- RESULTS OF LEACHING PROCESSES
mass balances based on known processes, e.g. limestone
weathering and deposition of calcrete
Estimations (assessment) of soil
degradation by erosion and
leaching are based on models,
extrapolating local results of
measurements onto larger areas,
taking into consideration factors such as
climatic influence, landscape, soil type,
vegetation cover, land use and others
(see USLE).
There are some dozen models available
for different physio-geographic regions
of the world.
CONCLUSIONS
1. Soil degradation is causing loss of soil material,
counterbalancing soil formation through weathering of
rock parent material;
2. Soil losses occur in solid (erosion) and liquid (leaching)
form;
3. Soil losses on one site mean at the same time deposition
of the lost material on other sites
- through aquatic/hydraulic transport and deposition;
- through wind transport and deposition;
- through precipitation after mobilisation and leaching;
4. Land and soil surfaces without protective vegetation
cover like deserts were always sources of soil material,
whereas others were sinks, gaining soil material, see e.g.
Sahara dust in South and North America