Land Use
What are the causes of land
degradation?
Soil is an important source of
nutrients
When soil is lost - where does it
go?
Salinization and Nutrient Loss
Picture from
the National Archives, taken during
the "Dust Bowl" in the 1930's
Land Use and Degradation
"The objective of this
convention is to combat desertification and mitigate the effects of drought in
countries... particularly in Africa ... in the
framework of an integrated approach ... with a view to contributing to the
achievement of sustainable development in the affected areas."
Convention to Combat
Desertification,
"A process that describes
human-induced phenomena which lower the current and/or future capacity of the
soil to support human life"
GLASOD - First United Nations
Global Assessment of Human-Induced Soil Degradation, 1988-90.
We wish to learn:
- What are the primary causes of land degradation?
- What is desertification and what management
strategies can be used to mitigate negative effects
1. Croplands
The
world's croplands are in decline due to the pressure of human activities. The
figure shows the regional and global trends in the total available area of the
world's croplands.
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Worldwide
the amount of cropland per capita has declined due to population growth.
North America and the former
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The first
global survey of soil degradation was carried out by the United nations in 1988-91. This survey, known as GLASOD - for
Global Survey of Human-Induced Soil Degradation, has shown significant problems
in virtually all parts of the world. The yellow line in each panel shows the
global cropland area per person. Obviously, this indicator is a function of two
factors: human population and cropland area. It has shown a steady decline in
the 30 years from 1961 to 1991, amounting to a decrease of between 20 and 30%.
The figure illustrates the regional changes that have accompanied this global
change. North and central America and the former
What are the causes of this degradation?
The loss
of arable land has been caused by a number of factors, many or most of which
are tied to human development. The primary causes are deforestation,
overexploitation for fuelwood, overgrazing, agricultural
activities and industrialization.
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Worldwide
the amount of cropland per capita has declined due to population growth.
North America and the former
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On the
global basis, the soil degradation is caused primarily by overgrazing (35%),
agricultural activities (28%), deforestation (30%), overexplotation of land to produce fuelwood (7%), and industrialization (4%).
The
patterns are different in the various regions. In North America, agriculture
has been responsible for 66% of the soil loss, while in
The
economic reasons for these processes are complex and are linked to the
particular characteristics of each region. Some of these issues will be
discussed later in the course, as part of case studies.
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Of
the world's 1.2 billion hectares with moderate to severe soil degradation,
the largest areas are in Asia and
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This
figure illustrates how pervasive is the problem of soil degradation. No
continent is free from the problem. Areas of serious concern include zones
where up to 75% of the topsoil has been lost already. The central portion of
the
2. Soils
All our
amino acids and nutrients eventually come to us from plant life (sometimes via
the meat of plant-eating animals). Plants synthesize amino acids from the
combination of sunlight, water and soils.
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Soil is
therefore of critical importance to life. Simply put: no soil, no life.
We first
define soil as a dynamic natural body capable of supporting a vegetative
cover. Where there is no soil, there is no plant life and we have barren
rock and/or sand. Soil is composed primarily of weathered materials,
along with water, oxygen and organic materials. Luckily for us, soil covers
most of the land surface with a fragile, thin mantle. Soil and agricultural
scientists have identified a huge number of different soil types.
Soil is layered
Soil is
layered into sections called "horizons". The figure shows a typical
soil profile developed on granite bedrock in a temperate region. The top
horizon is composed of humus and contains most of the organic matter.
This layer is often the darkest. The "A" horizon consists of tiny
particles of decayed leaves, twigs and animal remains. The minerals in the
A-horizon are mostly clays and other insoluble minerals. Minerals that dissolve
in water are found at greater depths. The "B" horizon has relatively
little organic material, but contains the soluble materials that are leached
downwards from above. The "C" horizon is slightly broken-up bedrock,
typically found 1-10 meters below the surface. While this is a typical soil
profile, many other types exist, depending on climate, local rock conditions
and the community of organisms living nearby. The U.S. Department of
Agriculture has classified 10 orders and 47 suborders of soils. If you include
other subsets, there are over 60,000 types of soil. The lunar surface, which
has been produced by meteoroid impacts, is not classified as a soil, but is
rather given the name "regolith" (derived
from the Greek words meaning cover and stone).
The
layered nature of soil indicates its long evolution under the effects of
atmospheric and biological processes. The process that creates soil from bare
rock is called "weathering". In the weathering process, the
atmosphere and water interact with bare rock to slowly break it down into
smaller and smaller particles. Rock climbers who encounter talus slopes
(regions of pebble-life rocks that form in great conical piles at the feet of
mountains) experience an intermediate step in the inexorable transition from
solid granite to sand and soil.
Soil is
generated by the process called weathering. This is a very slow process - and
one that depends a lot on the local conditions. For example, consider what
happens if you drop an iron nail in a garden in
Soil
itself is an important agent in weathering. When soil is present, rocks weather
much more quickly into soil. Thus, soil is both a factor in weathering - and a
product of weathering.
Rates of
weathering achange from place to place and from rock
type to rock type. The following weathering rates are in units of micro-meters
per thousand years:
- Basalt Cold climate: 10;
warm, humid climate: 100
- Granite Cold climate: 1;
warm, humid climate: 10
- Marble Cold climate: 20;
warm, humid climate: 200
The bottom
line on soil production is that it takes (on average) about 100 years to
generate a millimeter of soil. This is about a human lifetime. So, you can see
that soil geneation is a very slow process indeed on
human time scales.
Soil is an important source of nutrients
This table shows the results of a
famous experiment that was carried out at
From
experiments like this, it is possible to see that the pool of nutrients held in
the soil and vegetation is many times larger than the annual receipt of
nutrients from the atmosphere and from rock weathering. What this means is that
life husbands nutrients very effectively on land in the soil, storing
much of the total needed for plant life in the humus or topsoil.
So, we
can see that soil is very important for storing the essential nutrients needed
by plants. Simply put: No soil, no life!
The total loss of
arable land can be summarized in the following figure. Of the total available
(1500 million hectares, signifant components have
been lost due to the combined effects of desertification, salinization,
erosion, and development activities.
When soil is lost - where does it go?
When
topsoil is mobilized and gets into the hydrological cycle - it gets literally
washed to the sea. The topsoil becomes silt and traqvels
down streams and rivers, ultimately reaching the sea. The process is
responsible for a sink of topsoil. You can see a dramatic example of sediment output from
the
3. Soil Degradation Processes
Erosion
Erosion
is the term given to soil loss due to the mobilization of topsoil by the forces
of water and wind. Wind and water move the eroded particles to some other
location, where it is deposited as sediment. Soil erosion is a natural process
that removes soil from the land.
The
critical aspect of soil erosion for our purposes here is that the rate of the
process is highly dependent on human actions. Natural rates of soil erosion are
lower for soil with a good cover of vegetation than for bare soil. In fact, any
human actions that uncover soil (e.g., farming, logging, building, overgrazing,
off-road vehicles, fires, etc.) greatlty enhance soil
erosion rates.
During the last
dust bowl in the 1930's more than 30 million hectares were severely damaged in
parts of
Erosion
is a global problem, as we have already seen. Globally, topsoil is eroding
faster than it can be replaced over 1/3 of the world's croplands. In the
Desertification
Desertification
occurs whenever a non-desert area starts to exhibit the characteristics of a
true desert. The term was coined by the United Nations in 1977.
Over the
past 50 years, at the southern edge of the Sahara, an area the size of
Salinization and Nutrient Loss
Soil salinization is the concentration of salts in the surface
or near surface of soils. Human induced salinization
is a major problem in drylands and is often
associated with large-scale irrigation.
When drylands are irrigated, the water evaporates quickly,
leaving behind previously dissolved salts. These salts can collect, since there
is little rain to flush the system. The salt in the soil inhibits the uptake of
water by plant roots and the soil can no longer sustain a vegetative cover.
Nutrient
loss is an important problem in regions of low-input agriculture, such
as in
Restoration
We have
discussed some of the major problems in soil degradation. Can degraded soils be
restored to full function? This turns out to be a significant problem.
In
general lightly degraded soils can be improved by crop rotation, minimum
tillage techniques (next lecture), and other farm practices. More severely
degraded soils are more difficult to restore.
Moderately
damaged land takes more resources than an average farmer has to restore.
Changes in soil conservation practices can slow land degradation, but not
restore fertility often. National programs will be needed for such lands,
requiring major structural change (e.g., draining, contour banks, etc.)
Severely
eroded land generally is simply abandoned. Restoration efforts are
simply beyond developing countries - requiring deep ditches for drainage,
terraces to hold the soil in place, mechanized deep plowing to remove compaction,
reseeding programs, etc.
4. Summary
- Degradation of land includes
soil erosion, salinization, nutrient depletion,
and desertification. The rate of degradation has increased dramatically
with growth in human populations and technology.
- Severe land damage
accompanies large scale agriculture. Restoration is very
problematical.
- Continued loss of arable land
will jeopardize our ability to feed the world population.
- Land degradation is worldwide
- both developed and developing countries.
5. Suggested Readings
World
Resources 1992-1999: A Guide to the Global Environment,
F. T.
Mackenzie and J. A. Mackenzie, Our Changing Earth: An Introduction to Earth
System Science and Global Environmental Change, Prentice Hall, 1995