2 Distribution and importance of Ethiopian Vertisols and location of study sites

KL Srivastava, Mesfin Abebe, Abiye Astatke, Mitiku Haile and Hailu Regassa

Introduction
Land and soil features
Distribution
Land use
Study sites
Importance

Introduction

Vertisols are characterised by their extensive cracking from the surface to depths of 50 cm or more with seasonal drying and also gilgai microrelief or subsoils showing slicker-sides or spheroid structures as evidence of seasonal expansion and contraction (Probert et al, 1987). These soils generally have a weak horizon differentiation.

These soils are distributed around the 45°N latitudes, mainly in the tropical and subtropical areas of the world. Driessen and Dudal (1989) report an estimated 311 million ha of Vertisols or 2.4 per cent of the global land area. Vertisols occupy about 105 million ha in Africa (Blokhuis, 1982) and about 12.6 million ha in Ethiopia.

Ethiopia can broadly be divided into highlands (areas above 1500 m altitude) and lowlands (area below 1500 m altitude). The highlands cover 40% of the land mass but account for about 95% of all cultivated land, accommodating 88% of the total human population and 70% of the total livestock population. Also 90% of the economic activity is concentrated in these areas (Constable, 1984).

This chapter describes the distribution, use and importance of Vertisols in the country and indicates the location of study sites of the Joint Vertisol Project (JVP).

Land and soil features

Parent materials and landscape features

In the central highlands of Ethiopia, the main soil parent material is olivine basalt of tertiary age. Materials of more recent age like tuffs, cinders and finer ash, mainly of basic nature are also common parent materials (Ahmad, 1983; Mitiku Haile, 1987).

The largest extents of Vertisols are found on the volcanic plateaux. Colluvial slopes and sidelopes of volcanoes are found in the central highlands, on the colluvial slopes and alluvial plains along the Sudan border and on the vast limestone plateaux of central Hararge. They are also found in sites such as granitic colluvium in basins with seasonal drainage deficiencies in southern Sidamo. Sandstone colluvium are found in valleys in Tigray and the flood plains of the Wabe Shebele and Fafen rivers in the Ogaden (FAO, 1985).

The general slope range of the landscape on which Vertisols occur is 0-8% (Berhanu Debele, 1985). They are more frequent in 0-2% slope range and are usually found in the landscapes of restricted drainage such as seasonally inundated depressional basins, deltas, alluvia/colluvial plains, pyroclastic plains, undulated plateaux, valleys and undulating sideslopes (Berhanu Debele, 1985). Vertisols are usually associated with Nitosols, Cambisols, Lithosols, Andosols and Luvisols (Mitiku Haile, 1987). The association of Vertisols with other soils and their catenary positions are described by Fisseha Itanna (1992). Classification of some typical pedons is given in Table 1.

Soil characterisation

Soil characterisation helps in interpreting experimental results and serve as a guide to scientific soil management. Soil colour, depth-to-root-restricting layer, texture etc can be used to characterise soils.

Colour

Soil colour has been an important criterion for recognising and differentiating Vertisols by farmers and also by scientists (Kamara and Haque, 1988a; Mitiku Haile, 1991). Vertisol with a moist chrome of 1.5 or less within the upper 30 cm is grouped as pellic Vertisol; it is called a chromic Vertisol when the moist chrome is greater than 1.5. Fig. 1 shows major areas of pellic and chromic Vertisols in Ethiopia. The main study sites of JVP have also been characterised on the basis of soil colour (Table 2). Marked changes in colour as a function of topographic position, drainage and free iron content are also commonly observed (Table 3).

Texture

Vertisol profiles at several sites have been characterised by Kamara and Haque (1988a) and Mitiku Haile (1987). Table 2 includes information on clay content at nine sites.

Table 1. Classification of the pedons studied according to Soil Taxonomy and FAO/UNESCO systems.

Pedon	Order	Suborder	Great group	Subgroup	Family	FAO/UNESCO
P49	Alfisol	Ustalf	Haplustalf	Udic Ultic Haplustalf	Udic Ultic Haplustalf, Clayey-skeletal, mixed isothermic	Orthi-Umbric Nitosol
Holetta	Alfisol	Ustalf	Haplustalf	Ultic Haplustalf	Ultic Haplustalf fine mixed isothermic	Haplic Nitosol
Woserbi	Alfisol	Ustalf	Rhodustalf	Udic Rhodustalf	Udic Rhodustalf, very fine mixed isothermic	Dystri-Rhodic Nitosol
P47	Entisol	Orthent	Ustortbent	Lithic Ustortbent	Lithic Ustortbent, loam mixed isothermic	Entric Regosol
P34	Mollisol	Ustoll	Haplustoll	Lithic haplustoll	Lithic Haplustoll, loamy mixed isothermic	Haplic Phaeozem
P38	Vertisol	Ustert	Chromostert	Typic Chromostert	Typic Chromostert, fine montmorillonitic isothermic	Chromi-Entric Vertisol
P102	Ustoll	Ustoll	Argiustoll	Vertic Argiustoll	Vertic Argiustoll, very fine clay mixed isothermic	Verti-Luvic Phaeozem
Sokoru	Ustoll	Ustoll	Haplustoll	Pachic Haplustoll	Pachic Haplustoll fine mixed isothermic	Haplic Phaeozem
P4	Ustert	Ustert	Chromostert	Paleustollic Chromustert	Paleustollic Chromustert fine mixed isothermic (calcaereous) isothermic	Chromi-Dystric Vertisol
P100	Ustert	Ustert	Pellustert	Typic Pellustert	Typic Pellustert, very fine montorillontic (calcareous) isothermic	Pelli-Eutric Vertisol
P135	Vertisol	Ustert	Pellustert	Typic Pellustert	Typic Pellustert, very fine montorillontic (calcareous) isothermic	Pelli-Eutric Vertisol
P148	Vertisol	Ustert	Pellustert	Typic Pellustert	Typic Pellustert, very fine montorillontic (calcareous) isothermic	Pelli-Eutric Vertisol
P248	Vertisol	Ustert	Pellustert	Typic Pellustert	Typic Pellustert, very fine montorillontic (calcareous) isothermic	Pelli-Eutric Vertisol
Tulu Guracha	Vertisol	Ustert	Pellustert	Typic Pellustert	Typic Pellustert, very fine montorillontic (calcareous) isothermic	Pelli-Eutric Vertisol
P45	Vertisol	Ustert	Pellustert	Typic Pellustert	Typic Pellustert, very fine montorillontic (calcareous) isothermic	Pelli-Eutric Vertisol
P336	Vertisol	Ustert	Pellustert	Typic Pellustert	Typic Pellustert, very fine montorillontic (calcareous) isothermic	Pelli-Eutric Vertisol
P44	Mollisol	Ustoll	Haplustoll	Cumulic Udorthentic Haplustoll	Cumulic Udorthentic Haplustoll, fine mixed (calcareous) isothermic	Orthi-Calcaric Phaeozem
Tulu Dimtu	Inceptisol	Ochrept	Ustochrept	Vertic Ustochrept	Vertic Ustochrept, fine mixed isothermic	Orthi-Vertic Cambisol

Depth-to-root-restricting layer

Layers of bed rock, coarse sand and gravel occurring at shallow depths restrict root growth because they hold little water and few plant nutrients or offer mechanical impedance. For example, the subsoils of Were Ilu and Deneba included layers of rock, coarse sand and boulders at less than 1 m depth (Kamara and Haque, 1988a). Table 4 shows the available water capacity and soil physical characteristics of selected pedons in the central highlands.

Figure 1. Provisional soil association map of Ethiopia. Vertisols distribution and on-farm verification sites.

Source: Land Use Planning and Regulatory Department, Ministry of Agriculture, Addis Ababa, Ethiopia, 1984.

Table 2. Agroclimatic and general information for the main study sites of the Joint Vertisol Project.

Site	Altitude (m asl)	Annual rainfal (mm)	Vertisol great group (based on colour)	Soil depth (cm)	Clay (%) (surface layer)	Agro-ecological zone	Physiographic region	Zone
Debre Zeit	1850	870	Chromic	>100	50	M2	Central highlands	Central
Akaki	2250	1000	Pellic	>100	63	SH2	Central highlands	Central
Inewari	2600	844	Pellic	60-150	60	SH2	Central highlands	Central
Bichena	2500	1100	Pellic	>100	64	SH2	Central highlands	North eastern
Ginchi	2200	1108	Pellic	>100	67	SH2	Central highlands	Central
Alemaya	1980	880	Chromic	>100	65	SH2	Hararghe highlands	Eastern
Wachu	1970	1100	Chromic	>150	70	SH2	Chercher highlands	Eastern
Dogollo	2600	946	Pellic	60-150	60	SH2	Central highlands	Central
Sheno	2800	900	Pellic	>100	52	SH2	Central highlands	Central

M2 = tepid to cool, moist, mid- to high altitude.
SH₂ = tepid to cool, subhumid, mid- to high altitude.
Source: Kamara and Haque (1988a); Mitiku Haile (1987).

Table 3. Relationship between free iron, drainage class and colour of Vertisol pedons.

Pedon	Colour (Ap)	Drainage class	Free iron %
P 14	10 YR 3/2	Imperfect	5.67
P 148	10 YR 3/1	Imperfect	3.50
P 38	10 YR 3/2	Well drained	3.35
P 135	10 YR 3/1	Imperfect	2.58
P 45	10 YR 3/1	Imperfect	2.16
P 284	10 YR 3/1	Moderately well drained	2.08
P 336	10 YR 3/1	Imperfect	1.97
Tulu Guracha	5 YR 2/1	Moderately well drained	1.50
Andode	10 YR 2/1	Imperfect	1.39

Source: Mitiku Haile (1987).

Distribution

Physiographic regions

Ethiopia comprises of 12 physiographic regions (FAO, 1988), namely

1. Chercher highlands
2. Northern rift and coastal zones
3. North-eastern escarpment
4. Abbay gorge
5. Central highlands
6. Western lowlands
7. Northern highlands
8. Ogaden lowlands
9. Central rift
10. Southern lowlands
11. Mountains and plains around Lake Tana
12. Western highlands.

The Vertisols are highly important in the central highlands and western lowlands. Significant areas of Vertisols are found in other regions also. It is estimated that 7.6 million ha of Vertisol area are located in the highlands above 1500 m asl and of this about two minion ha are currently cropped (Jutzi and Mesfin Abebe, 1986). The remaining area (over five million ha) is located at elevations below 1500 m.

Table 4. Physical properties of three pedons.

Pedon	Depth (cm)	OM (%)	Clay (%)	Bulk density g/cc	Structure	FC	WP % wt	AWC
P 4	0-20	3.8	33.10	1.36	Fine Crumb	50.16	32.20	17.96
P 135	0-50	1.7	68.20	1.74	Prismatic	50.15	34.90	15.25
Tulu Guracha	0-50	1.0	76.10	1.70	Strong Prismatic	53.90	40.20	13.70

OM = organic matter;
FC = field capacity;
WP = wilting point;
WC = available water capacity.
Source: Mitiku Haile (1987).

Agro-ecological zones

Moisture regimes of Vertisols may be characterised by the length of growing period, as defined by FAO Agro-ecological Zones Project (FAO, 1978). Based on this and other data, the Land Use Planning Regulatory Department of the Ethiopian Ministry of Agriculture has divided the country into 16 major agro-ecological zones as shown in Table 5. This zonation is designed to assist in the process of planning development strategies and research programmes for optimum utilisation of land resources for agriculture. The SH₂ zone includes vast areas of Vertisols characterised by over 180 days of growing period. The agroclimatology of several Vertisol locations was described by Belay Simane (ICRISAT, Patancheru, India, unpublished data). As an example of climatic elements, Fig. 2 shows the monthly distribution of rainfall and temperature at Ginchi station (Wagnew Ayalneh, ILCA, Addis Ababa, unpublished data).

Geographic distribution

The geographic distribution of Vertisols and associated soils is shown in Fig. 1 and Table 6. Most of the Vertisol areas are located in the central, north-western and southwestern zones.

Land use

Land use of Ethiopian Vertisols varies widely depending on climate, soil properties, population density and other socioeconomic factors.

It is rare to find extended tree vegetation on Vertisols. The predominant natural vegetation is grassland. In some areas, trees are found scattered on better drained sites. Faidherbia albida trees are sometimes grown by farmers in their cropped fields for obtaining fodder and fuel and to improve soil fertility (Kamara and Haque, 1992). The common grass species on Vertisols are Andropogon, Sporobolus, Festuca, Eleusine, Hyparrhenia, Agrostic and Cynodon (Jutzi and Mesfin Abebe, 1986).

Most of the Vertisols in Ethiopia suffer from excess water and poor workability and are also underutilised. Berhanu Debele (1985) reports that where Vertisols are cultivated, the common crops grown under rainfed conditions are teff, durum wheat, chick pea, lentils, Vicia latyroides, linseed etc but the crop yields are quite low (Table 7). Animal traction is commonly used in this region.

In the highlands, about 70% of the Vertisol area is uncropped and most of it is used as grazing land. In the rift valley (lowlands), about 0.1 million hectares are irrigated. Most of the lowland Vertisol area supports native vegetation of grasses, shrubs and sometimes trees. Some Vertisol areas in the western lowlands have been reclaimed for arable cropping in recent years. A major constraint to the spread of animal-powered agriculture in this area is the prevalence of trypanosomiasis. Other constraints like nutrient deficiencies, drought and frost are also important in several areas.

Study sites

The Joint Vertisol Project (JVP) decided to focus its research and technology-transfer activities in the highland areas. The highland areas are characterised by use of animal-drawn plough for crop production. JVP placed particular emphasis on the central zone which includes vast areas of Vertisols. In addition, some sites were located in the eastern and north-western zones also. Table 2 gives the agro-climatic and general information for nine study sites. These sites represent much of the highland Vertisol properties, climatic features, cropping practices and general farming systems.

Importance

Vertisols have crucial importance for improving and sustaining food production in Ethiopia. Bull (1988) estimates that about 11.9 million ha (over 90% of total) of Vertisol area in Ethiopia are potentially arable (Table 8). Out of this, about eight million ha can provide above 150 days of growing period. Most of the Vertisol areas in the highlands of Ethiopia are traditionally planted late in the rainy season allowing only partial use of the potential growing period. Bull (1988) estimates that Ethiopian Vertisols can produce about 12 million tonnes of food grain if improved management practice could be widely adopted. This could be accomplished partly by expansion of the cropped area and partly by increasing yields per unit area At present, total grain production from Ethiopian Vertisols is probably less than two million tonnes and there is an urgent need for mounting research and development activities.

Table 5. Agro-ecological zones of Ethiopia.

Symbol	Major agro-ecological zone	LGP (days)	LGP (Types)	Temp. (°C)	Altitude (m as)
A1	Hot to warm arid/lowland	<45	Single & double	>27.5-21	<500-1600
A2	Tepid arid/mid altitude	<45	Single & double	21-46	1600-2400
SA1	Hot to warm semi-arid/low land	46-60	Single & double	>27.5-21	1300-1600
SA2	Tepid, semi-arid mid-altitude	46-60	Single & double	21-16	<500-600
SM1	Hot to warm, submoist low-land	61-120	Single & double	>27.5-21	1600-3200
SM2	Tepid to cold submoist	61-120	Single	21-11	3200-3800
SM3	Cold to very cold, sub-moist, subafroalpine to afroalpine	61-120	Single	11-7.5	500-1600
M1	Hot to warm moist lowland	121-180	Single & double	>27.5 -21	1600-3200
M2	Tepid to cool, moist mid to high altitude	121-180	Single & double	21-11	3200-3800
M3	Cold to very cold, moist	121-180	Single & double	11-<7.5	200-2400
SH1	Hot to warm, humid, lowland to mid-altitude	181-240	Single & double	>27.5-21	1600-3200
SH2	Tepid to cool, subhumid, mid- to high altitude	181-240	Single & double	21-11	1600-3200
SH3	Cold to very cold, subhumid subafroalpine to afroalpine	181-240	Single & double	11-<7.5	3200-3800
H1	Hot to warm, humid, lowland to mid-altitude	241-300	Single & double	>27.5-21	1000-2200
H2	Tepid to cool, humid, mid- to high altitude	241-300	Single & double	21-11	1600-3200
H3	Cold to very cold, humid, subafroalpine to afroalpine	241-300	Single & double	11-<7.5	3200->3800

LGP = Length of growing period.
Source: Land Use Planning and Regulatory Department, Ministry of Agriculture, Addis Ababa.

Figure 2. Mean monthly rainfall and temperature at Ginchi (1982-90).

Source: Wagnew Ayalneh, ILCA, Addis Ababa (unpublished data).

Table 6. Distribution of Vertisols and their associations in Ethiopia (hectares).

No	Zone	Pellic Vertisols	Chromic Vertisols	Vertic Cambisols	Other Vertic Soils	Total
1	Northern	na	512100	226100	450200	1188400
2	North-eastern	174100	na	na	na	174100
3	Eastern	na	1760000	831100	83600	2675500
4	Central	1607200	na	585400	576700	2769300
5	Southern	1378300	525200	237200	21000	2161700
6	North-western	433100	1579000	na	na	2012100
7	Western	1761600	25000	na	na	1786500
Total		5345300	4379600	1879800	1131500	12767600

na = nil or negligible area.
Source: Adapted from Berhanu Debele (1985).

Table 7. Grain yields of food crops on Vertisols under traditional management in the central Ethiopian highlands.

Crop	Grain yields (kg/ha)
Teff	530
Barley	860
Emmer wheat	680
Durum wheat	610
Horsebean	750
Linseed	300
Lentils	500
Chickpea	600
Field pea	730
Noug (Guizotia abyssinica)	290
Vetch (Vicia latyroides)	690

Source: Berhanu Debele (1985).

Table 8. Potential arable areas (in thousand ha) of Vertisols in the different thermal and growing period zones.

Thermal zone	Length of growing period (LGP)					Total
	L1	L2	L3	L4	L5
T1	230	170	630	690	40	1760
T2	704	1888	613	107	119	3431
T3	182	511	1279	1588	461	4021
T4	na	77	602	1310	634	2623
T5	na	36	15	47	3	101
Total	116	2682	3139	3742	1257	11936

L1 =	< 90 days
L2 =	91-150 days
L3 =	151-210 days
L4 =	211-270 days
L5 =	> 270 days
na =	not available.
T1 =	< 500 m	altitude
T2 =	500-1300 m	"
T3 =	1300-2000 m	"
T4 =	2000-3000 m	"
T5 =	> 3000 m	"

Source: Bull (1988).