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PLATE 1: SHEET EROSION

On grassland planted too late to Panicum maximum, the first May storms beat down on the poorly covered soil and separated the humus and clay from the coarse sand. Sheet runoff carried away the light topsoil particles (coloured dark grey), leaving behind sheets of red sand as evidence of eroded soil. ORSTOM Station, Adiopodoumé Côte d'Ivoire (5% slope).

As (simulated) rain falls on the soil, it causes sheet runoff, which moves slowly. If the surface is rough, runoff quickly becomes organized (here after 3 minutes) into a network of thin, faster streams which carve small temporary grooves in which the water (artificially coloured) travels as in wadis. ORSTOM Station, Adiopodoumé, Côte d'Ivoire (7% slope).

Sheet erosion has been allowed to develop on this bare plot. It has left behind small pedestals of soil (2 to 10 cm high) protected by a hard body (crust, roots, seeds). Runoff tries to carve away the base of these pedesals, forming "micro-cliffs". It carries away the fine topsoil particles (grey), leaving a coating of red (ferruginous) grains of sand on the surface. Adiopodoumé Côte d'Ivoire (7% slope).

A good mulch or a cover plant (here a pulse sown between rows of maize at the first hoeing) is enough to absorb the energy of raindrops, completely preventing sheet erosion and runoff. IITA Station, Ibadan, Nigeria.

PLATE 2: ERODIBILITY OF VARIOUS SOILS

Clayey vertisols rich in Ca are highly resistant to sheet erosion (K = 0.01 to 0.10). When waterlogged, they are prone to gullying ICRISAT Station, Hyderabad, India

Brown-red, sub-arid, alluvial soils, poor in organic matter, are very sensitive to rainsplash. Heavy runoff develops, leading to widespread gullying as soon as such soils are placed under cultivation. Sabouna, Burkina Faso.

Deep ferralitic soils are highly resistant (K = 0.1 to 0.2), but in the weathering zone, runoff activates gullying and lavakas which then develop through successive rock slides. Ambatomainty, Madagascar.

These schists are too shallow and should never have been cleared. During a heavy rainstorm (of the kind that strikes every ten years), the channel overflowed and stripped away the entire soil cover, degrading the soil for centuries. Capetown, South Africa.

Andosols are highly resistant to rainsplash, but when ground fine, the surface horizon floats in the runoff water. In order to protect such soils, the aggregates must be enclosed in a network of roots. Honolulu, Hawaii.

In the foreground, the sandy ferruginous soil derived from a calcareous sandstone is very fragile, being rich in fine sand and poor in organic matter. In the background, black, clayey, humus-rich vertisols derived from diorite are extremely resistant. Crops grow faster on this kind of soil. East London, South Africa.

PLATE 3: SOIL DEGRADATION

Fire and cropping quickly degrade the organic matter in these very poor, gravelly soils. A long fallow period is needed to re-establish the original vegetation. Koutiala, Mali.

Streambanks are often degraded by the continual passage of herds and flocks which come to drink and to graze the last green pasture in the dry season. Kaniko, Mali.

All cotton residues must be removed for pest reasons, causing organic imbalance in the soil and degradation of its productive potential, despite the good number of karité trees. Kaniko, Mali.

All the stumps had to be removed to allow mechanized tillage. After about ten years, the sandy topsoil horizon had been scoured, revealing the compacted impermeable crust of the plough pan. Runoff was then so heavy that it carried away seed and fertilizing organic residues, preventing any soil restoration. Baramandougou, Mali.

PLATE 4: EFFECTS OF FIRE

Fire can have five functions in Africa: clearing, hunting, parasite control, rangeland upkeep, and as an expression of dissatisfaction with government (particularly the forestry services). Korhogo, Côte d'Ivoire.

When fires are set earlier than usual, i.e. a month after the last useful rainfall, they spread quickly through the savannah, selecting for the fire-resistant tree species; this results in a shrub savannah made up exclusively of fire-loving species. Kokondekro Station, Côte d'Ivoire

When this same area is closed to livestock for thirty years, the savannah develops into dry forest in which creepers and forest species replace the grasses that can transmit fire. Kokondekro Station, Côte d'Ivoire.

Fire is an essential instrument in animal husbandry, allowing regeneration of forage resources during the dry season. This picture shows the boundary between the area that has been burnt off late in the season for thirty years and the surrounding tree savannah. Kokondekro Station, Côte d'Ivoire

A great deal of Africa that is at present under savannah would naturally revert to forest if it were protected from fire and grazing.

PLATE 5: MECHANIZED CLEARING

After the caterpillar ridger has overturned the trunks of the main trees, the rake then tears up the root network, separating it from the topsoil. The undergrowth, roots and trunks are all rolled along in a cloud of dust to the end of the plot, where the biomass and nutrients accumulated over the past 20 to 150 years are heaped up. Ibadan, Nigeria.

After the ridger and rake have done their work, the surface is level, but the litter and humus have vanished. The soil is bare, ready to suffer the onslaught of the rains and be turned into mire, for the root network has been pulled up and the soil pulverized by the caterpillar tracks.

When forest cannot be felled with a mechanical saw and progressively cleared, use of a special blade is recommended. This implement has a spur to split the stumps and a blade to saw them off at ground level.

After the special blade has been used, the soil surface is still covered with litter, and the stumps and root network are left in the soil, which suffers much less under this clearance method.

When branches cannot be left in place (since soil is tilled mechanically), it is better to burn the brushwood on the spot in order to release the mineral load rather than relegate it to the edges of the plot.

PLATE 6: REFORESTATION IN A SUDANO-SAHELIAN ZONE

The local chief gave the worst piece of land to be used for a village woodlot. After the land was cleared and fenceposts put in to close the area off to livestock, seed holes were made. This gravelly soil is very poor and stores very little water, and so nothing actually grows, and the land is even more denuded than before. If by good fortune the young trees grow before the fenceposts are eaten away by termites and collapse, nobody bothers about upkeep of the stand or the necessary thinning, for nobody knows who owns the wood - the State or the farmers. Yatenga, Burkina Faso.

Reforestation on a half-moon on gravelly ironstone. In this example, the water has mainly benefited annual grasses, although some trees still survive after six years (acacia, neem, eucalyptus). Outside the gravel area, the half-moons disappear in one or two years on these very fragile, sandy soils. Gourga, Burkina Faso.

Reforestation along stone lines. After unbroken lines of stones had been arranged in semicircles, the villagers planted various indigenous trees (barely visible in the foreground) and eucalyptus under wire netting. The women take care of weeding, and are allowed to grow a groundnut crop. The eucalyptus trees have profited considerably from runoff water. In the foreground, the posts supporting the netting have been destroyed by termites in three years. Cost of netting: 10000 FF or $US 2000/ha, obviating widespread use. Ilonga, Burkina Faso.

PLATE 7: REFORESTATION IN A SUDANO-SAHELIAN ZONE

Trees growing around homes protected by a bundle of thorn branches, a woven basket or a small lattice wall of bricks. Yatenga, Burkina Faso.

Some farmers have developed forest variations on the zaï method (see below). When hoeing, they leave some of the forest seedlings that spring from seeds contained in the dry corral dung that is dug into the zaï pit. When the first thinning is carried out after five years, some shoots are again left to create an acacia stand, while the rest are cut down for fuelwood. This system has a very positive effect on the restoration of soil fertility, runoff, and wafer and wind erosion, for the trees trap the leaves and alluvium blown by the dry harmattan wind. Gourga, near Ouahigouya, Burkina Faso.

Bocage of hedges. In a denuded area, a Regional Agricultural Centre (CRPA) project selected various hedge-forming species. Acacia nigritiana proved effective not only in creating a livestock-resistant hedge, but also in reducing wind erosion. Under its protection grass coyer developed naturally, slowing down runoff and sheet erosion. Elsewhere, farmers preferred Ziziphus mauritiana, for it is equally resistant but can also provide forage and fruit to sell at market. Ziga, near Ouahigouya, Burkina Faso.

PLATE 8: SPR

The Algerian diversion terrace was designed to evacuate runoff from fields made fragile by tillage to a protected spillway. To cope with the increased intake of water, the slope of the channel has to be increased from 0.2 to 0.4%, but here water collects at a low point, threatening to overflow, gullying the slope or causing landslides. Reforestation of communal lands with Aleppo pines by foresters is not respected: the best trees are removed before they reach maturity, and the pine litter does not improve the soil, or only very gradually. Milliana, Algeria.

Terraces have been built on the slopes of a calcareous plateau to encourage infiltration and the growth of fruit trees. There is no trace of runoff, either on the slope or in the spillway! The good condition of these terraces is insufficient justification for the investment. Is there a serious risk of runoff? Bel Mezioude, Algeria.

The calcareous crust of this brown soil was broken up by deep subsoiling. The stones were piled along the contour lines: not having prevented runoff, they were used to build new homes. It may be wondered whether such stones are more effective piled in rows or scattered over the ground where they intercept the energy of rainfall and runoff. Bel Mezioude, Algeria.

PLATE 9: SPR

This completely gullied hill was reforested with Aleppo pines 15 years ago, but overgrazing has left the soil still almost bare. Is such an investment in badlands an economic proposition? Why do farmers not respect such government efforts to protect their environment? Probably because they see the planting of trees as an attempt at expropriation by the State. Oued Isser, Algeria.

Reforestation of badlands after terracing of a marry hillside. After 12 years, the Aleppo pines have reached a height of 3 m on the terraces, but cover less than half the soil surface (too little for erosion control) and suffer the ravages of processionary caterpillars. It would be wise to diversify the species and introduce a leafy under-storey. Seghouane, Algeria.

Reforestation of a semi-arid hillside (annual rainfall 250 to 350 mm) after building bench terraces. After 17 years, the pines are growing again satisfactorily, although their height and the ground cover provided are slight because of grazing and drought. The forceful intervention of the forestry services on these degraded common grazing lands is little appreciated by the "beneficiaries": if the dam is to be protected, other strategies must be sought, and compensation envisaged for poor farmers whose only resource is animal husbandry. Relizane, Algeria.

PLATE 10: EROSION CONTROL STRUCTURES IN A SUDANO-SAHELIAN ZONE

Farmers lay out lines of stones, branches or grass in order to slow down sheet runoff, stem peak flows, and trap organic matter and sand, while allowing excess water through. These lines can also be used as boundary markers for property. Yatenga, Burkina Faso.

Line of stones reinforced with a line of grass. Farmers can reinforce their stone lines by sowing Andropogon, thus using 50% less stones, which can then be used for closer spacing of the lines, since the positive effects only extend 5 metres on 2% slopes. Andropogon fulfils a variety of functions: green forage in the dry season, straw for roofs, and various uses in artisanal crafts. Yatenga, Burkina Faso.

In gullies and wherever gullying sheet runoff is too fast, a semi-pervious dam should be built with large blocks of laterite. This flat-topped structure will slow down peak flows, help recharge groundwater, and trap nutrient matter. Filtering bund in Yatenga, Burkina Faso.

These earth bunds have been built over an area of 45000 ha in Yatenga province in the past 20 years. However, few are still functioning two years after construction. They act in fact as diversion bunds, leading runoff down into low-lying areas, particularly tracks! When farmers realize that the bunds waterlog upstream land while drying out downstream land, they break them and go back to irrigating their land with the water flowing down from the top of the hill. This method of diversion should be avoided in Sudano-Sahelian areas where stop-wash grass lines are more suitable. Ouahigouya, Burkina Faso.

PLATE 10: EROSION CONTROL STRUCTURES IN MOUNTAINOUS ZONE

Bench or Mediterranean terraces built in the 14th century by the Incas, irrigable and still used to grow cereals. This method requires a huge investment in labour (600 to 1200 days/ha) and upkeep (3 to 10 t/ha/3 yrs of manure + 2 to 5 t/ha/2 yrs of lime). It is acceptable only if land is scarce, labour abundant and cheap, and the crop economically viable. Machu-Pichu, Peru. [Photograph De Jaegher]

Stone risers in the valleys: systemas andenes in Peru. In order to make the best use of the colluvial deposits trapped in the valleys, the farmers have built stone risers which allow control of runoff water and protection of cultivated land. Cuzco, Peru. [Photograph De Jaegher]

In Nepal, slopes of up to 60% are converted in traditional style into narrow progressive terraces. The risers are grassed. Steeper slopes are covered with hay fields. Valley bottoms are irrigated and farmed intensively. Gulmi District, Nepal. [Photograph Ségala]

On the steep slopes around Lake Geneva, wine growers have built cemented stone risers as well as a network of stabilized roads which drain the whole slope. Lastly, the surface of the fields is protected by a bed of pebbles that absorb the energy of raindrops. The grapevines grow fast to start with, protected by this mulch of pebbles. Wine growing makes such large investments financially viable. Lake Geneva, Switzerland.

PLATE 12: MASS MOVEMENT

Sloughing in the form of a mudrock flow: a section of the hillside has collapsed into the gully during an exceptional rainstorm, forming a torrential red mud flow 1 km long. The gypseous marls of the hill had given rise to suffusion (tunnel erosion as the gypsum dissolved). Khef el Hamar, near Médéa, Algeria.

In a mountainous area, after a saturating downpour, gravity combined with runoff and alternations of frost and thaw to move huge boulders downhill. Ecuador. [Photograph De Noni]

Sloughing has produced a shell-shaped erosion scarp. On a steep schisty (or marry) slope, a section can break away, leaving a hollow with a reverse gradient. Water then collects at the bottom of the hollow and can give rise to a gully which will remove any trace of the original slide. Biscuicuy, Venezuela.

Such wholesale slides of soil cover over schist illustrate the dangers of working very steep slopes, which are further unbalanced by tracks and overgrazing. Gulmi District, Nepal. [Photograph Ségala]

PLATE 13: GULLYING

Deep V-shaped gully in a marry hill. The slope that catches the moist winds is in balance and is covered with vegetation; the dry slope is steep, unstable, undermined at the bottom, and stripped bare. The slopes recede as the marl weathers and as runoff washes away the buildup of sediment in the bottom of the gully. A simple wire netting barrier is sometimes enough to stabilize such slopes. El Azizia, Oued Isser, Algeria.

U-shaped gully. When the material sheared away by runoff lacks homogeneity, the gully develops vertical lips and grows as it caves in under pressure from the underlying water table. This is seen in the lavakas of Madagascar, where runoff first penetrates the resistant horizons, which are rich in clay and iron, then the ferralitic alterites, which lack almost all cohesiveness.

Trees do not stop gullying once it has started. Their roots can help to reinforce banks, but during the heaviest flood flows, the water swirls around the trunks, eroding the banks. Venezuela.

Tunnel gullying In gypseous marls, water seeps through cracks, dissolves the soluble salts, and hollows out tunnels, giving rise to gullying that is difficult to control. Similar phenomena can be found in deeply fissured vertisols and where surface water penetrates via the tunnels made by burrowing animals. Oued Mina, Algeria.

PLATE 14: GULLY CONTROL

The ONTF built large gabion structures to control a gully on marl located not far from a dam. After five years, no sediment has yet been trapped, and it would appear that this huge investment was quite unnecessary. However, the weirs could perhaps fill during a particularly exceptional rainstorm. Oued Sikak, Algeria.

On the central uplands of Madagascar, farmers are very skilled at transforming gullies into rice paddies. They use clumps of grass to build earthen retaining walls to hold the water and mud eroded from banks and hills that have suffered bush fires; they then devote all available manure to these areas. The hillsides support only meagre, fairly undemanding crops (cassava and extensive grazing). Madagascar.

A gully garden. Since basaltic rocks weather fast, fine sediment can be trapped behind sills of earth-filled plastic bags (which must be protected from the sun). The terrace thus formed is then manured and planted to coconut, banana, mango, sugar cane and various forage species. Petite Valley, Nippe, Haiti.

Runoff dug out a young gully in the soil cover starting at the ridge road. The farmer immediately planted banana, bamboo, sugar cane and various forage plants here. The runoff energy is absorbed by the plants, and the gully is stabilized. Jacmel, Haiti.

PLATE 15: GULLY CONTROL

A series of gabion, dry stone or wire netting sills were bulk at the head of the gully. By the second year, the structures had been covered by sediment, and had to be raised again to reach the equilibrium that would allow natural vegetation to cover the slope. Souagui, Algeria.

A treated gully behaves like a linear oasis. Three years after the sills were built and trees planted in the silt, the gully was covered with natural vegetation, in contrast with the arid surrounding area. Given the considerable cost of treating gullies, full advantage should be taken of such systems, ensuring the involvement of those living on the land alongside the gullies. Souagui, Algeria.

Several cubic metres of sediment have collected behind the dry stone sills. Water has infiltrated into the pores of this sediment, amounting to 20% of free water and a similar amount of absorbed water that can be used by plants. After two years, the mass of sediment gave birth to a spring, which was tapped to irrigate several trees. Souagui, Algeria.

Light sills made of wire or plastic netting (with 1-cm mesh) stretched between 2.5-metre angle irons stuck 50 cm into the soil, and held in place by galvanized bracing wires. They have proved at least as effective as gabion structures, which are more prone to piping under the sill that can evacuate all the accumulated sediment in just one peak flow. This type of sill costs 30 to 20% that of a gabion structure. Souagui, Algeria.

PLATE 16: WIND EROSION

Cloud of fine dust (in suspension) raised by the approach of a "tornado" at Déou, north-eastern Burkina Faso. [Photograph Ségala]

Formation of a small dune in the bed of an overflow basin. The soil is covered with a sedimentation crust, scored by the grains of sand sheeting over its surface. When grass manages to take root, the wind is slowed down and grains of sand are trapped. A small dune (nebkra) then forms, which will be the source of a recrudescence of vegetation (trapping seeds and water). Bani River, Mali.

A Texas landscape invaded by sand dunes: saltation at Big Sprint. [Photograph Fryear]

Reg on the surface of an eroded calcareous brown soil. These uplands of semi-arid calcareous brown soil are swept by wind and runoff which push fine particles down to the bottom of the slope and detach the calcareous crust, forming a reg. The olive is one of the last witnesses of the primary forest of this region, which was once the granary of the Romans. Darna, Cyrenaica Province, Libya.

TABLE 25
Cropping systems (C) efficiency surveys and erosion control practices (P) in Brazil (cf. Leprun, da Silveira and Sobral, 1986)

One of the main focuses of present research on soil conservation is the use of crop residues and tillage, and there is as yet no real proof of the long-term positive agricultural and economic effects of such techniques as minimum tillage, localized tillage with the space between rows being protected by stubble, partial ploughing in of stubble (stubble mulching) and no-till, leaving stubble on the surface (mulch tillage) - techniques that all seem to have a positive effect on water management and soil conservation. In any case, there are still various practical obstacles in the way of using these methods in which organic residues are left on the surface: weed control (herbicides are expensive), machinery to break up the soil without turning it over (vibrating teeth in place of a plough), machinery to sow through a mulch, and pest-control problems (particularly grasshoppers and snails).

In Brazil, Leprun, da Silveira and Sobral (1986) collated the results of experiments on erosion plots in the north-eastern, central-western and southern regions (Table 25). These show the remarkable efficiency of simple farming and biological practices that are easy and inexpensive for farmers to apply, and that ensure long-term productivity. In the best situations, these biological practices allow control of erosion and a decided reduction in runoff.

The most effective mechanized cropping techniques are minimum tillage, sod seeding in the mulch made up of residues from the previous crop, or else contour cropping. The best biological techniques are crop rotation, cropping on a mulch of crop residues or green manure, and permanent contour buffer strips.

TABLE 26
Erosion, runoff and yields as a function of soil preparation techniques (Saria Station near Koudougou, Burkina Faso: tropical leached ferruginous soil on ironstone, 0.7% slope)

TABLE 27
Effect of mounding on an almost bare soil (7% slope, Adiopodoumé 1956) (cf. Roose 1973)

In view of the difficulty of maintaining infiltration under major mechanized crops and reducing erosion through control structures on contour lines (Murundum in Brazil, the Monjauze embankment in Algeria), Séguy et al, (1989) worked with co-operatives to develop a holistic farming system that reduces tillage to a minimum and entails selection of disease-resistant seed, development of material for sod seeding plus fertilizer applications in stubble mulches (from manual planting canes to mechanized seeders), sowing legumes as catch crops under maize, dressings suited to the production level, a range of herbicides and pesticides compatible with mulch cropping, and a research and marketing network.

All these methods are at present being tested in Cameroon by IRA and CIRAD scientists within the framework of intensive cotton and cereal farming under Sudano-Sahelian conditions on typically fragile tropical ferruginous sandy soils.

SHALLOW TILLAGE (HOEING)

Since the formation of a thin slaked surface has a marked effect on infiltration, it might be hoped that shallow tillage would be enough to save both soil and water. And at Adiopodoumé (Roose 1973) it has been observed that the effects of hoeing a bare, sandy soil are similar to, though more ephemeral than, those of ploughing. Following a shallow scratching with the hoe, the soil can absorb only a single fairly gentle rainfall of 10 to 30 mm and erosion is contained for one to eight days after which it exceeds that on the control plots. While runoff is temporarily slowed, turbidity rises considerably, falling only with the formation of a new thin slaked surface.

The same conclusions could be drawn from cropping technique trials under simulated rainfall on steep-sloping loam-clay soil in the Lauragais region in south-western France. A rainfall of 40 mm in one hour produced a slight increase in infiltration, but the thin slaked surface then re-formed and with the higher turbidity soil loss in the end matched that on the control plot (Table 23).

At Bouaké (Table 21), shallow harrowing of bare soil barely reduced runoff compared to the untilled control plot but increased erosion considerably (Kalms 1975).

On the other hand, on the broad, gently sloping, tropical ferruginous pediments of Burkina Faso, Nicou, Ouattara and Some (1987) showed that yields close to those obtained after tillage could be obtained so long as the surface was broken up each time the slaking crust re-formed (Table 26). The point is that in these semi-arid Sudano-Sahelian zones, tillage necessarily entails later sowing than that traditional among the Mossi farmers, while simply scratching the soil allows the plants to take root faster and the runoff to start later if the slaking crust is broken up regularly. In places where people have never adopted tillage, scratching the surface with a donkey-drawn implement is a fast and inexpensive operation within the reach of small farmers.

Shallow tillage unblocks the macropores of the soil surface, and can thus improve infiltration in semi-arid zones and even in temperate zones, so long as the soil is kept free of a thin slaked surface until plant cover can take over. On the other hand, harrowing is a dangerous practice everywhere, especially on steep slopes; it serves very little purpose and should be avoided during the period of major rainstorms.

MOUNDING AND RIDGING

These techniques are widely used in Africa to ensure good root development (cassava, yam), and good drainage in temporary wetlands (including Sudanian areas), and to collect fertile soil around plants grown on the most degraded soils. Ridging also facilitates weed-control by giving the crop an advantage of 10 to 20 cm in height over the weeds. However, mounding - and, to a slightly lesser degree, ridging - is a dangerous practice, for although it theoretically increases the infiltration surface (hence in principle reducing runoff), it also increases the average slope of the land, reduces soil cohesiveness, and concentrates runoff along specific lines. It also increases erosion, which rises exponentially with the slope of the land (Table 27) (Roose 1973).

Two temporary experiments carried out during the 1956, 1967, 1968 and 1969 seasons at Adiopodoumé suggest a slight reduction in runoff and an increase in erosion and turbidity on a ridged soil under cassava or maize. However, these phenomena are not always very clear.

It would be easy to reduce soil and water loss for crops grown on mounds and ridges by tying and mulching them. However, it would then be impossible to avoid the formation of a very unfavourable surface structure in the furrows and pans they form which would reduce soil infiltration capacity at the end of the rainy season. In semi-arid Sudano-Sahelian areas, level planting on unridged ground followed by hoeing and hoe-mounding at three-weekly intervals, then by tying, allow broad, tropical, ferruginous pediments to absorb rainstorms of 50 to 70 mm - the levels to be expected at the start of the rainy season when the cover has not yet taken over. Studies by Rodriguez (1986) in Burkina Faso have shown that tied mounding allows considerable improvements in infiltration -and also in crop yields (+ 500 to 1000 kg/ha/yr for additional working days = 220 FF). Trials carried out by the CTFT at Gampela (Roose and Piot 1984) on gravelly soils have shown that tied contour ridging is in fact the only way of appreciably reducing runoff and erosion in Sudano-Sahelian areas. Unfortunately, on the relatively shallow gravelly soils on ironstone which are so common in the region, the water storage capacity and soil fertility are so low that the additional infiltration rarely has much effect on crop yields. Reference is made to the trials by Collinet and Lafforgue under simulated rain in the Lake Bam region (Figure 29), which showed that tied ridging on slopes of under 1% allows 60 mm/hr of rain to infiltrate, and more than 100 mm to be stored in the soil, i.e. three times more than if the soil had not been tilled.

TABLE 28
Effects of tied contour ridging on a sandy soil in southern Côte d'Ivoire under pineapple (cf. Roose 1973)

The effect of contour tillage, but especially contour ridging, is difficult to test on such small erosion plots (5 × 20 m long) - and such tests could in any case give unreliable results. However, many authors do recognize that tilling the soil along the contours considerably reduces the erosion risk, at least on slopes of less than 10%. On steeper slopes, the sheet of water retained by the contour ridges decreases, correspondingly increasing the risk of a succession of breaks in the ridges all down the slope. It is therefore vital to tie ridges in order to keep water and sand in place, and to set up spillways to lead off the excess (Table 28) (Roose 1973).

Deep drainage can also have an effect on runoff and erosion. On loamy soils in central France, Trévisan (1986) used simulated rain to show the considerable effect of the proximity of drains, which reduce persistent moisture in the macropores, improve structure, and maintain infiltration. More rainwater is retained and the final infiltration capacity is greater. However, in a good number of these soils with a plough sole or a fairly impervious B horizon, the improvement from such drainage is confined to the immediate vicinity of the drains.

The major role of crop residue management should also be emphasized here. When pineapple residues are burned and ploughed in, erosion and runoff increase much faster than when residues are simply ploughed in (Table 11), whereas when they are left on the surface, erosion and runoff become negligible, whatever the slope (Roose 1980a). In a semi-arid region (where increasing seed density does not increase yields because soil water storage capacity is too low), the future lies in better management of the soil surface, partly by eliminating the thin slaked surface and increasing the depth reached by crop roots, and partly by keeping as many crop residues as possible on the surface.

On the very rich volcanic soils of south-western Cameroon, the Bamiléké traditionally multicrop half a dozen species on large ridges running perpendicular to the contour lines on steep slopes (Fotsing 1992a) (Figure 31). Inexperienced agricultural scientists felt that these large ridges should be perpendicular to the greatest slope, but then saw that in heavy rains, water would collect at certain points on the slope, overflow the ridges, and form more serious gullies than in the traditional system. It must be emphasized that on slopes steeper than 25% the advantages are greatest if ridges are oriented in the direction of the greatest slope, which limits the catchment area and hence the volume of runoff between ridges. In the case of small and medium rainstorms, damage will obviously be greater when mounding follows the direction of the slope and will certainly lead to quite considerable erosion in the course of the year, but it does help to reduce the major risks of landslips or gullying. Thus the contour ridging method is not universally applicable. One elegant solution might be large ridges on a gentle slope (under 1%) toward a prearranged spillway, with ties between these ridges every 1 to 5 metres. Such ties must be lower than the ridges themselves in order to allow progressive lateral drainage during exceptional rainstorms. However, the secret of the success of the Bamiléké's ridging method lies in keeping a very thick permanent cover thanks to the combination of a large number of different crops throughout the year (see Part III).

In mountainous areas, tillage entails some serious hazards:

• it temporarily improves infiltration but reduces soil cohesiveness, thus heightening the risks of erosion and sliding;

• it allows organic matter to be turned in, but exposes the subsurface, which is poorer in humus, to rainfall impact;

• above all, it accelerates dry mechanical creep, since the implements move the clods.

The following solutions have been put forward:

• rough tillage by two to four people working together turning large clods in order to dig in plants, grass and manure;

• mounding is dangerous, for it concentrates runoff into rivulets which soon carve channels on steep slopes;

• ridging is often used to dig in the fallow and crop residues:

• it collects a great deal of well-drained, friable soil for tubers;

• it stores water (60) to 22 mm if the slope increases from 2 to 40%) if it is perpendicular to the slope (a risk of landslips in the heaviest rains);

• it drains slopes if it is oblique or follows the direction of the slope,

• it gives crops an advantage in height over weeds.

In Peru, depending on the season, the farmers may choose full tillage, ridging prior to sowing, ridging a considerable time after sowing, or tractor tillage (Figure 32). It has been seen that yields can be increased while reducing tillage time - and hence increasing the benefit for farmers. For tractor tillage, however, this obviously greatly increases the risk of degradation, for tillage has to follow the direction of the greatest slope if the tractor is not to overturn.

FIGURE 31
Direction of ridging in relation to slope

Gentle slope:

• tied ridging
• contour ridging, slope up to

Steep slope > 25%, the effect of roughness on runoff quickly decreases,

due to the reduction in water storage capacity,
but risks of overspill and sliding increase

• during mild rainfall, erosion is less with contour ridging

• during heavy rainfall, there is a danger that runoff will spill over, causing a break: all the water held back by the ridge will then flow out at this point, forming a gully - which is much more difficult to eliminate than all the small rills that drain ridges set in the direction of the slope (smaller catchment basin).

Gully

Rill

• In PERU, at altitudes of 1500 to 4000 m, the farmers try to adapt their cropping methods to local conditions of season and climate:

Figure

• In CAMEROON, in Bamiléké country, at between 1000 and 2000 m, the farmers make:

• large contour ridges on gentle slopes

• wide, short ridges on steep slopes, in the direction of the slope, or staggered
(effectiveness depends on the plant cover provided by mixed cropping)

IN CONCLUSION, giving advice on the orientation of ridges is a tricky matter!
On gentle slopes, ridging and tied mounding are very effective.
On steep slopes, depending on the greatest risks, there is a choice:

If draining is required

: oblique ridges draining towards a grassed spillway
: well-covered, wide, short, staggered ridges

if landslides are likely

: step microterraces or slanting ridges

FIGURE 32 Direction of ridging in mountainous areas (Peru and Cameroon) as a function of rainfall risk

Moreover, in dry years the farmers make their ridges perpendicular to the slope in order to store as much water as possible, whereas if it looks like a very wet year, they follow the direction of the greatest slope in order to facilitate drainage, and if the year looks uncertain, they make one square set perpendicular to the slope and the next in the direction of the slope, creating a patchwork of little plots that allows runoff to circulate slowly.

The C factor (influence of plant cover and cropping techniques in Wischmeier's equation)

In Wischmeier's equation, the C factor is the relationship between erosion measured on a bare fallow reference plot under a given crop. It expresses the interaction between the crop and cropping techniques and how this affects the reaction of a soil type to rainfall. The C factor changes as the plants grow and the state of the soil surface alters, and can be calculated for each of the main periods of the cropping cycle and the region under consideration: five periods are recognized in the United States, and up to nine in high-rainfall tropical areas with two cropping seasons. Taking account only of an annual overall measurement, the following figures have been obtained in West Africa (Roose 1973) (Table 29) and Tunisia (Table 30).

TABLE 29
Importance of plant cover and cropping techniques (C) for various crops in West Africa Annual average C

TABLE 30
Importance of plant cover (C) in Tunisia

The fundamental effect of plant cover and of the adaptation of cropping techniques to regional environmental conditions are subsumed under the C factor in the USLE model. Taking account only of an annual overall measurement, this factor varies from 0.9 to 0.1 for the main crops grown in West Africa. It can fall to 0.01 under a forest crop with cover plants and under grassland, and to 0.001 under a mulched crop and under forests of varying densities.

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