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Introduction
Soil degradation is the decline in quantity and quality of soil. It includes:
- Erosion by wind and water
- Biological degradation (the loss of humus and plant/animal life)
- Physical degradation (loss of structure, changes in permeability)
- Chemical degradation (acidification, declining fertility, changes in pH, salinisation and chemical toxicity.
Fig. 1 The Universal soil loss equation
Causes of degradation
The Universal soil loss equation A = RKLSCP is an attempt to predict the amount of erosion that will take place in an area on the basis of certain factors which increase susceptibility to erosion (Fig. 1). These are discussed in Table 1 below.
January 2006 Number 191
SOIL DEGRADATION – A CREEPING CONCERN?
G F
eo actsheet
Table 1 Factors relating to the Universal soil loss equation (USLE).
Factor Ecological conditions Erosivity of soil R
Erodibility K
Length-slope factor LS
Land use type Crop management C
Soil conservation P
Description
Rainfall totals, intensity and seasonal distribution. Maximum erosivity occurs when the rainfall occurs as high intensity storms. If such rain is received when the land has just been ploughed or full crop cover is not yet established, erosion will be greater than when falling on a full canopy. Minimal erosion occurs when rains are gentle, and fall onto frozen soil or land with natural vegetation or a full crop cover. The susceptibility of a soil to erosion. Depends upon infiltration capacity and the structural stability of soil. Soils with high infiltration capacity and high structural stability that allow the soil to resist the impact of rain splash, have lowest erodibility values. Slope length and steepness influence the movement and speed of water down the slope, and thus its ability to transport particles. The greater the slope, the greater the erosivity; the longer the slope, the more water is received on the surface.
Most control can be exerted over the cover and management of the soil, and this factor relates to the type of crop and cultivation practices. Established grass and forest provide the best protection against erosion, and of agricultural crops, those with the greatest foliage and thus greatest ground cover are optimal. Fallow land or crops that expose the soil for long periods after planting or harvesting offer little protection. Soil conservation measures, such as contour ploughing, bunding, use of strips and terraces, can reduce erosion and slow runoff water.
wind
precipitation
mass movement
R
C
S
L
P
K
A = the predicted soil loss
R = the climatic erosivity or the rainfall erosivity index
K = soil erodibility
L = slope length
S = slope gradient
C = cover and management
P = erosion control practice
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Soil degradation encompasses several issues at various spatial time scales.
- Water erosion accounts for nearly 60% of soil degradation. There are many types of erosion including surface-, gully-, rill- and tunnel-erosion.
- Wind erosion
- Acidification is the change in the chemical composition of the soil, which may trigger the circulation of toxic metals.
- Groundwater over-abstraction may lead to dry soils, leading to physical degradation.
- Salt affected soils are typically found in marine-derived sediments, coastal locations and hot arid areas where capillary action brings salts to the upper part of the soil. Soil salinity has been a major problem in Australia following the removal of vegetation in dryland farming.
- Atmospheric deposition of heavy metals and persistent organic pollutants may make soils less suitable to sustain the original land cover and land use.
- Climate change will probably intensify the problem. Climate change is likely to affect hydrology and hence land use.
Climate change, leading to higher average temperature and changing precipitation patterns, may have three direct impacts on soil conditions:
- The higher temperatures cause higher decomposition rates of organic matter. Soil organic matter is important as a source of nutrients and it improves moisture storage.
- More floods will cause more water erosion.
- More droughts will cause more wind erosion.
Human activities
Human activities have often led to degradation of 15% of the world’s land resources (Table 2). These impacts frequently lead to reduction in yields. Land conservation and rehabilitation are essential parts of sustainable agricultural development. While severely degraded soil is found in most regions of the world, the negative economic impact of degraded soil may be most severe in the countries most dependent on agriculture for their incomes.
Table 2 Human activities and their impact on soil
erosion.
Action Removal of woodland or ploughing established pasture Cultivation
Grazing
Road or tracks
Mining
Effect The vegetation cover is removed, roots binding the soil die and the soil is exposed to wind and water. Particularly susceptible to erosion if on slopes. Exposure of bare soil surface before planting and after harvesting. Cultivation on slopes can generate large amounts of runoff and create rills and gullies. Overgrazing can severely reduce the vegetation cover and leave surface vulnerable to erosion. Grouping of animals can lead to over-trampling and creation of bare patches. Dry regions are particularly susceptible to wind erosion. Collect water due to reduced infiltration that can cause rills and gullies to form, e.g. when the west coast highway was built in St. Lucia, Tropical Storm Debbie led to massive erosion as water was channelled along exposed, bare road. Exposure of the bare soil. Degradation from chemical dumping.
Fig. 2 Types and causes of land degradation.
Types of land degradation Water and wind erosion account for more than 80% of the 20 million km 2 degraded worldwide.
Causes of land degradation Overgrazing and agricultural mismanagement affect more than 12 million km^2 worldwide. 20% of the world’s pasture and rangelands have been damaged and the situation is most severe in Africa and Asia. Huge areas of forest are cleared for logging, fuelwood, farming or other human uses.
Water erosion 56%
Deforestation/ fuelwood consumption 37%
Industry & urbanisation 1% Agricultural mismanagement 27%
Overgrazing 35%
Wind erosion 28%
Chemical degradation 12%
Physical degradation 4%
Managing soil degradation
Abatement strategies, such as afforestation, for combating accelerated soil erosion are lacking in many areas. To reduce the risk of soil erosion, farmers are encouraged towards more extensive management practices such as organic farming, afforestation, pasture extension and benign crop production. Nevertheless, there is a need for policy makers and the public to intensify efforts to combat the pressures and risks to the soil resource.
Methods to reduce or prevent erosion can be mechanical, e.g. physical barriers such as embankments and wind breaks, or they may focus on vegetation cover and soil husbandry. Overland flow can be reduced by increasing infiltration.
Mechanical methods include bunding, terracing and contour ploughing, and shelterbelts such as trees or hedgerows. The key is to prevent or slow the movement of rain water downslope. Contour ploughing takes advantage of the ridges formed at right angles to the slope to act to prevent or slow the downward accretion of soil and water. On steep slopes and in areas with heavy rainfall, such as the monsoon in South-East Asia, contour ploughing is insufficient and terracing is undertaken.
The slope is broken up into a series of flat steps, with bunds (raised levees) at the edge. The use of terracing allows areas to be cultivated that would not otherwise be suitable. In areas where wind erosion is a problem shelterbelts of trees or hedgerows are used. The trees act as a
barrier to the wind and disturb its flow. Wind speeds are reduced which therefore reduce its ability to disturb the topsoil and erode particles.
Preventing erosion by different cropping techniques largely focuses on:
- Maintaining a crop cover for as long a possible
- Keeping in place the stubble and root structure of the crop after harvesting
- Planting a grass crop. Grass roots bind the soil, minimising the action of the wind and rain on a bare soil surface. Increased organic content allows the soil to hold more water, this preventing aerial erosion and stabilising the soil structure. In addition, care is taken over the use of heavy machinery on wet soils and ploughing on soil sensitive to erosion, to prevent damage to the soil structure.
There are three main approaches in the management of salt affected soils: 1 Flushing the soil and leaching the salt away 2 Application of chemicals, e.g. gypsum (calcium sulphate) to replace the sodium ions on the clay and colloids with calcium ones. 3 A reduction in evaporation losses to reduce the upward movement of water in the soil.
Equally specialist methods are needed to decontaminate land made toxic by chemical degradation.
Case Study 2: Land degradation in Barbados
The most significant area of land degradation in Barbados is within the Scotland District (Fig. 4) which falls within the parishes of St. Andrew, St. Joseph and St. John. This area represents 14% of the island’s topography and is geologically unique, since it is the only area on the island where the limestone cap has been removed. Changing land use practices and the application of inappropriate agricultural techniques (growing sugar cane on very steep slopes, for example) have also resulted in significant and visible loss of soils in the limestone areas of the island. The Scotland District is particularly prone to landslides and slippage because of its elevation and soil structure. The soil in this area comprises clays and shales.
Causes of land degradation
- The reduction of the natural vegetative cover which renders _the topsoil more susceptible to erosion.
- Unsustainable land use practices such as excessive_ irrigation, the inappropriate use of fertilisers and pesticides and overgrazing by livestock.
The removal of vegetation and topsoil has resulted in:
_- Increased surface runoff and stream discharge
- Reduction of water infiltration and groundwater recharge
- Development of erosionial gullies and sand dunes
- Change in the surface microclimate that enhances aridity
- Drying up of wells and springs, and
- Reduction of seed germination of native plants_
Fig. 4 The Scotland District
Controlling land degradation
One of the most effective ways in which land degradation can be controlled is through increasing the vegetative cover within the affected area. The Soil Conservation Unit located within the Scotland District is involved in educating farmers about sustainable farming practices. The farmers are taught methods which include keeping the soil covered, incorporating organic matter to assist with percolation and reducing the use of fertilisers.
0 km 5000
ST. JOHN
ST. JOSEPH
Speightstown ST. ANDREW
Holetown
Bridgetown
N (^) Key: Degraded land
Conclusion
Soil degradation is a complex issue. It is caused by the interaction of physical forces and human activities. Its impact is increasing and is having a negative effect on food production. Some areas are more badly affected than others but in a globalised world the impacts are felt worldwide. The method of dealing with soil degradation depends on the cause of the problem, but also the resources available to the host country. Degradation is likely to increase over the next decades as a result of: climate change; population growth; the use of increasingly marginal areas for living and food supply.
References
- Technical Report on Soil Degradation. van den Born, G.J. et al, EEA (2000)
- Europe’s Environment , European Environment Agency (1998)
- Physical geography: a human perspective. Huggett, R. et al. Arnold (2004)
- A useful website is www.unepo.org
Exam question
Study Table 4. Describe and suggest reasons for the variations shown between continents in the amount of severely graded land.
Answer guidelines
Work on the statistics to calculate the percentage of severely affected land by area and by cause. Note the very high % of eroded land in Europe. Also note that in Europe only 25% of land from agricultural activities. Look for factors such as chemical contamination, urbanisation and industrialisation or impact of deforestation (e.g. Tennessee Valley or the Dustbowl in the Great Plains) as a result of over-cultivation, overgrazing and farming very marginal lands.
Use this Factsheet to interpret the differences between amount and causes, always supplying precise % when interpreting the statistics.
Acknowledgements This Factsheet was researched by Garrett Nagle. Curriculum Press, Bank House, 105 King Street, Wellington, TF1 1NU. Tel. 01952 271318. Geopress Factsheets may be copied free of charge by teaching staff or students, provided that their school is a registered subscriber. No part of these Factsheets may be reproduced, stored in a retrieval system, or transmitted, in any other form or by any other means, without the prior permission of the publisher. ISSN 1351-
Fig. 5 Controlling soil erosion - identify the methods marked 1 - 6.
Sub-Saharan Africa North Africa and Near East North Asia, east of Urals Asia and Pacific South and Central America North America Europe World
Land area 000s km 2
23, 12, 21, 28, 20, 19, 6, 134,
Total affected by severe/very severe land degradation 000s km 2 5, 4, 4, 8, 5, 3, 3, 35,
Amount of severe/very severe land degradation due to agricultural activities 000s km 2 1, 759 1, 3, 1, 2, 727 12,
Table 4 Human-induced land degradation.
