Ethiopia is a country endowed with diverse natural resources such as agroecology, landscape configurations, soils, geomorphologies, water bodies, micro- and macro-fauna, and animal biodiversity, well differentiated wet and dry seasons, precious minerals, vast potential agricultural areas, cultural and livelihoods practices of its populations. These all together have enabled the country to have diversified potential resources on which the country largely depends on for its strategic development. Among these potentials, the agricultural potential is the main one and it holds the first place in developing the country’s economy. It plays greater role in meeting the livelihoods of over 85% of the rural community, 40% of the national GDP, 80% of the export earnings, the row inputs supply of the agriculture-based industries and the supply of food to the urban residents of the country (source).
However, the productivity of the sector has been low and it is the main cause for the country’s food/livelihood and nutrition insecurity and slow economic development. The low agricultural productivity is partly attributable to the inappropriate use and management of soil resources by the farming community. This has become the main leading anthropologic cause of the country’s soil degradation where the main actors are erosion, acidification, salinization, organic matter depletion.
As a result, there are issues with the physical, chemical, and biological health situations of the soil and hence, its fertility leading to unbalanced agroecological systems in which the nutrient and water supply capacity and function of the soil is highly impaired. Therefore, to solve the above-mentioned situation, the ministry of agriculture has been studying the country’s soil resources to know the nature, diversity, distribution, and potentials for agricultural use. Accordingly, the Ethiopian soil resources have been classified under 18 Soil Reference Groups (SRGs).
The main problems of the Ethiopian soils are soil degradation, acidification, salinization, soil organic matter depletion and seasonal waterlogging of heavy clay soils especially on 0-2% slope range of the cultivable land. For example, soil erosion due to rainfall runoff is very serious and it is estimated to be up to 220 t ha-1 y-1 while the sediment yield is estimated to range between 2 and 70 t ha-1 y-1.
On the other hand, area of land affected by acidity covers over 7.5 million ha, of which over 4 million ha is estimated to be strongly acidic (pH < 5.5 ) on which normal growth, productivity and production of most crops is highly affected especially in the central, north-western, western, and southwestern highland areas of the country where the average annual rainfall is high. In addition, water logging problem of Vertisols is very high in the highland areas of the country where the average annual rainfall is high and the slope ranges between 0-2%. Further, in the arid and semi-arid areas of the country where evapotranspiration rate is higher than the average annual precipitation rate, soil salinity /sodicity problem is estimated to cover over 11 million ha and it is created mainly due to inappropriate irrigation practices and the use of low irrigation water quality.
Acrisols (Soil Taxonomy (ST) = Ultisols): Acrisols are formed from parent materials of acid rocks (mostly of Pleistocene age or older), crystalline and basic basalt rocks. They are characterised by their argic B-horizon, and dominance of stable, low-activity clays covering about 238 km2 (0.02%) of the Ethiopian landmass. They are potentially suitable for growing perennial crops like cashew, and mango plantations and they are most common for shifting cultivation while continuous cultivation requires recurrent input in terms of fertilisers and lime.
Alisols (ST = Ultisols): Alisols are developed from a wide variety of basic rocks and unconsolidated materials on undulating to hilly landscapes in warm to cool Ethiopian humid environments. The loss of iron oxides and clay minerals lead to a bleached eluvial horizon between the surface horizon and the argic horizon. They are moderately well-drained to well-drained soils with a reddish-brown to dark brown colour covering about 1,766 km2 (0.16%) of the total land area of the country. They are potential for Al-tolerant crops such as tea plantations in western and southwestern parts of the Ethiopian highlands.
Andosols (ST = Andisols): Andosols are developed in volcanic ash, tuff, pumice, and other volcanic ejecta of various compositions. The genesis of Andosols is based on the rapid weathering of pyroclastic materials (tuffs, pumice, cinders, lahars, and ash or other volcanic ejecta) in the presence of organic matter. They are characterised by having loamy, dark-coloured, and often very humic surface horizons with a fluffy, fine crumb ('floury') structure. They cover about 3,542 km2 (0.31%) area of the country. They are potential for agricultural production, however, so far, most of them are underutilised.
Arenosols (ST = Psamments): Arenosols are formed from sandy soils developed in residual sands, in situ after weathering of old, usually quartz-rich soil material or rock, and soils developed in recently deposited sands. They are characterized by moderately deep to very deep, somewhat excessively drained, coarse-textured soils and by having low water retention, rapid permeability, low natural fertility, and low CEC. They cover about 5,024 km2 (0.4%) of the total landmass of the country. They have limited potential for agriculture due to their poor inherent fertility, low water holding capacity, limited to moderate rooting conditions and excessive drainage.
Calcisols (ST = Calcids): Calcisols are found in plains/lowlands being developed from the eolian, alluvial and colluvial deposition of limestone origin. They have deep profile although recurrent drought slows down soil development. They are also formed in-situ from calcareous parent materials having shallow depth. The most prominent soil forming process in Calcisols is the translocation of calcium carbonate from the surface horizon and its accumulation in the subsurface horizon and the partial removal of carbonates by low annual rainfall conditions. They are characterized by brown surface horizon and yellowish brown subsurface with calcium carbonate concretions and by having low organic matter. Mostly they have a crumby structure; but platy
Calcisols (ST = Calcids): Calcisols are found in plains/lowlands being developed from the eolian, alluvial and colluvial deposition of limestone origin. They have deep profile although recurrent drought slows down soil development. They are also formed in-situ from calcareous parent materials having shallow depth. The most prominent soil forming process in Calcisols is the translocation of calcium carbonate from the surface horizon and its accumulation in the subsurface horizon and the partial removal of carbonates by low annual rainfall conditions. They are characterized by brown surface horizon and yellowish brown subsurface with calcium carbonate concretions and by having low organic matter. Mostly they have a crumby structure; but platy structures can also occur due to a high percentage of adsorbed divalent cations. They are naturally prone to surface crusting, leading to water logging and aeration problem. They cover about 30,108 km2 (2.7%) of the landmass of Ethiopia. These soils are frequently used for extensive grazing and do not have many fertility issues and can be used for irrigation agriculture with careful management practices.
Cambisols (ST = Inceptisols): Cambisols are formed under different environments: (i) calcareous limestone, colluvial and alluvial deposits, and volcanic rocks (in the north); (ii) Trap series volcanic rocks and basalts (in central, northcentral, and southwestern Ethiopia); and (iii) mixed volcanic rocks and limestone (in the south-eastern parts) of the Ethiopian highlands. These soils have wide range of variation among themselves, and it is very difficult to give a comprehensive quantified physical, chemical and fertility status for them. In Ethiopia, most Cambisols are medium-textured soil with good structural stability, high porosity, and good water holding capacity, and high content of weatherable minerals in silt and sand fractions. They cover about 387,982 km2, i.e., 34.2% of the total land area of the country. They have good potential for agriculture in the highland areas where annual rainfall is sufficient. Currently, these soils are intensively used for crop production and grazing. In the drier part of the country (southern, southeastern, northeastern, and rift valley regions), these soils have limitations mainly due to moisture deficits and can be enhanced using appropriate irrigation practices.
Fluvisols (ST = Fluvents): Fluvisols develop on flood plains, riverbanks, and toe slopes of mountain areas throughout Ethiopia. They also develop where lacustrine and lake deposits are found, such as around Lake Tana and some rift valley lakes of Ethiopia. They regularly receive fresh sediments during rainy seasons and form a stratified material which form stratified layers with clear or diffused boundaries between horizons depending on the nature and particle distribution of the transported materials. The transported materials mostly do not possess a significant amount of organic matter. Properties of Fluvisols depend on the nature of the transported soil. They are mostly found as surface or subsurface depositions or a mixture of both soil horizons, and hence, characterised by a weak horizon variation (A-C). Their structure varies from single grain to crumby or massive depending on the particle size distribution and land management while their textures is dependent on the nature and origin of the deposited sediments. They cover about 113,014 km2 (10 %) of the landmass of Ethiopia. Fluvisols have good fertility both in low laying locations of the highlands and lowlands. They respond well to management. However, they are susceptible to accumulation of salts (salinization) under faulty irrigation systems and/or occurrences of high evaporation that brings salts to the surface in lowland areas.
Gleysols (ST = Ultisols): The development of Gleysols is conditioned by disproportionate wetness at a depth less than 50 cm from the soil surface in some periods of the year. The wetness in the presence of organic matter causes a low-redox condition. Thus, Gleysols have greyish or bluish to greenish matrix colours with mobilised or removed iron compounds. Consequently, oxidation of transported Fe2+ and/or Mn2+ compounds can take place in the soil cracks, along living roots and former root channels where there is a supply of oxygen. They are characterized normally by having spongy profiles or matted litter layer resting on a dark grey Ah-horizon that changes sharply into a mottled grey subsoil. The topsoil is typically a mixed organic and mineral materials. Gleysols have blocky or crumby structure at the surface and a prismatic structure at the subsurface horizon. They cover about 89 km2 (0.01%) of the landmass of Ethiopia. They have potential for grazing in drier periods and with careful management practices for highland rice cultivation and horticulture.
Leptosols (ST = Entisols): Leptosols (ST = Entisols): Leptosols are genetically young soils with weak profile development over various kinds of continuous rocks, or unconsolidated materials with less than 20% (by volume) fine earth. They mainly occur on rugged topographic highlands where parent materials are almost exclusively volcanic, except in the south-eastern highlands in which limestone mixed with pre-cambrian basement materials are the dominant. They are characterised by incomplete and shallow solum, and they are extremely rich in coarse fragments possessing little available soil volume for root extension. They have also low water holding capacity and offer scant potential for water and nutrients for crops growing on them. They cover about 226,742 km2 (20%) of the total landmass area of the country. They are potentially good for growing trees and shrubs compared to annual crops because they have more extensive roots that penetrate deep strata to extract nutrients. They can be also used for grazing and forest development, or alternatively for agroforestry while they are less suitable for deep-rooted crop cultivation.
Lixisols (ST = Alfisols): Lixisols are derived mainly from unconsolidated, strongly weathered and strongly leached, finely textured parent materials. They occur in subtropical and warm temperate climates of Ethiopia, notably on old erosional or depositional surfaces. They are characterised by a clay accumulation in the argic horizon in combination with the occurrence of low activity clays and with a moderate to high base saturation. They are mostly deep and medium-textured soils. The eluvic horizon of these soils is massive in structure., the eluvic horizon may become very hard (so-called 'hard setting’) when dry. They are also characterised by low to medium water holding capacity, and low aggregate stability. They cover about 193 km2 (0.02%) of the total area of the country being distributed in the residual landforms of low plateau with hills, and moderately dissected topography. These soils are often potential for extensive grazing and perennial crops like cashew, mango, citrus, and other fruit trees are well adapted to deep or very deep soils. Supplementary irrigation may be required during the drier season for crop production. Continuous cultivation is quite possible through proper management of recurrent inputs of fertilisers and/or lime application.
Luvisols (ST = Alfisols): Luvisols are derived mostly from the Pleistocene epoch: (i) Trap series volcanic rocks and basalts (in central, northcentral, and southwestern Ethiopia); and (ii) mixed volcanic rocks and limestone (in south-eastern highlands of Ethiopia). The major pedogenetic process of Luvisols is the formation of their argic horizon by translocation of clay from the surface soil to the subsoil without marked leaching of base cations, or advanced weathering of high-activity clays. Luvisols typically have a brown to dark brown surface horizon over a (greyish) brown to strongly brown, or red argic subsurface. They have a moderately deep to deep profile, with granular or crumb surface soil structures that are porous and well aerated, and their argic horizon has a stable blocky structure, contributing to a good water holding capacity. These soils are the sixth most abundant soils in Ethiopia covering 70,717 km2 (6.2%) of the total land area of the country. Luvisols are good agricultural soils due to their deep rooting depth and large nutrient reserve. They are intensively used for crop production and grazing. Most Luvisols have lost their topsoil and consequently have become poor in organic matter and nitrogen content due to prolonged and continuous cultivation. Their potential could be enhanced by liming of acidic Luvisols integrating with organic and inorganic fertilisers.
Nitisols (ST = Inceptisols and Oxisols): Nitisols are formed from basic iron-rich rocks such as basalt. Most Nitisols exhibit an A-B(t)-C horizon sequence. Nitic properties come into existence because of different processes. Firstly, there is very strong weathering, which is called ferralitisation. Secondly, the shiny peds are formed by micro-swelling and shrinking. Thirdly, there is the process of biological pedoturbation by termites, ants, worms, and other soil fauna, which leads to the subangular soil structure in the topsoil and diffuse soil boundaries. These soils are characterized by having dusky red or dark red colour and by having clayey to fine clayey texture. They are deep, well-drained soils with a typical nutty or polyhedral blocky structure and shiny ped faces having high total porosity. They have an argic horizon with a deeply stretched clay bulge which does not show a relative decrease from its maximum of more than 20% within 150 cm of the surface. They cover about 73,309 km2 (6.5%) of the country’s land area. These soils are far more productive than most red soils, as they have deep and porous solum with stable soil structure. They have high P-retention features, making them poor P-level soils. However, they respond well to P and N fertiliser application.
Phaeozems (ST = Mollisols): Phaeozems develop from fine-textured basic parent materials on flat to sloping landscapes in warm to cool humid environments. In Ethiopia, they have formed under warm temperate climatic regions with natural vegetation cover producing enough contents of organic carbon to darken the topsoil. A low-intermediate level of leaching partially removes calcium carbonate from its topsoil, mostly to be accumulated in its subsoil. This favours the presence of sufficient faunal activities, such as earthworms and burrowing mammals to homogenise the soil, as observed in the central Ethiopian rift valley. Phaeozems are characterised by Mollic A horizon and a base saturation of > 50% in the upper horizons. They are porous, well-aerated, with moderate to strong and very stable crumb to angular blocky structure and have high organic carbon, medium carbon to nitrogen ratio, and good workability. They cover about 171 km2 (0.02%) of the total landmass of the country. These soils are highly productive and are currently used for cultivation of cereals, pulses and Enset (false banana). They are found on sloping landscapes having shallow, eroded, stony and less fertile nature and they are used for grazing and browsing.
Planosols (ST = Ablaqualfs and Albaquults): The genesis of Planosols is dependent on parent material and environmental conditions such as poor drainage and aquic moisture regime created due to regular inundation during rainy seasons. The ‘abrupt textural change’ from coarse-textured surface soil to finer subsoil in Planosols can be caused either by an initial abrupt lithological discontinuity (geogenetic) and/or a particular soil-forming process (pedogenetic). Geogenic layering results from alternating periods of geomorphologic activity. Within the pedogenetic processes, there are two possibilities: selective eluviation-illuviation of clay in soil material with low structure stability, and chemical pedogenetic processes; a process proposed under the name ‘ferrolysis’, an oxidation-reduction sequence. The most consistent morphological characteristic features of all Planosols profiles are their clay-poor “albic” E horizons that abruptly overlie a clayey subsurface horizon. Most of them exhibit an A-E-B horizon sequence and water logging results in the formation of iron and manganese nodules in E horizon. Their textural distribution within profiles is generally identified by distinctly higher clay content in the B horizon than the overlying A and E horizons. They cover about 463 km2 (0.04%) of the total area of the country. Planosols are mainly used for extensive grazing. The bleached E-layer is usually used for brick making. The problem in the management of Planosols for crops is related to the hampered growth of plant roots in the slowly permeable clay subsoil and their fertility problems in some areas. The problems are usually related to aluminium toxicity in highly acid Planosols and slight salinity or high content of sodium in salt-affected soils. Fertilization of Planosol soils is extremely difficult due to their low capacity to retain nutrients.
Regosols (ST = Entisols): Regosols are young soils without subsurface diagnostic horizons. Soil forming processes have a minimal effect on the properties of Regosols and conditioned by climate (dry and hot climate) and topography. Most of these soils develop on the erosional surface of the highlands’ slope and in some locations where their formation and development are retarded by the nature of the parent material. Profile development is limited to the formation of a thin A-C horizon sequence. The great variation among these soils makes impossible to give comprehensive characteristics of Regosols. They have shallow, medium-textured, and non-differentiated mineral soils with low water holding capacity and high permeability in Ethiopian situation having a minimal expression of A-C diagnostic horizons and low coherence of the matrix material. These soils cover around 3,995 km2 (0.4%) of the Ethiopian total landmass. They are commonly used for extensive grazing and for cultivating perennial cash crops and agroforestry. In mountainous areas, these soils are under natural vegetation covers.
Solonchaks (ST = Salids): Solonchaks (ST = Salids): Solonchaks are formed from parent materials with moderate to high concentrations of salts such as residual or fossil salts of former alluvial, colluvial, or lacustrine materials, and/or marine sediments. Solonchaks are common in areas where evapotranspiration is considerably greater than precipitation, at least during part of the year. Salts dissolved in the soil moisture remain behind after evaporation/transpiration of the water and accumulate at the surface of the soil ('external Solonchaks') or at some depth ('internal Solonchaks'). They are characterised by A-C or A-B-C horizon sequence. They also distinguished by their extremely low microbiological activity and a small number of individual microorganism groups. They have high content of soluble salts which have good physical properties and structural stability with high water holding capacity and hydraulic conductivity. The intensive alkalization process and very low inherent fertility of these soils results in high soil reaction within the entire soil horizons. These soils cover 9,958 km2 (0.88 %) of the total land area of the country. They have good potential for forage and pastures production. The toxicity of specific ions and high osmotic pressure on plants for water uptake from the soil are the major constraints of Solonchaks. Therefore, these soils have limited potential for agricultural use.
Solonetz (ST = Aridisols): These soils are associated with the formation of a natric subsoil horizon, which is linked with humus-rich surface horizons. The formation of the saline subsoil is unique to Solonetz, conditioned by the presence of high surface sodium levels responsible for dispersing the soil, and translocate to the subsurface with or without organic matter. The natric horizon is a dense subsurface horizon with a higher clay content than the overlying horizon(s) having a high amount of exchangeable sodium and/or magnesium, like the argic horizon. It is characterised by A-Btn-C and A-E-Btn-C horizon sequences. They generally show differentiation within the profile in respect to colour, structure, bulk density, and particle size distribution. They exhibit alteration of soil physical properties resulting from the swelling and dispersion of colloidal soil particles caused by the presence of excess sodium, and finally results in poor water and air circulation, water-holding capacity, restricted root penetration, and seedling development problems. They cover about 491 km2 (0.02 %) of the total landmass area of the country. These soils have potential for forage production and commercial trees (e.g., gum), and can be used as pastures despite they create problems to be used for crop production. Salt-tolerant crops like mustard and sorghum can be cultivated on these soils with a humus-rich surface horizon without amelioration. However, in most cases, these soils need reclamation.
Vertisols (ST = Vertisols): The major processes in the formation of Vertisols are alternating swelling and shrinking of expandable clays resulting in deep cracks in the dry season. Shrink-swell behaviour may also form gilgai micro-relief, especially in drier climates. Vertic horizons are clayey and when dry, they often have a hard to very hard consistency. Most Vertisols exhibit an A-C or A-(B)-C horizon sequence. Polished, shiny surfaces (slickenside), often at sharp angles, are distinctive. Vertisols of humid Ethiopian highlands (e.g., pockets of central and western highlands) exhibit shallower and narrow cracks, whereas, in semiarid and sub-humid zones of parts of the rift valley, Vertisols exhibit wider and deeper cracks. Calcium carbonate nodules are commonly found in drier surface and sub-surface Vertisols e.g., around Jigjiga, eastern Harerghe and northern Ethiopian highlands. A large proportion of Vertisols occur at slopes <8%, but they are also found up to 15%. They are formed largely on alluvial materials, whereas some developed on weathered basaltic rocks. They are commonly deep (>100cm) and they are typically characterised by their deep and wide cracks during the dry season. All Vertisols have high clay content (>30%) and they have angular to sub-angular blocky structures with high-water holding capacity. Their horizons are characterised by wedge-shaped soil aggregates, slickenside, and shrink-swell cracks and they have hard to very hard consistency. They cover about 205,208 km2 (18.1%) of the Ethiopian landmass. They have high potential for natural fertility having high water holding capacity and being suitable for a wide range of crop production. Nevertheless, they are commonly constrained by their imperfect drainage and hardness which impose workability problems. Their main features are critically limiting the agricultural production and management systems due to their heavy textural composition and shrinking properties which inhibit the growth of deep-rooted trees and crops.
The Ethiopian soils have physical, chemical, and biological problems, of which, the followings are the major ones.