Agroecological assessment of Ethiopian Vertisols

T.A. Bull

AACM Advisory Team Agricultural Development Department Ministry of Agriculture
PO Box 60147, Addis Ababa, Ethiopia

Present address: AACM Company Pty Ltd. 11-13 Bentham Street, Adelaide, SA 5000, Australia

Abstract
Introduction
Methods
Results and discussion
References

Abstract

A simple agroecological zonation system developed by the AACM (Australian Agricultural Consulting and Management) Agricultural Advisory Team in the Agricultural Development Department of the Ethiopian Ministry of Agriculture has been used to define the distribution of Vertisols in Ethiopia. Vertisols are found in all but two of the 25 agroecological zones (AEZ). In addition, although Vertisols occur in all eight administrative zones, they predominate (>25% of the arable area) in only three, the central, northwest and southeast zones. Hence, programmes to support adaptive research and extension of improved Vertisol management practices should be centred on these AEZs and administrative zones.

If appropriate surface/subsurface drainage measures were implemented on 25-50% of the Vertisol areas in the main AEZs, then a conservative estimate of potential food grain production would be about 12 million t. This figure highlights the critical need to make better use of these soils in a country which is striving for food self-sufficiency.

Introduction

The AACM (Australian Agricultural Consulting and Management) Agricultural Advisory Team was attached to the Ethiopian Agricultural Development Department (ADD) of the Ministry of Agriculture in late 1984. The team reviewed past trials and demonstrations conducted by the ADD in order to formulate a new trials programme of improved crop production technologies designed for the peasant sector. One prerequisite for this programme was to develop a suitable agroclimatic zonation system so that past data could be aggregated on a rational basis to provide a framework for selecting new trial sites. The prime requirement was for a simple system which could be readily understood by field officers and be useful to national and regional planning programmes

The agroecological approach has been used to formulate the framework for a National Field Trials Programme which has been implemented by the ADD. This programme locates adaptive trial sites in the major agroecological zones (AEZ)/soil units of each region so that representative fertilizer and agronomic practices can be developed for extension to peasant farmers.

This paper concentrates on the assessment of Vertisol areas found in the various AEZs and the implications for trials and agricultural production.

Methods

Several agroecological type zonation systems have been utilised in Ethiopia in the past. The Ministry of Agriculture grouped agricultural areas according to loosely defined altitude classes (high, medium and low) and soil colour (red or black). An equally broad approach based on three land use classes (high potential cereal cropping, high potential perennial cropping and low potential cropping) was adopted by the Ethiopian Highland Reclamation Study (Cloutier, 1984) and by ILCA (Amare Getahun, 1978; Gryseels and Anderson, 1983). Neither of these systems is sufficiently detailed for planning research, development and extension programmes

A more realistic approach, based on a combination of altitude, rainfall and soil colour was proposed by Pinto (1984), but it contained too many classes to be practical at a field level. However, the concept was very useful and a similar but more refined approach has been developed by the Land Use Planning and Regulatory Department (LUPRD) of the Ministry of Agriculture for the preparation of land resources maps (Hendrickson et al, 1984). In this approach the major factors considered in assessing land resources are:

· length of growing period (LGP) - a function of rainfall, evapotranspiration, soil water storing capacity and meteorological hazards. The calculation of LGP has been developed by FAO (1978) and although it cannot account for local conditions like runoff during high intensity rains, soil water augmentation from subsurface drainage, variable soil water storage characteristics, etc. it remains a useful concept at regional and national levels.

· thermal zone (TZ) - a function of temperatures prevailing during the growing season and closely related to altitude in Ethiopia.

· landscape units-these combine aspects of the prevailing landform, distribution of slope classes and the major soil types.

Unfortunately the LUPRD land resource classification is based on relatively scant meteorological information and on satellite imagery which has not been fully verified on the ground. However, it remains the most complete and up to date information available for the whole country.

Consequently the land resource maps and supporting documents have been used to define a simple system of agroecological zonation The length of the growing period and thermal zone were chosen as the basic climatic factors to define the main AEZs. Five classes of LGP and five TZ were selected to define 25 possible AEZs representative of the whole country:

Length of growing period	Thermal zone
LI = <90 days	T1 = <500 metres
LII = 91-150 days	T2 = 500-1300 metres
LIII = 151-210 days	T3 = 1300-2000 metres
LIV = 211-270 days	T4 - 2000-3000 metres
LV = >270 days	T5 = >3000 metres

Within each AEZ the areas of the various landscape units have been measured, and from these data the areas of the individual soil classes and slope classes have been estimated. From an aggregation of these data, these areas have been calculated:

· gross area-total land area within a given AEZ.

· arable area-area remaining after deduction of the areas of Lithosols, lithic phases, swamps, lakes and land with slopes of >30%.

· weighted area-area calculated by applying a population density factor to highlight those locations already intensively developed for agriculture. (Weighted area (WA) is calculated from arable area (AA) and population density per km² (PD) by WA - AA x PD/200.)

· soil classes-total area occupied by given soil classes in each AEZ.

· slope classes-the areas located within each of the four main slope classes (0-8, 8-16, 16-30 and >30%) in each AEZ.

The full details of the system and the results obtained on a regional and national basis have been summarised (AACM, 1987).

Results and discussion

Vertisols occupy almost 12 million ha, or nearly 19% of the arable area of Ethiopia and 22% of the weighted area currently being intensively farmed. As the third most common soils after Nitosols and Cambisols, they clearly represent a major soil resource in the country which is vastly underexploited due to management difficulties using the traditional cultivation practices.

Zonal distribution of soil classes

Vertisols occupy more than 10% of the soils in all administrative zones of the country, but are the most important component (>25%) in the central, northwest and southeast administrative zones (Table 1). Hence, the development of improved management practices for Vertisols will have important implications for increasing crop production in all administrative zones.

Table 1. Areas (expressed in million ha) of the major soil classes in the administrative zones in Ethiopia.

Soil class	Administrative zonea
	CEN	NW	W	S	SE	E	NE	N
Nitosols	0.8	3.7	8.1	1.2	0.4	0.1	0.1	-
Cambisols	1.3	0.8	0.5	3.1	1.8	1.2	1.0	2.3
Vertisols	1.6	2.7	2.0	1.6	1.6	1.2	0.3	0.9
Luvisols	0.4	2.3	0.1	1.3	0.7	0.5	0.1	0.6
Fluvisols	0.2	0.1	1.6	1.4	0.2	1.0	0.2	1.3
Xerosols	-	-	-	0.9	0.9	2.5	-	1.1
Solonchaks	0.1	-	-	0.1	-	-	-	-
Acrisols	-	0.4	1.3	0.1	-	-	-	-
Others	0.2	0.1	0.4	-	0.3	1.3	0.1	-
Total	4.6	10.1	14.0	9.7	6.1	9.0	2.9	7.2
% Vertisol	35	27	14	16	26	13	10	13

a. Zones:

CEN - Central (Shewa)
NW = Northwest Gojam & Gonder)
W = West (Kefa, Ilubabor & Wellega)
S = South (Sidamo & Gamo Gofa)
SE - Southeast (Arsi & Bale)
E - East (Harerge)
NE = Northeast (Wello)
N = North (Eritrea & Tigray)

Agroecological distribution of Vertisols

National arable areas

Vertisols occur in all but two of the AEZs in Ethiopia (Table 2), but tend to be concentrated in four main AEZs:

LII T2 1.89 million ha LIV T3 1.59 million ha
LIV T4 1.31 million ha LIII T3 1.28 million ha

These four AEZs contain more than 50% of the total area of Vertisols in the country.

Although these four AEZs are important on the basis of Vertisol area, these soils comprise only 20-40% of the total arable soils in each AEZ (Table 3). Vertisols assume much greater relative significance in the AEZs LII T1, LIII T1, LIV T1 and LV T1, where they occupy 56-79% of the arable area. The total area of these AEZs is relatively small, but proper Vertisol management will be critical to enhanced agricultural production.

Zonal arable areas

When the distribution of Vertisol occurrence in the AEZs of the administrative zones is considered (Table 4), the relative importance of the AEZs containing large areas of Vertisols differs (Table 5).

These are the AEZs which must be considered in each region when assessing the type of improved Vertisol management practices required for future development.

Table 2. Potential arable areas (expressed in thousand ha) of Vertisols in the different agroecological zones.

Thermal zone (TZ)b	Length of growing period (LGP)a					Total
	LI	LII	LIII	LIV	LV
T1	230	170	630	690	40	1760
T2	704	1888	613	107	119	3431
T3	182	511	1279	1588	461	4021
T4	-	77	602	1310	634	2623
T5	-	36	15	47	3	101
Total	1116	2682	3139	3742	1257	11936

a. LI = < 90 days	b. T1 = < 500 metres
LII = 91 - 150 days	T2 = 500 - 1300 metres
LIII = 151 - 210 days	T3 = 1300 - 2000 metres
LIV = 211 - 270 days	T4 = 2000 - 3000 metres
LV = > 270 days	T5 = > 3000 metres

Table 3. Vertisols as a percentage of the total arable area in the different agroecological zones.

Thermal zone (TZ)b	Length of growing period (LGP)a					Mean
	LI	LII	LIII	LIV	LV
T1	5.6	63.2	79.4	55.8	62.5	27.2
T2	5.7	29.6	15.5	3.8	8.1	12.7
T3	6.5	12.4	30.0	20.3	10.9	18.7
T4	-	13.3	35.0	41.9	25.3	32.7
TS	-	40.0	4.2	19.9	1.0	12.6
Mean	6.3	23.5	29.0	24.6	14.6	18.7

a. and b. Refer to Table 2 footnotes.

Table 4. Arable area (expressed in thousand ha) of Vertisols in the different agroecological zones.

AEZb	Administrative zonea
		CEN	NW	W	S	SE	E	NE	N
LI	T1				190	40
	T2				595	26	59	3	21
	T3				46		136
	T4
	T5
LII	T1								108
	T2	1	911	7	71	332	1	5	560
	T3				128	56	120	70	137
	T4							43	34
	T5							36
LIII	T1			603
	T2		545	30			34		4
	T3	169	385		85	107	481	52
	T4	432				58		112
	T5	11						4
LIV	T1			653
	T2			107
	T3	412	353	319	84	47	373
	T4	466	478	50		316
	T5	47
LV	T1
	T2			12	107	46
	T3	27		193	195
	T4	17		8	68	541
	T5					3
Total	1582	2672	1982	1569	1572	1204	325	864

a. Refer to Table 1 footnote.
b. Refer to Table 2 footnotes.

Table 5. Relative importance of the agroecological zones containing large areas of Vertisols.

Administrative zones	Agroecological zones		Percent of Vertisol area in administrative zone
Central	LIV T4	LIII T4	57
Northwest	LII T2	LIII T2	54
West	LIV T1	LIII T1	63
South	LI T2	LV T3	50
Southeast	LV T4	LII T2	56
East	LIII T3	LIV T4	71
Northeast	LIII T4	LII T3	56
North	LII T2	65

a. Refer to Table 2 footnotes.

National weighted areas

Population weighting of the Vertisol areas causes a slight change in emphasis of the respective AEZs (Table 6). In this case the four major AEZs are:

LIV T3 0.57 million ha	LIV T4 0.57 million ha
LIII T3 0.30 million ha	LIII T4 0.25 million ha

which account for over 68% of the total weighted Vertisol area.

These AEZs represent the areas where most farmers are already attempting to farm Vertisols. Hence, national research and development programmes should concentrate initially on the above four AEZs in order to achieve the maximum immediate benefit from improved Vertisol management and farming practices.

Table 6. Weighted area (expressed in thousand ha) of Vertisols in the different agroecological zones.

Thermal zone (TZ)b	Length of growing period (LGP)a					Total
	LI	LII	LIII	LIV	LV
T1	-	7	8	10	-	25
T2	15	165	62	11	12	265
T3	12	72	303	573	183	1143
T4	-	25	253	567	165	1010
T5	-	13	7	-	-	20
Total	27	282	633	1161	360	2463

a and b. Refer to Table 2 footnotes.

Zonal weighted areas

The distribution of the weighted Vertisol areas in the individual zones (Tables 7 and 8) shows only relatively minor divergence from the AEZs identified on a national basis.

Only in the north zone is there a need to address AEZs with shorter growing periods and somewhat lower altitudes.

Table 7. Weighted area (expressed in thousand ha) of Vertisols in the different - agroecological zones.

AEZb		Administrative zonea
		CEN	NW	W	S	SE	E	NE	N
LI	T1
	T2				10	1	2	3	1
	T3				3		9
	T4
	T5
LII	T1								7
	T2		101		5	10		1	48
	T3				12		15	18	27
	T4							13	12
	T5							13
LIII	T1			8
	T2		51	3			8
	T3	69	83		35	22	75	18	1
	T4	179				22	2	50
	T5	5						2
LIV	T1			10
	T2			11
	T3	226	114	69	68	11	85
	T4	235	212	12		108
	T5
LV	T1
	T2			2	10
	T3	30		49	97	6	1
	T4	22		3	33	106	1
	T5
Total	766	561	167	273	286	198	118	96

a. Refer to Table 1 footnote.
b. Refer to Table 2 footnotes.

Table 8. Distribution of weighted Vertisol areas in the individual agroecological zones.

Administrative zonesa	Agroecological zones		Percentage of Vertisol area in administrative zone
Central	LIV T4	LIV T3	60
Northwest	LIV T4	LIV T3	58
West	LIV T3	LV T3	71
South	LV T3	LIV T3	60
Southeast	LV T4	LIV T4	75
East	LIV T3	LIII T3	81
Northeast	LIII T4	LIII T3	58
North	LII T2	LII T3	78

a. Refer to Table 1 footnote.
b. Refer to Table 2 footnotes.

Thus a national programme to develop and promote improved surface drainage, revised land preparation patterns, more productive cropping patterns, better conservation and erosion control practices and other related soil, water and crop management procedures should be concentrated in four target AEZs (LIV T3, LIV T4, LIII T3 and LIII T4), and three administrative zones (central, northwest and southeast) for maximum immediate impact.

Vertisol-related cropping systems

In order to simplify the types of farming and cropping systems which should be considered when promoting the development of Vertisols in Ethiopia, the broad grouping of relevant agroecological zones shown in Table 9 is useful.

The longer growing period/higher altitude grouping includes the AEZs LIV T3/T4 and LIII T3/T4. It includes 50% of the total area of Vertisols and regions of high population density. Within this grouping, farming systems are based on rainfed production. The major objective is to improve soil surface drainage in order to avoid waterlogging and better exploit the longer growing season.

The second largest grouping, short growing period/low altitude, includes the AEZs LI T1; LI T2, LII T1 and LII T2. This group includes 25% of the Vertisol area, but is located in regions of low population density. In general, most of this group is currently suited to grazing, but with proper surface and subsurface drainage, irrigated cropping is possible, particularly large-scale industrial or import substituting crops, since most Vertisols are associated with the flood plains of the larger rivers.

The third grouping, longer growing periods/low altitude, includes AEZs LIII to LV at T1 and T2, and represents 18% of the Vertisol area. Rainfed cropping predominates in this zone and, with proper soil and water management, a broad range of crops can be grown.

The final group, short growing period/higher altitude, is relatively minor and represents only 7% of the total Vertisol area. It includes AEZs with LGPs of LI and LII and thermal zones T3, T4 and T5. These highland valley bottoms are currently mainly used for grazing, but if surface drainage could be improved in the T3 and T4 areas, these could be readily used for the irrigated production of vegetables and other horticultural crops.

Table 9. Grouping of the agroecological zones (AEZ) to simplify the selection of suitable cropping patterns for research and development.

Group	Area (ha x 1000)	Agroecological condition	Farming system/cropsa
I	5939	Longer growing period (151>270 days) Higher altitude (1300>3000 m)	Rainfed cropping (maize, wheat, barley, teff, oats, haricot, linseed, noug, rape. faba)
II	2992	Short growing period (<150 days) Low altitude (<1300 m)	Grazing and irrigated cropping (cotton, kenaf, sugar-cane, sesame, rice, sorghum, maize)
III	2199	Longer growing period (151- >270 days) Low altitude (<1300 m)	Rainfed cropping (maize, sorghum, sesame, cotton, sugar-cane, sunflower)
IV	806	Short growing period (<150 days) Higher altitude (1300- >3000 m)	Grazing and irrigated cropping (vegetables, spices, fruit trees, flowers, maize)

a. Cropping assumes that improved surface drainage and land shaping can be achieved.

In general terms, if 25% of Groups II and IV could be irrigated and 50% of Groups I and III could be brought under improved surface drainage, the following potential food grain production levels could be postulated for Ethiopian Vertisols.

Group I	6 million t (at 2 t ha-1)
Group II	3 million t (at 4 t ha-1)
Group III	2 million t (at 2 t ha-1)
Group IV	0.8 million t (at 4 t ha-1)
Total	11.8 million t

Hence, the possible benefits from improving the management of Vertisols in Ethiopia are enormous in a country aiming at food self-sufficiency.

References

AACM (Australian Agricultural Consulting and Management). 1987. A simple system for defining agroecological zones in Ethiopia. Final report - a national summary. AACM Advisory Team, Working Paper No. 30. Agricultural Development Department, Ministry of Agriculture, Addis Ababa, Ethiopia.

Amare Getahun. 1978. Zonation of the highlands of tropical Africa: the Ethiopian highlands, Working Document. ILCA (International Livestock Centre for Africa), Addis Ababa, Ethiopia.

Cloutier P E. 1984. Assessment of the present situation in agriculture, Working Paper No. 11. Ethiopian Highlands Reclamation Study, FAO/LUPRD (Food and Agriculture Organization/Land Use Planning and Rural Development), Addis Ababa, Ethiopia.

FAO (Food and Agriculture Organization). 1978. Report on the Agro-Ecological Zones Project, Vol. 1, Methodology and results for Africa. World Soil Resources Report No. 48. FAO, Rome.

Gryseels G and Anderson F M. 1983. Research on farm and livestock productivity in the central Ethiopian highlands. Research Report No. 4. ILCA (International Livestock Centre for Africa), Addis Ababa, Ethiopia.

Hendrickson B L, Ross S. Sultan Tilimo and Wijntje-Bruggeman H Y. 1984. Ethiopia: geomorphology and soils, Field Document No. 3. Assistance to Land Use Planning, FAO/LUPRD (Food and Agriculture Organization/Land Use Planning and Rural Development), Addis Ababa, Ethiopia.

Pinto F W. 1984. PADEP Preparation Report, Annex 3: Gojam and Gondar. IBRD (International Bank for Reconstruction and Development), Nairobi, Kenya.