4    Land degradation: Form, extent and related factors

4.1 Soil erosion as a form of land degradation

4.1.1 Topography affecting erosion

4.1.2 Rainfall and wind affecting erosion

4.1.3 Soil properties affecting erosion

4.1.4 Vegetation and land cover affecting erosion

4.1.5 Land use and management affecting erosion

4.2 Biological degradation of soil

4.3 Chemical degradation (nutrient depletion) of soil

4.4 Physical degradation of soil

Land degradation can be triggered by various processes that lower the potential productivity of land leading to long-term (sometimes irreversible) deterioration. These processes are numerous but for the purpose of this study, the primary focus is on processes of soil erosion and biological, chemical (nutrient depletion) and physical degradation as forms of land degradation. These processes are interrelated and could occur due to natural causes but they are invariably accelerated by human intervention in the natural environment (Barber 1984; Agray-Menash 1985). Human intervention increases with population growth and pressure. For example, based on his study in the Gauche catchment area (234 ha) in West Hararghe zone, Thomas (1991) reported that of all the visible erosion incidents, 81% were caused by human induced factors while the rest were by natural factors. The most common human-induced factors that cause accelerated erosion include deforestation, inappropriate agricultural practices such as over cultivation and overgrazing, and inappropriate institutional and policy applications, e.g. land tenure, input supply and forest regulation policies. Poorly designed and constructed roads, defective conservation measures, cattle tracks and footpaths are also some of the human induced factors causing visible degradation.

Extreme diversity in natural resources and ecological conditions within the region causes the processes of land degradation to vary significantly. Fertile soils and abundant rainfall attracted early farmers to these highlands. As soil degradation in the northern highlands of Ethiopia advanced, people moved southwards, particularly to the Oromiya highlands. This situation is still putting pressure on the highlands of the region. Increased demand for trees/forests for construction and fuel, and expansion of farmlands to steep and marginal areas have also contributed to degradation. The present extent of soil degradation which is over a very large area of the Hararghe highlands, North and East Shewa, and Wellega, Arsi and other zones is evidence of the unabated spread of soil degradation in the Oromiya region.

In this section, we will discuss the extent of various forms of degradation and their proximate, mainly natural, causes. First, the different forms of degradation and natural causes are discussed in the following order: soil and water erosion, biological degradation, chemical degradation (nutrient depletion), and physical degradation. It should be noted that the various forms of degradation are to some extent inter-linked, so there may be some repetition in the discussion. The impacts of these various forms of degradation are discussed in Section 5 while Section 6 discusses some of the underlying causes including socio-economic, institutional and policy-related causes.

4.1 Soil erosion as a form of land degradation

The major physical agent in environmental degradation in the settled highlands of the Oromiya region is soil erosion. Topography, rainfall, wind, lack of vegetation cover, soil properties, and land use and management practices are the immediate causes of soil erosion (Barber 1984). There are also underlying or distant causes, such as population pressure, poverty, high cost and inaccessibility of inputs, insecure land tenure, lack of appropriate production and conservation technologies and many of these are further influenced by various government policies or lack of them (Figure 3). In this section, the nature and extent of soil erosion and the proximate causes will be discussed.

Source: Fitsum et al. (1999).

Figure 3. Causes of soil erosion.

4.1.1 Topography affecting erosion

Oromiya region's topography consists of high altitudes and rugged landscapes, as described earlier. The rugged topography and steep slopes affect soil erosion rate through its morphological characteristics. Two of these, namely gradient and slope length, are essential components in quantitative relationships for estimating soil loss (Wischmeier and Smith 1978). On sloping lands, more than one-half of the soil particles that are dislodged by raindrops during rainfall are carried downhill. Erosion increases dramatically because the increased angle facilitates water flow and soil movement. It is not surprising, therefore, that areas like the Charchar highlands in Hararghe, MartiJaju areas in Arsi, central Shewa etc. suffer some of the region's highest erosion rates.

Data for assessment of the effect of slope gradient and length on soil erosion is limited. However, it is generally accepted that an increase in slope and slope length will increase erosion because they lead to an increase in overland flow volume and velocity. Runoff on low slopes flows slowly and quickly forms a water layer deep enough to act as surface mulch. Increasing slope length enhances soil loss as more runoff can accumulate on long slopes. Thomas (1991) identified that slope shape together with ground/field attributes exercise a strong influence on the nature and extent of visible erosion damage.

On steep slopes, soils are generally shallower and their nutrient and water storage capacities are limited. Thus, soils in these areas, when exposed to soil eroding agents, face greater degradation consequences compared to soils in flat areas. Since most of the terrain of the highlands of the region is undulating and hilly, most agricultural land is situated on sloping ground. Increasing population has resulted in an increasing demand for cultivable land which has increasingly moved on to steeper slopes previously covered (through cutting and burning) by forests.

A survey in three Peasant Associations in West Hararghe showed that due to population pressure farmers were forced to cultivate lands over 50% slope even though there was a directive from the woreda not to cultivate lands over 35% slope (Adugna et al. 1996). The Soil Conservation Research Project (SCRP) has shown that an increase in population as a result of the resettlement programme in Illubabor has forced the community to clear forests on steep slopes for maize cultivation. As a result erosion and leaching due to heavy rainfall decreased soil fertility (Hagmann 1991). In the Ginchi watershed in West Shewa, analysis of aerial photographs shows that in 1950 only 34% of the watershed was under cultivation, mainly in the lower and middle part of the undulating landscape, 60% was under pasture and woodland, covering the medium and higher slopes, and 6% was roads, pathways and water bodies. In 1990, the situation has completely changed. Crops are now grown on over 60% of the land area extending up to 35% slope while pasture and woodland has been reduced to half its previous size. Furthermore, the length of gullies increased 14 times between 1950 and 1990 and they have become wider and deeper because of severe erosion (Saleem 1995). Similar situations exist throughout the region.

4.1.2 Rainfall and wind affecting erosion

The major components of climate that affect soil erosion are rainfall and wind. Erosive processes are set in motion by the energy transmitted from either rainfall or wind or a combination of these forces. Although the effects of erosion are not easily observed on a daily basis, water and wind are both capable of quickly damaging the soil. Sheet and rill erosion are by far the most widespread kinds of accelerated erosion and impact agricultural production more than other kinds of erosion. Soil erosion by rainfall and wind consists of two principal sequential events: the detachment of soil particles from the soil mass and the transportation of the detached particles (Young and Wiersma 1973). Raindrops hit exposed soil launching soil particles into the air. When rainfall is intense and rapid runoff occurs, gullies ranging from 1 to 100 m deep may form and large volumes of water and soil may be swept away (Pimentel et al. 1998). The power of rainfall to produce erosion is related to rainfall amount, intensity and distribution. Rainfall intensity is more important than rainfall amount in causing erosion. Rainfall with an intensity equal to or exceeding 7.6 cm/h in 5 minutes, 3.6 cm/h in 15 min, 2.5 cm/h in 30 min, or 2.0 cm/h in 60 min is classified as excessive (Krauer 1988). When erosion increases, less water enters the soil matrix and is available for the crop. In the tropics, erosion may reduce infiltration by up to 93% and thus, increases runoff (Lal 1976, cited in Pimentel et al. 1998).

The average amount of rainfall in the Oromiya region decreases from west to east. The highest is in Gore (2212 mm), followed by Arjo and Hurumu each receiving 2140 and 2025 mm, respectively. The lowest amount of rainfall occurs in the eastern border area, where the average yearly amount is 200 mm (Berhanu et al. 1998a). In most of the highlands of the region, the largest proportion of rainfall occurs during the main rainy season (June–August). The rainfall erosivity is also highest during this period. For example, out of 1082 mm of rainfall recorded in 1988 at Hunde Lafto (West Hararghe) station, 64% occurred during July–September. Out of the calculated 642 J/mh erosivity index, 78% was imparted during the same period. At Dizi Research Station (Illubabor), out of 1654 mm of rainfall, 86% was registered during June–September which accounted for 89% of the 1227 J/mh erosivity index. Furthermore, rainstorms of 1.2 cm/h lasting 20 minutes were recorded in Dizi, and 14.1 cm/h intensity rainfall in six minutes was recorded in September 1984 at the SCRP station based in Suke, Hararghe. Although of short duration, these rainfall events of high intensity produced about 50% of the total soil loss from test plots at the research sites (Krauer 1988; Hagmann 1991).²

2. These results from the Dizi research station by SCRP in 1988/90 indicated that among the various erosivity indices, the rainfall erosivity factor used in the Universal Soil Loss Equation (USLE) was the most appropriate rainfall erosivity index for the Ethiopian environment (Solomon 1994). The reason could be that it combines the effect of rainfall amount, maximum intensity and duration of all significant rains.

The agricultural practices in most highland areas of the region leave the soil bare and loose at the onset of the rains to facilitate planting and weeding. Therefore soil loss could be high at this time even during low erosive rainfall. For most of the highlands of Genale Dawa Basin, the estimated soil loss rate is 0–50 t/ha per year. However, in some parts of Sidamo and Bale highlands, high soil loss rates of 51–200 t/ha per year (predominantly 51–100 t/ha per year) have been reported (FAO 1986).

The estimate of average annual soil losses for all types of land cover in the highlands of Ethiopia lies between 10 and 35 t/ha and average values for croplands vary between 20 and 100 t/ha. These values of erosion rates are much higher than the rate of soil formation and conversion of parent material into soil. The rates of soil formation in Ethiopia, as estimated by Hurni (1983), vary between 2 and 22 t/ha per year. A comparison of erosion rates by country or region within a country is often misleading because average erosion rates obscure the high degree of variability within each area or watershed. For example, in Ethiopia an average of 42 t/ha per year is estimated to be lost each year from agricultural land (Hurni 1988b, 1993), but in the Illubabor area, soil loss on slopes under cultivation is estimated to be a minimum of 100 t/ha per year, with modal values between 150 to 200 t/ha per year for different locations (Hagmann 1991). These rates might be higher compared to the highlands of Hararghe and East Shewa, where there is less intensive rainfall and where erosion has already removed most fertile topsoil.

Wind erosion is determined by soil erodibility, surface soil roughness, wind velocity, wetness of soil, vegetation cover and management practices (with regard to windbreaks). Usually, when wind speed reaches 25 mph, the wind detaches soil particles from unprotected soil (Pimentel et al. 1998). Wind erosion is accelerated when soil is dry, weakly aggregated or less cohesive and bare. Due to the decrease in vegetation cover, an increase in tillage methods that leave the surface smooth, frequent trekking of large number of livestock for water and grass and poorly constructed roads, wind erosion is becoming a serious problem in the region. Although the severity is not as serious as water erosion, it is posing a threat in Rift Valley areas and associated lowlands with light texture soils. In some parts of these lowlands deforestation either for cultivation or settlement is in progress.

4.1.3 Soil properties affecting erosion

The major soil classes found in the highlands of Oromiya are Vertisols, Nitosols, Luvisols, Acrisols, Cambisols and Phaeozems. Each of these has properties that affect soil degradation differently (OESPO 1999). Soils vary in their resistance to erosion partly based on texture and amount of organic matter. The resistance also depends on soil condition and depth. Soils high in silt and low in clay and sand are highly erodible (Nill et al. 1996). The high erodibility of silty soils is explained by their weak structural stability. They rapidly form surface seals upon the impact of rain drops. Erosion is less on clayey soils due to better aggregation and on sandy soils due to the non-sealing surface.

Some soils like Inceptisols which cover the Charchar highlands (Hararghe) are fragile in nature and sensitive to both geological and man made activities. The reddish brown to red clay soils of tropical and subtropical areas, known as Alfisols or Nitosols are widespread in the region where intensive cultivation is practised. They are common in the coffee, tea, oil crops and fruit-growing areas of Hararghe, Borana, Illubabor, Jimma, Wellega and Bale zones. Andosols found in coffee growing areas are generally young volcanic soils and have high water absorbing capacity, which create high water pressure in pore spaces. River bank erosion and road excavation aggravate the situation.

Nitosols have high moisture storage capacity, a stable soil structure and hence are less susceptible to erosion than many other soils. Vertisols are characterised by their extensive cracking from the surface to depths of 50 cm or more with the advance of the dry season (El Wakeel and Abiye 1996). Vertisols have developed in the central highlands and basins in the western Oromiya region where rainfall reaches 2000 mm (OESPO 1999) and soil erosion takes place even on slightly sloping plains (Driessen and Dudal 1991). Field observations have shown that these areas very often suffer from rill and gully erosion that cut deep gullies into the soil and aggravate the rate of land degradation.

If soil depth is inadequate, the water holding capacity and rooting anchorage of the soil may decrease below the critical levels. As soil depth decreases, croplands revert to weedy grasslands and ultimately degrade to bare rock. The sloping areas in the highlands of Hararghe, Arsi, Shewa and Wellega are some of the areas which have lost most of their fertile topsoil and have become susceptible to land degradation.

Organic matter in the soil improves soil structure, root penetration, water-holding capacity and infiltration. With increasing organic matter, erodibility decreases (Wischmeier and Smith 1978). The increasing reliance of the rural population on animal dung and crop residue for fuel has reduced the amount of organic matter which should have been added into soils as organic fertiliser. Consequently soil structure has deteriorated and soils have become fragile and prone to erosion.

Soil conditions, e.g. antecedent moisture content, vegetation cover, slope and tillage system generally influence soil erodibility and land degradation. On moist soils, rainfall starts and causes higher runoff volumes than on dry soils. For this reason rains occurring at the onset of the rainy season generally cause less runoff and soil loss than rains at the end of the rainy season. However, better vegetative cover at the end of the rainy season also helps to reduce runoff and erosion from occasional heavy rains.

Sodium has a pronounced dispersion influence on soil structure. Soils in the Rift Valley area have a high sodium content and are easily dispersed and susceptible to wind and water erosion. Deposition of sediment (silt and sand) caused by runoff on the surface plains within the channels in the drainage system and on the lower slopes of colluvium or alluvium plains is generally observed. The soils in these areas are dominantly coarser in texture, easily detachable but difficult to transport.

4.1.4 Vegetation and land cover affecting erosion

Throughout the world, the lowest erosion rates, ranging from 0.004 to 0.5 t/ha per year, are found in undisturbed forests (Pimentel et al. 1998). However, once forest land is converted to agriculture, erosion rates increase because of vegetation removal, over-grazing, and tilling. Vegetation cover reduces erosion. Living and dead plant biomass reduces soil erosion by intercepting and dissipating raindrops and wind energy. Above-ground foliage slows the velocity of water running over the soil decreasing the volume of water and soil lost in surface runoff. Plant roots physically bind particles, thus stabilising the soil and increasing its resistance to erosion. Plant roots also enhance water conservation by creating pores in the soil surface that enable water to enter easily into the soil matrix. The uptake of water by plant roots also depletes the soil water content and thereby further increases infiltration rates.

It has been estimated that closed forests covered about 40% of the western, southern and central parts of the country (what is now Oromiya region) about a century ago. However some argue based on chronicles of the many travellers to Ethiopia in the last 500 years that forest cover was not really that high, particularly in the north of the country which has been densely populated for a long time. This inconsistency has resulted in several different estimates about the current volume of forest cover in the region. For instance, the estimates (in the mid 1990s) vary from 7.2% to 8.2%, while that of wood and bush lands vary from 32% to about 60% (Berhanu et al. 1998a). However, continued extensive deforestation in places like the Rift Valley since the 1990s could have reduced the forest area further.

The Oromiya regional state has identified 43 high natural forests as regional forest priority areas covering a total area of nearly 3 million hectare (Table 6). Forests found in the Bale, Borana, Arsi, Shewa and Hararghe areas are dominated by Juniperous, Podocarpus, and Juniperous-Podocarpus mixed forests. There are also mixtures of pegeum africanum, ekebergia ruppeliana, schefflera abyssinica, and apodyties dimidiata spp. On the other hand, those found in Wellega, Jimma and Illubabor areas are dominantly mixed broad leaves, consisting of aningeria adolfi friederici, edebergia, albizia, bosqueca, fagaropsis, pegeum, syzygium, croton, celtis, polyscias and schefflera spp. These areas are homes to coffee arabica, which accounts for about 66% of the country's foreign exchange earnings. The coffee-growing area increased from 345 thousand hectare in 1993–94 to 435 thousand hectare in 1998–99. This expansion has been taking place by removing forest cover including wild coffee plants, thereby posing a threat to biodiversity in coffee. In some areas such as east and west Hararghe, coffee and other crop-growing areas are being converted to chat plantations because of its quicker cash generation potential and because of the high incidence of coffee berry disease. Between 1993–94 and 1998–99, the area growing chat increased from 68 to 78 thousand hectare. It is becoming an important source of cash income for farmers and foreign currency for the country. It is claimed that growing chat contributes to reducing erosion as fields are prepared and trees are planted so moisture is retained and runoff is reduced.

Table 6. Regional forest priority areas (identified so far) in the Oromiya region.

The wood and bush lands of Oromiya region are restricted to agro-pastoral and pastoral areas of Jimma, Illubabor, Wellega, Borana, Shewa, Bale, Hararghe and Arsi zones. They are found in a variety of forms depending on the altitude, topography, ground water level and associated vegetation types. The wood and bush lands are found on slopes, along rivers, on mountain tops and on plains and generally include various species of acacia, boswellia, commiphora, balanites, euphorbia, combretum, croton, oxythantera, protea, erica arborea, hypericum, poor stands of juniperous procera and hagenia (EMA 1988 cited in Berhanu et al. 1998a).

There are also man-made forests in the region including industrial and peri-urban plantations. Most of the industrial plantations are found in and around the natural forests in Arsi, Jimma, Wellega, Illubabor, Shewa and Borana zones. The main planted species include eucalyptus, cypresses, juniperous and pinus making up 53.2%, 30.8%, 5.4% and 2.3%, respectively. The remaining 8.3% is covered by other minor species (Berhanu et al. 1998a).

The mixed broad-leafed forests in the region provide almost all the lumber marketed in the whole of Ethiopia. The exploitation rate of forest resources is so high that in the last decade many of the natural forests have shrunken in size while others have degraded in terms of quality or have been converted to other land use types. Between 1989 and 1998, plantation forest areas declined by 93% in the Jalo-Muktar forest in east Hararghe and 32% in the Gara Gada forest in West Wellega (Table 7). The density of Syzygium guincense has declined drastically in West Wellega and Cordia africanum and Anenjeria adolfi-friedrici, the species very good for timber, have become endangered in the western zones of the region because of uncontrolled logging (Devendra et al. 1998).

Table 7. Changes in forest resources in the Oromiya region, (1989 and 1998).

In recent times, felling of trees for fuel, wood and charcoal without replacement has become a serious problem contributing to the loss of vegetation and hence to increased soil erosion. The increase in the human population has reduced land holding per capita and created pressure on limited land for agricultural production. Those who cannot produce enough cut forests and trees for fuel, wood and charcoal to earn a living. For example, 22% and 33% of households in Melkedera Peasant Association (PA) in Ambo woreda depend occasionally and regularly, respectively, on nearby forests for their livelihood (Mirgissa 1994). Also, the removal or destruction of vegetation cover through overgrazing and bush burning etc. leads to land degradation as such practices leave soils bare and exposed to erosion and other degradation processes.

Recent estimates show that 3.1% of the natural forests is lost annually due to shifting cultivation, commercial agriculture, fuel wood collection, urbanisation, forest fires, poor utilisation and logging (Berhanu et al. 1998a). A study in Bura Adele, Berisa and Daneba Peasant Associations of Adaba Dodola district (Bale zone) shows that the annual rate of deforestation was 1.6, 9.4 and 5.6%, respectively, during the period 1993–97 (Abdurahiman 1998). Another study on the Belete and Gera forests of Jimma zone shows that the annual rate of deforestation was 9.5 and 4.7%, respectively, during 1996–98 (MoA 1998).

In rural areas in the region, woody biomass provides 70% of the energy while crop residues, animal dung, charcoal, kerosene and electricity account for 19.5%, 10% and 0.5%, respectively. In the small and medium urban centres, charcoal, kerosene and electricity provide 6.7% and 22% of energy, respectively; the remainder is provided by woody biomass and dung cakes imported from rural areas. Under 9% of the population in the region have access to electricity (Berhanu et al. 1998b). The relative importance of fuel sources varies widely across the region depending on the availability of alternative sources but in general the importance of woody biomass has been declining while that of dung and crop residues has been increasing. It is estimated that an equivalent of about 15,000 ha of forest is being lost annually in the region due to fuel needs alone (Haile-Yesus 1996). In Ada woreda in East Shewa, for example, lack of fuel wood induced farmers to use dung as the main source of energy instead of fertiliser. In 1983, an average household in the area used 10 kg of wood and 41 kg of dung cakes as fuel (Gryseels and Anderson 1983). With an increasing population, the situation has most likely worsened in this and similar areas.

Among other factors that contribute significantly to deforestation are property rights. Private ownership protected forests to some extent during the imperial reign (Adugna et al. 1996). The ownership right was passed to the Peasant Association during the Derg regime for management as a community resource. These forests were not only poorly managed but they were sometimes exposed to accidental fire and even reportedly set on fire deliberately which then allowed free grazing rights and free cutting of fuel wood after burning. Such practices have a negative effect on proper management of forest resources and ultimately the land is easily degraded (Asefa 1994).

Overgrazing in some parts of the region has changed grassland from a high cover perennial species to a low cover annual species, and from more palatable to less palatable species. Expansion of farmlands has not only led to forest or bush clearing and burning, but also restricted the area for overgrazing. Due to the shortage of grazing lands in many areas of the highlands, croplands are usually used for uncontrolled grazing immediately after crop harvesting. Livestock roam, feeding on weeds and grasses and creating stresses on agricultural lands. Although this practice is not entirely new or recent, the intensity and the duration of such common access grazing has apparently increased in recent times due to a feed shortage for an increasing livestock population. This kind of livestock grazing and the resulting traffic causes soil crusting and reduced infiltration which makes the vulnerable to erosion.

Cropping need not necessarily cause erosion, even on steep slopes. Perennial crops, for example, can protect the soil in the same way that the natural vegetation does. Areas in the Sidamo highlands (Uraga, Bensa, Bule etc.) show little evidence of erosion as there is good vegetation cover and perennial field crops such as enset, chat, coffee and fruit trees are grown. In western Oromiya region, more and more forests are cleared to open new farmlands because of rapid population growth and new settlements. Owing to the high rate of rainfall and sloping terrain, these areas are now under the threat of severe land degradation. However, because of its relatively better vegetative cover, people often underestimate the problem and hence less effort is made to control land degradation there.

In coffee growing areas, where Nitosols predominate and in other Rift Valley areas (especially around Zuway lake), vegetation loss due to termite infestation which leads to erosion has become a problem. Termites build galleries and mounds and damage buildings, crops, coffee, eucalyptus trees, pasture grasses, tree regeneration, the wood quality and the life span of large trees plus they leave the area bare. Areas devoid of their vegetation are vulnerable to soil erosion and degradation. Overgrazing and the decrease in soil fertility aggravate termite infestation and damage. For example, in West Wellega several districts have been suffering from the infestation of termites for the last twenty years but the problem has worsened in recent years (Devendra et al. 1998; EARO 2000). Manasibu, Nadjo, Jarso and Bojji districts are the worst hit. In Manasibu alone, about 66 thousand hectares of land have been taken out of production and more than 33,367 farmers have abandoned their lands due to termite damage. The problem in the above-mentioned districts is exacerbated by land degradation due to soil erosion and poor crop and livestock husbandry (ICRA 1998). The recent spread and intensification of termite damage are largely the result of changes in the balances within the agro-ecosystems following human interference, poor land use and mismanagement of natural resources. The termites have existed for a long time but the ecological changes that occurred as a result of expanding human and livestock population has promoted a change in termite survival strategy inflicting heavy damage on agricultural land.

4.1.5 Land use and management affecting erosion

Croplands and pastures are susceptible to erosion but croplands are more vulnerable because the soil is repeatedly tilled and left without a protective cover of vegetation. The socio-economic situation in rural areas often leads people to use their environment inappropriately which induces land degradation. In any area the type of land use affects the level of soil protective cover and consequently the rate of erosion and erodibility. Deforestation and the removal from the fields of dung and crop residues for fuel and feed causes a steady reduction in the organic matter content of highland soils, rendering them less productive and more easily erodible.

Fallowing has been traditionally used as a soil management and fertility restoration strategy as vegetative regrowth during fallowing helps these processes. Where there has been persistent population pressure on arable land, the length of the fallowing period has shortened over time leading to continuous cropping. For example, in Dizi catchment in Metu area, 30% of available land was under cultivation in 1957, which increased to 41% in 1982. In 1957, one year of cropping was followed by two years of fallow, while in 1982, cropping was done every other year (Solomon 1994). A survey in Agucho village in West Hararghe showed that cultivated land per capita decreased from 0.29 ha in 1983 to 0.12 ha in 1988 (Thomas 1991). Another survey in three Peasant Associations in Chiro woreda in West Hararghe showed that in 1995 average farm size in the lower midlands, midlands and highlands was 0.70, 0.50 and 0.40 ha, respectively, and land per capita was 0.11, 0.07 and 0.07 ha, respectively. Moreover land was more fragmented in the highlands compared to the lower midlands and midlands. A survey in Tiyo woreda revealed that 98% farmers wanted 2.5 ha more land to earn a good living (Gavian and Amare 1996). About 31% of the households in five PAs around Chooman dam did not have any land to cultivate (Asefa 1994). In all these cases of increased pressure on land, increased continuous cropping and a shortened or the absence of fallowing for soil management would be normally expected. When land is used more intensively without better quality inputs such as manure and fertiliser, fertility loss and erosion might be exacerbated.

Tillage operations are sometimes carried out along slopes. Furrows formed along slopes cannot slow down runoff compared to those made along contours. Production of teff, the main cereal crop in the region, requires fine land preparation to allow the small teff seeds to germinate. However, fine tillaging also makes the soil vulnerable to erosion during the early part of the main rainy season. For example, in Metu woreda, two test plots with teff and maize at the same slope (18%) exhibited runoff rates of 437 mm and 112 mm, respectively. On the other hand, a plot under coffee forest at a 51% slope in the same woreda showed only 36 mm of runoff. Over 81% of soil erosion in Chiro woreda has been attributed to inappropriate practices such as steep slope cultivation, runoff from surrounding fields, cattle tracks and footpaths and defective soil conservation measures (Thomas 1991). Some farmers ignore erosion because it is difficult to measure the extent of erosion visually in one storm or even in one season. The government also ignores erosion because of its insidious nature; that is, there are no major crises because the soil is gradually lost year after year.

Management of animals and their feed sources is also a major contributor to soil erosion in parts of the region. Main feed sources are savannah grasslands, bush lands, temperate pastures, fallowed farmlands, crop residues and by-products, and the aftermath of grazing cropland. In the perennial zone in the highlands, tree foliage is also a major source while in other areas it is a minor source. However, taking all these sources together, there is an estimated 24% deficit in feed supply for the current stock. Consequently there is overuse of some grazing resources leading to soil erosion, compaction and other forms of degradation (Berhanu et al. 1998b). For example, in Mekro PA in Tiyo woreda, 32% of PA-allocated private pastureland, 33% of rented pastureland and 52% of common/shared pastureland were rated by the community as 'poor' in terms of the quality defined by density of pasture cover, soil condition and species dominance in pasture. Since 91% of the total pasturelands are common pastureland, the overall quality of land and pasture in the community is likely to deteriorate rapidly, given present trends (Getachew 1997).

Mass movement of soil can be caused by human activity and land use change. In Oromiya region, mass movement of both solifluction (earth flow) and landslides occur in steep areas where the natural balance is upset due to the removal of root binding forces through clearing of forests and bush for cultivation on steep lands. When soils are saturated with water during periods of exceptionally heavy rain, mass movement will occur. Places like Omo Nada (Jimma), Dadar highlands (East Hararghe) and other steep areas in Oromiya are vulnerable to landslides.

4.2 Biological degradation of soil

Biological degradation refers to the process that leads to a decline in the humus content of soil through mineralisation (Solomon 1994). Decomposition of organic matter is a function of microbial activity. The majority of organic matter is concentrated near the soil surface in the form of decaying leaves and stems so erosion of topsoil results in a rapid decrease in soil organic matter levels and therefore causes a loss of food for soil micro organisms. Once the organic matter layer is depleted, soil productivity and crop yields decline because of the degraded soil structure and depletion of nutrients.

The stability of soil aggregates is dependent on microbial biomass. Thus, elimination of soil micro organisms (by erosion, burning etc.) causes physical damage to the soil ecosystem. These physical effects may in turn lead to increased erosion, organic matter depletion, and further reduction in microbial activity. All factors that favour the production and decomposition of organic matter will minimise the risk of biological degradation.

A decline in organic matter has a far-reaching effect on both chemical and physical properties of soils. It affects soil physical properties through its influence on soil structure and aggregate stability which therefore influences soil erosion. The availability of nitrogen and phosphorous is dependent on the organic matter content of the soil.

Because of the concentration of organic matter on the surface and its low density, it is one of the first to be removed by erosion and is the hardest to replace. Solomon (1994) reported that based on his study in Metu area Illubabor zone soil organic matter content dropped from 20% to 7% in less than three years of continuous cultivation due to mineralisation. In the warm humid areas the rate of mineralisation is faster than in the drier areas. However, in the western part of the region particularly in Ilubabor, Jima and parts of Wellega the turnover of biomass is better and thus the organic matter content of the cultivated soil may stabilise at 3–4%.

The rate of mineralisation is high in the absence of natural cover when topsoil is exposed to unusually extreme temperatures and humidity. Getachew (1991) showed that organic matter content increased as the length of the fallow period increased, and decreased as the cultivation period became longer. However, due to increases in pressure on cultivated land, land holdings have continuously shrunk leading to short and/or no fallow period which can lead to nutrient depletion and soil structure deterioration.

Removal of grain and crop residues from the fields, without replacement of nutrients such as manure and fertiliser, tends to deplete the soil of nutrients, as the natural replenishment cannot compensate for the nutrients removed. In large parts of Oromiya highlands fuel wood deficits were and still are mainly made up by substituting dung, grass, and straw as sources of fuel. Stalks of maize and sorghum in Hararghe are used as an energy source. This is also evident in the northern and central parts of the region. Besides home consumption, preparing and selling dung cakes has become a source of household income especially for women in rural areas. The use of dung and crop residues as household fuel rather than to maintain soil fertility and structure is likely to contribute to land degradation.

An equivalent of about 6 million G cal/year of animal dung is used for fuel in the region (Berhanu et al. 1998b). This is equivalent to about 1.5 million tonnes of dung or 14.9 thousand tonnes of nitrogenous fertiliser or about 29 thousand tonnes of urea which is the amount of urea distributed in the region in 1997. If applied at the rate of 50 kg/ha (equivalent to about 100 kg urea/ha), this amount could cover 286 thousand hectares of land. Use of dung for fuel means denying the soil of its effective conditioner and fertiliser. This practice is most pronounced in areas where forest cover has more or less disappeared and where acute fuel shortage is being felt like in East Shewa, North Shewa, West Shewa, Arsi and Bale.

In order to compensate for the lost soil nutrients, farmers in areas around 2500 m elevation (in northern Oromiya) practice guie (soil burning) as a traditional fertilisation practice. It increases the levels of available P and K in the soil and improves the structure of the surface horizon allowing better water movement in the plough layer at temperatures low enough to prevent drastic change in particle size and organic matter. It is widely known that yields of barley and wheat increase because of soil burning in the first few years. Nevertheless, yields continuously reduce after a few years and the land has to be fallowed thereafter. Roorda (1984) mentioned that the loss of organic matter content and dehydration of lattice clays, which lead to changes in clay mineralogy, are some of the disadvantages of soil burning. The effect of such indigenous knowledge is temporal but the farmers are doing it in the absence of a better option, for guie is also very labour intensive.

In western Oromiya (Wellega, Illubabor and west Shewa) animals are kept in 'dallaas', a fence-like temporary structure constructed on farmlands where animals are kraaled for systematic collection of manure to increase soil fertility. Asefa (1994), based on his socio-economic study in the Fincha'a watershed area in East Wellega, showed that 98% of the farmers interviewed use a traditional fertilisation method called ciicata baasuu, whereby a movable wooden barn is prepared to kraal animals and this is moved from place to place within or between plots. In other parts of Oromiya highlands, farmers generally produce blends of compost manure to increase efficiency of nutrient release for crop production.

4.3 Chemical degradation (nutrient depletion) of soil

Generally nutrients are lost through erosion in runoff and in the eroded sediment. Finer soil fractions are the most vulnerable to erosion. Nutrients, being abundant in these finer soil fractions, are also lost to erosion. Further nutrient losses occur through chemical degradation, i.e. deterioration of properties of the soil, that occur as a result of acidification and salination or sodification. The latter is common in arid and semi-arid areas where rainfall is inadequate to leach excess salts down through the profile but is not a concern in this study. The acidification process may be accelerated through burning and clearing of vegetation, continued use of acid containing fertilisers and excessive irrigation (Thomas 1997). There may be other underlying causes of chemical degradation (Figure 4). In general, soil erosion has received the most attention in Ethiopia as this is seen as the principal form of soil degradation and nutrient loss. Therefore little is known about other nutrient losses processes. For example, Pol (1992) based on a study in southern Mali, reported that loss of nitrogen by erosion accounted for 17% of total nitrogen export and the remainder is lost through other mechanisms. The relationship between soil erosion and nutrient depletion is not widely understood with respect to the Ethiopian situation. It was estimated that the highlands of Ethiopia lost about 41 kg of Nitrogen/ha from agricultural lands between 1982 and 1984 and that the projected loss would reach 47 kg Nitrogen/ha by the year 2000 (Stoorvogel et al. 1993). Other studies have reported nitrogen deficits of over 100 kg/ha per year for the Ethiopian highlands (e.g. Steinfeld et al. 1998). However, how much of these losses and deficits are due to soil erosion and how much is due to chemical degradation is unclear. In this section, some of the direct or immediate causes of chemical degradation are discussed.

Source: Fitsum et al. (1999).

Figure 4. Causes of nutrient depletion.

Leaching, a process of translocation of nutrients beyond the reach of crops, occurs in areas of heavy rainfall where there are lengthy periods of rain. The Soil Conservation Research Project has indicated that the highlands of Metu area (Illubabor) experienced chemical degradation due to leaching. Although no quantitative evidence is available to substantiate this, there is reason to suspect chemical degradation in areas like these. The nature of the soils, which varies from moderate to strong acidic, is an indication of leaching with more cation absorption sites being occupied by aluminium ions. This also implies potential aluminium toxicity and a decline in available nutrients. Actual aluminium toxicity, however, is not present (Hagmann 1991).

Kefeni (1992) found that the loss of nutrients from eroded soil in a 100 ha catchment area in Anjeni in the Amhara region was about 210 kg N, 680 kg P and 160 kg organic matter per hectare per year. Tadesse (1992) found that out of 1000 soil samples collected and analysed from Wellega and Assosa, 68% were classified as strongly acidic having a pH range of 4.5–5.5. At Nejo, liming of acidic soil improved yield significantly. Soils having a low pH can fix nutrients such as P, Mo and Ca thus not making them available to plants while they release Mn and Al into the soil solutions leading to toxicity in crops and animal feeds.

Nutrient depletion can be reduced, if not reversed, if adequate additional nutrients are applied to crops to replace potential losses through leaching, uptake by plants and other processes. The problems related to reduced organic manure application were highlighted earlier. Inorganic fertiliser application has been increasing slowly and Oromiya region is the largest consumer in Ethiopia utilising over 50% of the imported urea and di-ammonium phosphate. This is in a setting where the recommendations have neither been location specific nor periodically assessed for fine tuning (OESPO 1999). Fertiliser use is still not widespread and those farmers who apply inorganic fertilisers continuously to their soils to replace depleted nutrients cannot sustain high crop yields everywhere, perhaps because soil erosion exacerbates nutrient losses that are not fully compensated by current application rates.

The length of the fallow period in the cropping cycle also influences the chemical properties of soils. Continuous cultivation leads to deterioration of the essential nutrients. Getachew (1991) showed a sharp decrease in total nitrogen content of Lixisols in Dizi catchment (Illubabor zone) in the first 3 to 5 years of continuous cultivation. This is obviously connected to a decline in organic matter under the same practice.

4.4 Physical degradation of soil

Physical degradation may occur as a result of sealing, compaction, reduction in aeration and reduced permeability etc. Lack of organic matter and a high percentage of very fine sands and silt in soils are some of the factors contributing to surface sealing.

Crop production requires finely prepared seedbed with the maresha which affect soil structure, leave the soil devoid of vegetation exposing the latter to kinetic energy exerted from rain drops. In such cases the clods dislodge and seal soil pore spaces. A decrease in soil pore spaces reduces infiltration and increases overland flow volume and velocity, leading to soil crusting especially when it is dry.

The situation is worse when it comes to sowing fine seeds like teff (Eragrostis tef) which demand fine seedbeds and cattle trampling to compact the soils for better germination and weed control. A teff-seedbed preparation at Jima (where the rainfall is over 1500 mm per year) resulted in a soil loss of about 37 t/ha per year on a 9% slope (unpublished data), while the same type of soil at Holetta (rainfall above 1000 mm per year) had a soil loss of 16 t/ha year on a 6 % slope (Asrat 1992). The former is 4.5 times higher while the latter is 2 times higher than a tolerable level of soil erosion of a given field.

Overstocking and overgrazing including grazing of leftover residues on cropland after harvesting cause soil compaction due to heavy and continuous trampling by livestock. Watering points and cattle routes are particularly vulnerable to soil compaction, which leads to excessive runoff and reduced water infiltration. Revegetation in these areas is therefore impeded. Unimpeded water flowing down slopes causes rills and gullies. The bulk density of grazing land in Illubabor area was measured to be 1.34 g/cc. This is a high figure compared to 0.83, 0.79 and 1.12 g/cc for a coffee forest, ungrazed grass fallow, and crop land, respectively (Solomon 1994).