Special Public Works Programmes - SPWP - Soil Conservation - Project Design and Implementation Using Labour Intensive Techniques (ILO - UNDP, 1982, 220 p.) CHAPTER A. GENERAL PRINCIPLES A.1. THE DIFFERENT FORMS OF SOIL DEGRADATION A.2. RAINFALL EROSION 2.1. Factors in rainfall erosion (introduction...) 2.1.1. Rainfall 2.1.2. Nature of the soil 2.1.3. Slope of the land 2.1.4. Vegetation 2.1.5. Man 2.2. The effects of rainfall erosion 2.2.1. Mechanical effects 2.2.2. Chemical effects 2.3. Integration of rain water erosion factors A.3. SOIL EROSION BY WIND 3.1. Wind erosion factors 3.2. The effects of wind erosion A.4. OTHER FORMS OF SOIL DEGRADATION 4.1. Excessive humidity 4.2. Excess of toxic salts 4.3. Unsuitable agricultural practices 4.4. Socio-economic aspects of soil degradation A.5. LAND USE 5.1. Land employment 5.2. Land classification 5.2.1. Classification system developed by the Soil Conservation Service of the US Department of Agriculture 5.2.2. Other classification systems

Special Public Works Programmes - SPWP - Soil Conservation - Project Design and Implementation Using Labour Intensive Techniques (ILO - UNDP, 1982, 220 p.)

CHAPTER A. GENERAL PRINCIPLES

A.1. THE DIFFERENT FORMS OF SOIL DEGRADATION

Soil degradation may be due to rain run-off from the soil, the effect of wind, excessive humidity, poor irrigation practice or unsuitable farming techniques.

The most spectacular forms of soil degradation are due to rainfall and wind which reshape the ground relief. Rain water running off over the soil may carry away the main fertile components and even totally strip the top soil which is the basis of agricultural production. The wind may have the same effect by carrying away the fine particles of an unprotected soil, including the main fertile components. The wind may also carry away larger particles, depositing them at a distance and thus covering with sterile sand deposits regions which were previously fertile.

Excessive humidity in the soil is another cause of degradation. The sources of this humidity may be numerous and varied and due either to topographical conditions (low-lying land), rivers overflowing into alluvial plains, excessive rainfall or over-generous irrigation. The consequences are degradation of soil structure and leaching of their chemical components, defective soil aeration with resultant effects on cultivation, low yields or the impossibility to continue agricultural production. Other forms of soil degradation are more insidious, less spectacular but nevertheless just as devastating for the region’s agricultural economy. In this case, it is the soil’s chemical content that is degraded. These forms of degradation may be due to excessive exploitation as the result of high demographic pressures. In many countries, periods of fallowing which allowed the natural regeneration of the soil have been reduced. The soil’s nutrients are not renewed and fertilising is not sufficient. This degradation may also be the result of poor irrigation in an arid climate where the salts brought in by irrigation build up in the soil resulting in salination and alkalinisation. Thousands of hectares of fertile land have been rendered unfit for cultivation in this way.

A.2. RAINFALL EROSION

2.1. Factors in rainfall erosion

Atmospheric precipitation is the main cause of rainfall erosion which produces surface run-off that has considerable destructive force. Other factors affecting erosion are the nature of the soil, the slope, vegetation and human activity.

2.1.1. Rainfall

The main characteristics of precipitations are the amount of rain, the intensity and the frequency. Rainfall intensity is one of the most important factors in soil erosion. Rainfall erosivity is the result of the kinetic energy in raindrops striking the soil; the amount of kinetic energy increases with rainfall intensity; it leads to soil compaction and demolition of aggregates.

Rainfall intensity is measured by means of a recording rain gauge.¹

¹ Recordings in Madagascar have shown that rainfall intensities of less than 1.5 mm/min are rarely erosive, whereas rainfall intensities of over 2 mm/min are always erosive. The figure of 2 mm/min is the cut-off point above which erosion occurs.

In Arkansas, USA, it is estimated that on uncovered, loamy soil with a slight slope (6 per cent), erosion occurs as soon as the rainfall reaches 2.5 mm in 5 minutes.

The influence of rainfall intensity increases with increasing soil humidity, i.e. with increasing rainfall frequency. A soil covered by a film of water will disaggregate more readily and will have more intense rain water run-off.

Annual precipitation variations also have an effect on soil loss and years of heavier rainfall produce wash away larger quantities of soil.

2.1.2. Nature of the soil

The susceptibility of soil to erosion depends on the soil’s nature and is called erodibility.

Erodibility is difficult to assess since it depends on numerous parameters, the most important of which are soil structure, texture, chemical composition and organic-matter content.

Texture refers to the proportion of different size particles in the soil. The smallest particles are clays and the largest are stones or gravel.

The international system classifies soil texture as follows:

- clays with a particle size range less than 0.002 mm
- silts with a particle size range between 0.002 and 0.02 mm
- fine sands with a particle size range between 0.02 and 0.2 mm
- coarse sands with a particle size range between 0.2 and 2.0 mm
- gravels with a particle size range greater than 2.0 mm

Structure refers to the arrangement of these individual particles in the soil into separate aggregates of different size and shape.

The cohesion of the structure or “structural stability” can be determined by use of Hénin’s “instability index”, the factors of which are:

- the mean percentage of stable aggregates,
- the fraction of dispersed clay plus loam,
- the fraction of coarse sand.

It is expressed by the equation

Structural stability is an important factor in water run-off and erosion. Stability is due in particular to humic and clayey colloids of soil which hold together sand and alluvial particles. The chemical nature of the bases which are linked to the absorbant complex also plays a role in the structural stability.¹

¹ Calcium and magnesium ions allow flocculation and, consequently, greater stability; sodium ions cause dispersion and structural disaggregation.

Disaggregation of the structure results in reduced soil permeability and porosity.

2.1.3. Slope of the land

The speed of rainfall run-off on soil increases with increasing slope, and soil erosion increases with increasing run-off speed.

Run-off flow rate and the amount of particles carried away also vary in relation to the length of the slope.

With a given angle of slope, erosion intensity will depend on:

- the nature of the soil
- vegetation cover
- precipitation Intensity

2.1.4. Vegetation

This is a major factor in controlling soil degradation and acts in several ways:

- By protecting the soil from the direct impact of water drops. When rain water is intercepted by the plant covering before it reaches the soil, the height of its fall is reduced and, consequently, its kinetic energy and destructive effect are smaller.

- By intercepting some of the rainwater which then remains on the foliage and evaporates without increasing the ground run-off volume.

- By inhibiting ground water run-off due to the matting of roots and accumulated vegetable matter.

- By enriching the soil with organic material which improves structure and porosity.

Roots and, in particular, the matting of fine roots increase the cohesion of soil particles. Their effect is even greater if they grow densely close to the surface. Dead roots increase the porosity of the soil surface and promote water infiltration. Organic matter resulting from leaf decomposition improves soil structure.

The effects vary depending on the type of vegetation.

Forest growth has the greatest effect in protecting soil from water erosion. Forest soil contains 2 to 3 per cent of organic material.

A grass covering may also have a significant effect provided the plant coverage is dense.

Fallowing also plays a conserving role depending on the type of vegetation involved (forest fallowing, crop fallowing, bare fallowing).

Crops have a less conservational effect. Resistance to erosion increases with increased density of crops. Forage pasture offers better protection than cereals or hoed crops.

Orchards do not constitute a sufficiently dense vegetation to effectively control erosion.

Research carried out around Lake Aloatra, Madagascar (ref. 1), on the Aristida pastures with slopes of 20 to 36 per cent have shown the following topsoil losses in relation to the density of vegetation coverage:

	- 100 per cent covered soil	0.026 t/ha/year
	- 40 to 60 per cent covered soil	4 t/ha/year
	- 20 per cent covered soil	12 t/ha/year

2.1.5. Man

Man is a prime factor in soil degradation since irrational use of the soil is often at the origin of erosion.

Abusive use of forests and pastures may lead to their destruction; the same applies to abusive clearing of the soil on very steep slopes, unsuitable crops, ignorance of the mechanisms by which organic material is lost from the soil and how it is replaced.

2.2. The effects of rainfall erosion

2.2.1. Mechanical effects

These are due to the impact of water droplets on the soil and the erosive force of rain water run-off. Primary or “splash” erosion is due to the impact of the raindrop on the soil which breaks up the soil particles and may project them up to 60 cm vertically and 1.50 cm horizontally. The energy released by soil particle disaggregation increases with rain intensity.

The finer soil particles are more readily dispersed by the “splash” effect. These particles block the pores in the soil surface decreasing the soil absorption capacity, and the excess water runs off carrying away the fine particles in suspension.

“Splash” erosion can be reduced or prevented by a covering of vegetation which absorbs a large part of the raindrop’s kinetic energy.

Water which does not filter through the soil on a watershed runs down the slope. Initially, the run-off is diffuse or forms a sheet of water in minute anastomosing streams. Gradually, these streams hollow out small grooves a few centimetres in width and depth which may carry away fine components up to sand particles of 0.2 mm diameter. This type of erosion may progressively strip the top and most fertile layers of the soil. Cultivation and biological practices may reduce this erosion by modifying soil stability or aggregate size. If such processes are not adequate, mechanical processes may be employed by levelling the land, breaking it down into small fields in which erosion is no longer a danger.

As erosion continues, run-off collects in small rills or channels where its erosive and transporting powers are enormously increased. The rills become gullies and the gullies become progressively deeper and wider forming ravines. If the process is allowed to continue all the top soil may be stripped off.

Where rill or gully erosion takes place, cultivation or biological processes are no longer sufficient to retain the soil. Dividing the land into small fields significantly reduces rill erosion; however, when gully erosion occurs, more extensive collective measures are required. The erosion sediment is carried by gullies, streams and rivers to lakes, dams or, finally, to the sea. Local deposition of sediment can cause great damage to growing crops and silt up drainage and irrigation channels, increasing the danger of flooding.

The end result of uncontrolled water erosion is the loss of soil and destruction of its productive capacity.¹

¹ To give an example of the proportions that this type of erosion can attain, it is estimated that in India, with a total surface area of 3.3 million km², 1.4 million km² are subject to significant soil loss and 6,000 million tonnes of soil are lost each year from a surface area of only 800,000 km² (UNESCO Courrier, May 1980).

2.2.2. Chemical effects

Added to soil loss, there are significant losses of fertile components,² in particular mineral salts. Drainage water may contain up to 50 g of calcium nitrate in the case of cultivated land and up to 150 g in the case of bare land.

² In India, it is estimated that 6 million tonnes of fertile components disappear each year, i.e. more than is applied in the form of fertiliser.

A rainfall of 10 mm on a soaked soil may carry off 5-15 kg of this fertiliser per hectare.

2.3. Integration of rain water erosion factors

An attempt at an integration study of rain water erosion factors is given in Wischmeier’s formula.

Fig. A.1: Diagram of the main natural factors in run-off erosion

After M. Deloye and H. Rebour 1953 (ref. 11)

This formula, when applied to a specific region, makes it possible to estimate soil loss and determine what erosion control methods should be implemented to ensure that erosion does not exceed the threshold at which it becomes dangerous.

Wischmeier’s formula is as follows:

A = R (K L S C P)

Where:

A is the soil loss in tonnes per acre¹ (1 American short ton = 0.907 kg);

¹ 1 acre is equal to approximately 4,000 m².

R is the precipitation erosivity factor or “rain index”.

It can be calculated for a rainfall or for the rainfalls over a given period. Generally, a mean annual rainfall index is used.

For a given rainfall:

where	Eg = kinetic energy of the rain in feet/tonne/year;
	Im = maximum Intensity of the rain in 30 min in inches/hour.

To calculate the kinetic energy of rain, it is necessary to have a hyetogram recording in which the rain is broken down into segments of equal intensity in order to establish a duration-intensity ratio.

The relationship between kinetic energy of a rainfall (of regular intensity) am intensity is given by the formula:

Eu = 916 + 331 Log I_h

In which:

Eu = unit kinetic energy in feet/ton/year
I_h = intensity in mm/h

The energy in segment E_h is equal to Eu multiplied by the number of millimetres which have fallen during the segment. The energy is cumulative.

In order to calculate Im, it is necessary to mark on the recording the 30 min section of the curve in which the largest number of millimetres of rain fell.

K or the “soil index” is a dimensionless factor which measures the relative resistance of a soil to erosion. These values are obtained experimentally.

L.S or the “slope index” is a dimensionless factor; it indicates the effect of the angle and length of the slope (see fig. A.2).

C or the “cultivation index” is the ratio of earth loss of cultivated land under well-defined conditions to that of a continually worked fallow land where C = 1 (fig. A.3).

P or the “water and soil conservation index” is the ratio of earth loss on a field in which soil conservation is practised to that of a cultivated field along the line of maximum slope (fig. A.4).

Fig. A.2: Wischmeier’s universal equation (graph giving the values of the factor L.S as a function of the length and percentage of the slope (ref. 23))

Fig. A.3: Crop coefficient values - C

Type of crop		C
In the United States
	non-hoed crops (rice-cereals)	0.6-0.8
	plant covering, green manure	0.3-0.6
	fallow and depending on the condition, location, climate	0.3-(1.5)
In Tunisia:
	bare earth - bare fallow land	1
	orchards	0.90
	wheat	0.71
	rotation with cereals	0.40
	fodder	0.47
	rotation with fodder	0.15-0.23
	improved pastures	0.01

For mechanised cultivation, these values should be subjected to a coefficient of 1.3-1.8.

Fig. A.4: Erosion reduction coefficient of soil conservation remedies

Value of P factor: water and soil conservation index in %
			Terraces
Slope %	Contour line cultivation L value to be considered: length of field slope	Rotational field strip cropping value of L to be considered: length of field slope	the earth in the channel is considered lost Value of L to be considered: distance between channels	the earth in the channel is not considered lost Value of L to be considered: distance between channels
1.1 - 2.0	60	30	60	30
2.1 - 7.0	50	25	50	25
7.1 - 12.0	60	30	60	30
12.1 - 18.0	80	40	80	40
18.1 - 24.0	90	45	90	45

A.3. SOIL EROSION BY WIND

3.1. Wind erosion factors

The erosive effect of wind varies depending on the nature of the vegetation and soil.

The ability of wind to move soil particles depends on wind intensity and particle size. At soil level, wind speed is zero, flow is laminar for a height of a few millimetres and thereafter wind speed increases as the distance from the soil increases as a function of the logarithm of height.

It is estimated that the wind speed required to move the finest soil particles is 15 km/h. The effect of wind varies depending on particle size.

The smallest particles are carried in suspension in air and may form dust storms that move over considerable distances.

Medium-size particles of 0.05-5 mm diameter are carried in a bouncing action over the surface of the ground by a phenomenon called saltation.

The larger particles roll or “creep” along the surface. The saltation effect increases the number of particles in motion as they are carried along; this has an avalanche effect. The amplitude of the phenomenon increases the greater the area of land exposed to the wind.

Vegetation is the best protection against wind since it breaks the force of the wind and reduces the area of exposed land, thus limiting the saltation process.

The soils most susceptible to wind erosion are those of coarse texture and, in particular, fine sands. The soil’s level of humidity also plays a role and dry soils offer the lowest resistance to the effect of the wind.

Wind erosion occurs, in particular, in arid and semi-arid regions in which there is a major dry period and light vegetation - part of which disappears totally during the dry season due to over grazing of pastures. The constant action of cattle hooves tends to break up the soil surface and make it more susceptible to wind erosion.

In temperate climates, wind erosion is also encountered on sandy coastal areas due to the soil texture and the absence of sufficiently dense vegetation.

Crops and various farming techniques can also cause erosion. Repeated working of the soil and excessive soil fragmentation during the dry season tend to increase the danger of erosion whereas cultivation techniques which maintain or increase soil surface roughness (ploughing, banking) have a protective effect.

3.2. The effects of wind erosion

Wind erosion has a deleterious effect on the soil:

- by loss of fine soil components, and fertile components in particular, which leads to structural degradation and a reduction in water retention capacity;

- by moving the coarser components which build up behind various obstacles and form dunes which can cover and make sterile entire regions.

Wind erosion also effects vegetation itself. Windborne sand particles have an abrasive effect on grass and crops. The wind increases vaporisation and tends to exhaust the soil’s usable water content more rapidly.

A.4. OTHER FORMS OF SOIL DEGRADATION

4.1. Excessive humidity

Excessive humidity in the soil leads to degradation and reduced fertility. Water is an essential component for soil and plant life but excessive quantities have disadvantages due to:

- reduction of chemical and bio-chemical action resulting from oxygen deficiency which prevents oxidation and certain micro-organism life;

- the reduction of soil temperature as a result of excessive surface evaporation;

- its action on plant roots which can no longer penetrate deeply into the soil and which often suffer from parasitic disease promoted by the high humidity;

- the difficulty of cultivating wet soil;

- reduction in crop yield may range from a slight fall to total crop loss.
Excessive humidity can be controlled by drainage.

4.2. Excess of toxic salts

In arid and semi-arid climates, irrigation may lead to the build up of toxic salts in the soil. Each new irrigation flow brings with it a certain quantity of mineral salts. If the water flow is not sufficient to leach the soil, i.e. take the salts down to lower levels where they will not be harmful to plant life, these salts gradually build up in the soil due to vaporisation until they reach concentrations harmful to crops.

In addition to salt concentration (Salinisation), the phenomenon of alkalinisation may occur, i.e. calcium ions are replaced by sodium ions in the absorbant soil components and this leads to degradation of soil structure and reduced permeability.

These phenomena can be controlled by leaching the soil and by suitable draining to evacuate water with a high dissolved-salt content.

4.3. Unsuitable agricultural practices

Intensive agriculture and failure to apply adequate amounts of fertilisers exhaust the soil of its plant nutrients. This form of soil degradation¹ is noted only in passing since measures for remedying this do not come within the framework of labour-intensive work.

¹ There are other forms of degradation such as sedimentation or soil acidification,

4.4. Socio-economic aspects of soil degradation

The main effect of soil degradation is the damage to agricultural activities that result. The harm to agricultural land may be irreversible, or the cost of returning the land to its fertile state may be so high as to be not economically viable.

The farmer’s profits must be sufficient to allow him to live and pay other expenses such as fertilisers, seeds, fuel, etc. They must also be sufficient to invest in soil conservation and improvement.

This situation is of course more difficult to achieve in smallholdings with poor soils than on largeholdings with good soils.

As erosion progresses, the farmer’s work becomes more difficult, more expensive and less profitable and finishes by becoming impossible. At the regional level, the deleterious effects finally undermined the total structure of social and economic life.

In order to avoid such situations it becomes necessary, wherever possible, to encourage conservation measures which bring together the largest number of farmers in work of value to the collectivity.

A.5. LAND USE

5.1. Land employment

Depending on its characteristics, land is normally classified into two large categories: production land and protection land.

Production land is used for cultivation.

Protection land usually has natural forest or pasture vegetation and plays a major role in the conservation of the cultivated land that is situated downhill.

The balance between production and protection land will vary depending on the country’s level of development and may change depending on technical, social and economic conditions.

Although protection land is of less significance from the economic point of view than production land, it has a decisive role in maintaining the country’s biological balance.

5.2. Land classification

5.2.1. Classification system developed by the Soil Conservation Service of the US Department of Agriculture

The classification is divided into:

- capability units;
- capability subclasses;
- capability classes; see fig. A.3, crop coefficient values.

The capability units group soils that have about the same influence on crop production and respond in about the same way to the management requirements of common crops.

The subclasses group capability units having similar limitations (erosion hazard, wetness and climatic limitations).

The capability classes describe progressively, in eight stages, the degree of risk to erosion and limitations of use.

The simplified classification is as follows:

Simplified classification

Unit	Subclass	Class
I			Land suitable for cultivation
	1		Land with annual cultivation
		I	Land that can be permanently cultivated and which, when normally rotated, is treated with fertiliser or lime. These are lowlands for which conservation practices are not necessary.
		II	Less fertile land with lower yield, often on a slight slope (3%), where erosion has already taken place by reducing the depth of the arable land. Moderate conservation practices necessary.
	2		Land with intermittent cultivation
		III	The soil has to be reconstituted periodically by allowing a vegetation covering to occur, and cultivation takes place only from time to time. This is the case of eroded land with slopes of 10-16%.
		IV	Land where the slope is steeper than in class III and more seriously eroded. Is suitable only for occasional or limited cultivation.
II			Land requiring permanent cover
		V	Land not suitable for ploughing but relatively resistant to erosion. Suitable for permanent pasture. Requires careful exploitation.
		VI	Land of the above type but which has poor erosion resistance due to physical properties or topography. It can be used for pasture with conservation techniques required from time to time.
		VII	Exhausted land. Pronounced erosion that can be reconstituted by grassing or planting with total conservation practices.
III			Non-productive land
		VIII	Soils of class VIII are suitable only for natural vegetation, forests, etc. Should not be cleared.

Fig. A.5: Land classification on the basis of susceptibility to run-off erosion (based on a drawing published in the USA)

Fig. A.6: Deep gully. Advanced stage of erosion (Upper Volta)

Figure

Figure

Examples of advanced stage of erosion

Niger

Upper Volta

Upper Volta

5.2.2. Other classification systems

5.2.2.1.	Classification by degree of run-off erosion (ref. 11), see fig. A.7.
5.2.2.2.	Classification of land by slope in tropical Africa (ref. 1), see fig. A.8.
5.2.2.3.	The classification of BEEK and BENNEMA (FAO 1972) (ref. 28). This is a new classification system designed more specifically for developing countries. It incorporates social and economic factors into the technical capability classification.

Fig. A.7: Diagramatic classification of degrees of run-off erosion

Current or potential state	Apparent effects of erosion	Soil characteristics	Types of remedy
I. Stage of stability	Insignificant, clear run-off water. No apparent erosion	Flat or almost flat land or very slight slopes less than 3% High permeability: 20 cm/hour (cf. 3.2.4, Ch. D) Dense cover of vegetation Good fertility Good soil cohesion Land well supplied with humus	All types of crop possible No treatments
II. Stage of insidious erosion	Slight run-off of turbid water at very low speed during heavy precipitations No apparent erosion or traces of rills	Slight uniform slopes of less than 3%, or very intersected slopes of 5-8% Good permeability: 12-15 cm/hour Light covering of vegetation Moderate fertility Relatively good soil cohesion Land with a relatively good humus content	All crops on contour lines Well planned rotation From time to time, bench terraces Reduced grazing density
III. Stage of initial apparent erosion	Run-off already quite pronounced with moderate precipitation; muddy water flowing at moderate speed Appearance of light patches and stones on surface Shallow gullies appear especially after the soil has been broken up, but do not hinder machines Slight reduction in fertility	Uniform slopes of 5-8%, or intersected slopes of 10-16% Small collection basins (1 or 2 hectares) Moderate permeability: 8-10 cm/hour Very slight cover of vegetation Moderate fertility Poorly coherent soils Land with only slight humus content	Alternating crops on contour lines with ½ in annual cover crops Bench terraces often necessary Reduced grazing density
IV. Stage of intense erosion	Heavy run-off of muddy water with moderate and heavy precipitation, speed quite high to very high Increase in the number of patches and stones Deep gullies which begin to Impair mechanical cultivation	Uniform slopes of 10-16% or intersected slopes of 20-30% Watershed of several hectares Low permeability: 2-5 cm/hour No vegetation covering	Alternating permanent grass and cereal cultivation, for example cover crops must dominate in the rotation system Terracing essential Back-sloping terracing necessary on slopes over 15% Ploughed crops are possible between terraces Low grazing density
V. Stage of dangerous erosion	Pronounced run-off at the slightest rainfall Water carries gravel or aggregates moving at high speed in the event of heavy precipitation Deep gullies preventing the movement of heavy machines Land carried away in blocks	Uniform slopes of 20-30% or intersected slopes of 45-65% Large watershed Very low permeability 0.5-1 cm/hour No vegetation cover Mediocre fertility Unstable soils	“Algerian terraces” essential Cover cultivation everywhere, with permanent grass cover on large surfaces Pasture, woods very light grazing density with periodic prohibition
VI. Final stage of erosion	Top soil entirely stripped away	Very steep slopes Very large watersheds Virtually zero permeability No vegetation cover Fertility completely lost Unstable soils	Diversion channel above and below to protect cultivated areas Prohibition of grazing Trial tree-planting on back-sloping terraces

Observations

1st column:	A given area of land may go successively through the six stages described, commencing with the stage of stability.
2nd column:	There is a constant danger that the features may deteriorate, but they can regress if the treatment given in column 4 is suitably applied. Where the proposed remedies prove Inadequate, it would be necessary to immediately apply the remedies of the next, more serious stage.
3rd column:	This is merely a list of different features, some of which must exist in combination; however, it is not necessary that all be present.
4th column:	For each category, there is listed a number of treatments by order of effectiveness; the choice should be made depending on the stage to which the erosion has progressed. Obviously, general remedies (working, fertiliser, etc.) should be applied at all stages. Various stages may coexist in a single plot of land. In general, the erosion starts at the lowest part (watershed effect) and moves progressively upwards.

Fig. A.8: Classification of land on the basis of slope (in tropical Africa)

Reference 1

Slope %	Land use	Possible cultivation methods	Crops to be avoided	Protective measures
0-3	Various crops	Mechanised cultivation	-	-
3-12	Crops alternating with grass cover	Mechanised cultivation	Precautions to be taken for bush-type culture on bare soil	Absorption or diversion network Contour cultivation
12-25	Crops grassland woodland	Manual cultivation Animal-drawn machines	Bush-type crops on bare soil	Anti-erosion networks Terraces
25	Pasture forestland	Manual cultivation	All	Anti-erosion ditches