Physical properties of Ethiopian Vertisols

Asnakew Woldeab

Holetta Research Centre Institute of Agricultural Research (IAR)
PO Box 2003, Addis Ababa, Ethiopia

Abstract
Introduction
Importance of Vertisols in Ethiopian agriculture
Physical properties of Ethiopian Vertisols
Problems
Research results
Conclusions
References

Abstract

Vertisols are important to Ethiopian agriculture. They account for 24% of all highland soils that are cropped, but their high yield potential has not been realised. Production constraints are related to the physical properties of Vertisols and their moisture regime. The heavy texture and expanding clay minerals narrow the range between drought stress and excess moisture. Workability of these soils is hampered by their stickiness when wet and hardness when dry, and waterlogging and erosion greatly affect crop production. These soils are important to agricultural production; research priority should be given to realising their production potential.

Introduction

Vertisols are dark-coloured clays which develop cracks when expanding and contracting with changes in moisture content. They are geographically widespread, but it is only in the past decade or two that they have received scientific attention. Finck and Venkateswarlu (1982) indicated that Vertisols have an enormous yield potential but that this is often not realised.

Vertisols represent a vast crop production resource. It is estimated that there are at least 280 million ha of these montmorillonitic clays in the world, located mainly in Africa, Australia, India and the USA. Many of these soils are underutilised because they are difficult to manage-hard and cloddy when dry, and very sticky when wet (Willcocks and Browning, 1986).

Of the total Vertisol area, 126.5 million ha are found in three developing countries (Sudan, Chad and Ethiopia), where resources, facilities and trained scientific manpower are scarce, and where food is in short supply.

Berhanu Debele (1985) reported that of the 25 FAO/Unesco soil orders, 17 exist in Ethiopia. Lithosols, Cambisols, Nitosols, Vertisols, Xerosols, Solonchaks, Fluvisols and Luvisols cover more than 80% of the country, and are the most important soils. Vertisols cover 12.6 million ha, or 10.3% of the country; 7.6 million ha are found in the highlands. One quarter of these soils are presently cropped-24% of all highland soils cropped in Ethiopia (Jutzi and Mesfin Abebe, 1986)-which indicates their importance in Ethiopian agriculture.

The physical characteristics of Vertisols, coupled with the limited resources of small farmers, limit crop production on these soils.

Importance of Vertisols in Ethiopian agriculture

The largest Vertisol areas are on the volcanic plateaux, colluvial slopes and side slopes of volcanoes in central Ethiopia; on the colluvial slopes and alluvial plains bordering Sudan; and on the vast limestone plateaux of central Harerge province. Limited areas are found in such varied sites as the granitic colluvium in basins with seasonal drainage deficiences in southern Sidamo; on sandstone colluvium in valleys in Tigray; on the floodplains of the Wabi Shebele and Fafen rivers in the Ogaden; and in basins in western Ethiopia, where rainfall reaches 2000 mm (FAO/LUPRD, 1984).

Donahue (1972) reported that of 29 randomly sampled pedons in four major agricultural areas of the country (Setit Humera in Gonder, Gambela in Ilubabur, Chilalo in Arsi, and Middle and Lower Awash river basins in Harerge regions), 19 were classified as Vertisol and 10 as Entisol. Some site characterisations of Vertisols are given in Table 1.

Rainfed crops such as teff (Eragrostis tef), durum wheat, chickpea, lentils (Lens culinaris Med), linseed, noug (Guizotia abyssinica), and bread wheat are generally grown on Vertisols. Wherever drainage conditions are favourable, faba bean, field peas and barley are cultivated. In the lowlands, irrigated crops such as cotton, sugarcane, citrus, and some vegetables are grown on these soils. Small farmers grow sorghum, haricot beans, maize and other lowland crops.

Average yields on these soils are low: 500800 kg ha^-1 for cereals, 500-700 kg ha^-1 for highland pulses and 300 kg ha for oil crops.

Physical properties of Ethiopian Vertisols

Texture

Vertisols in Ethiopia generally contain more than 40% clay in the surface horizons and close to 75% in the middle part of the profiles. The sand fraction is low, often less than 20%, and is found in the bottom and the surface (plough layer) horizons. In the highland Vertisols where soil burning (guie) is practiced, the sand fraction is normally high in the surface horizon because the clay bakes into sand-size particles (Table 2, Berhanu Debele, 1985).

Table 1. Site characterisation of some Vertisol areas in Ethiopia.

Site characteristics	Awash	Melka Werer	Wonji	Ambo	Ginchi
Altitude (m)	700-750	750	1540	2060	2240
Physiography	Piedmont plain? Upper terrace?	Alluvial plain (back swamp)	Alluvial plain	Ambo depression	Lava plateau
General slope	3%	<1%	<1%	3-4%
Local slope	3%	<1%	0.5%	2-3%	0-2%
Erosion	Slight sheet wash	None, there may be deposition from floods	None	Slight sheet wash	Slight sheet and gully erosion
Drainage	Imperfect	Imperfect-poor	Imperfect-poor	Imperfect-poor	Imperfect
Land use	100% grassland (game reserve)	100% cultivated at station	100% cultivated to sugar-cane	100% cultivated to cereals and pulses	Mostly cultivated: wheat, chickpea, teff noug, lentil, linseed
Parent material	Colluvium/alluvium	Alluvium	Alluvium	Colluvium?	Weathered basalt
Rainfall (mm)	500	540	-	875	±900
FAO class	Pellic Vertisol (sodic phase)	Pellic Vertisol	Pellic Vertisol	Pellic Vertisol	Pellic Vertisol

Source: Morton (1977).

Clay mineralogy

In Ethiopian Vertisols the dominant clay minerals belong to the smectite group. Since both the free and total iron contents of Vertisols are high, it is believed that Nontronite is the most prevalent smectite. Berhanu Debele (1985) indicated that illitic minerals also constitute a significant proportion.

Bulk density

Because few data on bulk density of Ethiopian Vertisols are available, it is not possible to characterise bulk densities of very widely distributed Vertisols. Reports from elsewhere show that Vertisol bulk density is usually high, 1.5-1.8 g cm^-3, and may reach 2.05-2.1 g cm^-3 (Murthy et al, 1982). These variations in bulk density are caused by swelling and shrinking with changes in soil moisture content. The soils have high bulk density when dry and low density when wet (Virmani et al, 1982).

Consistency

When dry, Vertisols are hard and impossible to plough with oxen-drawn implements and may even be difficult to cultivate with heavy machinery. Seedbed preparation is therefore difficult; the seedbed is generally rough even after repeated cultivation. When wet these soils become plastic and sticky. Tillage and seedbed preparation are only possible within a narrow soil-moisture range.

Table 2. Particle size distribution in some Ethiopian Vertisols.

Location	pH (H2O)	Depth (cm)	Sand (2-0.05 mm)	Silt (0.05-0.002 mm)	Clay (<0.002 mm)
Wonji	6.9	0- 10	20.0	3.0	77.0
	7.4	10- 95	17.0	3.0	80.0
	7.5	95-160	17.0	13.0	70.0
	7.6	160-200	18.0	5.0	77.0
Awash	8.2	0- 5
	8.5	5- 30	26.0	25.0	49.0
	8.6	30- 65
	8.1	65-100	8.0	42.0	50.0
	8.2	100-170	8.0	42.0	50.0
Melka	7.8	0- 30	15.5	22.5	62.5
Werer	7.8	30- 75	17.5	15.0	67.5
	7.6	75-100	17.5	15.0	67.5
	7.6	100-145	32.5	32.0	34.5
	7.5	145-170	17.5	30.0	52.5
Ginchi	6.8	0- 22	12.0	24.0	64.0
	6.8	22- 50	12.0	20.0	68.0
	7.5	50- 80	10.0	16.0	74.0
	7.7	80-125	14.0	12.0	74.0
	7.6	125-155	13.0	30.0	57.0
	8.0	155-210	25.0	52.0	23.0
Ambo	6.1	0-10/15	13.7	17.5	68.8
	6.1	0-10/15- 35	11.2	20.0	68.8
	6.5	35- 65	11.2	20.0	68.8
	7.7	65-125	25.0	75.0
	7.7	125-200+	30.0	70.0
Sheno		0- 8	24	42	36
		8- 40	14	22	64
		40- 65	-	-	-
		65-100	14	24	62
		100-130	18	22	60
		130-200	14	25	61
Sheno
(guie)	5.6		29	46	25
(no guie)	5.6		18	32	50

Sources: Morton (1977) and IAR (1979).

Structure

In the dry season, surface horizons are characterised by huge, strongly developed prismatic primary structures separated from each other by deep vertical cracks, of various sizes, at intervals of 20-30 cm. These prisms break into strongly developed, often coarse, angular to sub-angular, secondary aggregates. In the wet season, both primary and secondary structures are almost completely destroyed, reducing the surface horizon to a massive block. At this time only shiny pressure faces and/or well-developed slickensides are visible (Berhanu Debele, 1985).

Pores, except for the cracks developed during the dry season and occasional root channels, are limited. The plant roots are confined to cracks and slickenside faces.

Available water

Vertisols have a relatively high water storage capacity in the root zone because of their depth and high clay content. The available water range has been reported as 110-250 mm for the top 1 m of the soil profile (Virmani et al, 1982). Virgo and Munro, as quoted by Virmani et al (1982), observed that the moisture content in deeper layers of the soil profile is lower, apparently due to compression effects on metric potential.

The high water-storage capacity of Vertisols is important in regions with uncertain rainfall. The growing season on deep Vertisols is usually longer than on other soils; on the highland Vertisols, wheat, lentil, chickpea and vetch grow to maturity entirely on residual soil moisture after establishment at the end of the rainy season. Farmers practice late-season planting to avoid the serious drainage problems characteristic of these soils during the rainy season.

Problems

Cultivation and seedbed preparation

Ethiopian Vertisols have a high content of clay, particularly expanding lattice clays. High clay content, type of clay mineral, unfavourable consistency and absence of pores make them difficult to work in both dry and wet conditions. A substantial amount of rainfall is needed to wet a dry Vertisol. The rain tends to move into cracks rapidly and wets the deeper layers of the soil profile, leaving the surface relatively dry. Achieving optimum moisture conditions for cultivation is difficult under present management practices. Once the rainy season starts and the surface is wet, cultivation is virtually impossible.

To overcome cultivation difficulties, seedbed preparation for all crops in the Ethiopian highlands starts with two ploughings during the short rainy season (March/April), when workability is relatively good. Up to six passes are made to prepare a seedbed for teff and durum wheat. It is not always possible to prepare a fine seedbed. Even after repeated cultivations the seedbed is rough. For the other crops, two or three passes are considered sufficient.

Drainage

The highland Vertisol areas are generally characterised by smallholder mixed cereal-livestock farming systems with a marked subsistence orientation. Land cultivation is almost exclusively done using oxen-drawn implements. The area is characterised by high rainfall (>900 mm year^-1) and low evaporative demand due to moderate temperatures, which vary widely with altitude, but might average 15°C annually. As a result, most vertic soils are severely waterlogged (estimated at 2.5 million ha, especially vertic Cambisols and vertic Luvisols) (Jutzi and Mesfin Abebe, 1986).

As the result of poor drainage, crops sown in early June suffer from prolonged waterlogging-they are stunted and show signs of poor aeration and nutrient deficiency. Grain yields are low.

Erosion

Vertisols in Ethiopia are located on either relatively flat or slightly sloping land. Erosion is a serious problem under present management, especially on fallow cultivated during the rainy season and on some sloping land in the highlands.

Research results

Research on black clay soils at two Institute of Agricultural Research (JAR) sub-centers (Ginchi and Sheno) showed that drainage and fertilizer application increase yields.

Soil burning practiced in the highlands can be replaced by adequate drainage through deeper ploughing with a tractor or planting on cambered beds. The advantages of cambered beds or ploughing with a mouldboard plough are more pronounced if high levels of fertilizer (60 kg N and 26 kg P ha^-1) are applied.

These recommendations are not within the reach of small farmers and have not resulted in the expected impact. Farmers still practice traditional methods of improving the drainage of Vertisols.

The Vertisols management project for the Ethiopian highlands, a joint project of ILCA, ICRISAT (International Crops Research Institute for the Semi-Arid Tropics), JAR, Alemaya University of Agriculture, the Ministry of Agriculture and Addis Ababa University, is developing technologies within the reach of the traditional farmer to drain excess moisture, improve soil fertility and develop a sustainable farming system.

Conclusions

The need for more intensive and applied research on Vertisols is apparent. Any research geared towards increasing productivity should start with a clear understanding of the physical characteristics of Vertisols. Research should address two important soil physical issues: workability and drainage.

Aspects that require urgent research attention include:

· identification and characterisation of Vertisols;

· tillage and land configuration: surface drainage and seedbed preparation;

· water management and use: rainfall pattern and probability studies, and water harvesting from catchments; and

· soil conservation: design and development of conservation structures and practices.

References

Berhanu Debele. 1985. The Vertisols of Ethiopia: their properties, classification and management. In: Fifth Meeting of the Eastern African Sub-Committee for Soil Correlation and Land Evaluation, Wad Medani, Sudan, 5-10 December 1983. World Soil Resources Reports, No. 56. FAO (Food and Agriculture Organization), Rome. pp. 31-54.

Donahue R L. 1972. Ethiopia; taxonomy, cartography, and ecology of soils. Michigan State University Press, Monograph No. 1. Michigan, USA.

FAO/LUPRD (Food and Agriculture Organization/Land Use Planning and Rural Development). 1984. Ethiopia: geomorphology and soils. AG: DP/ETH/78/003. Field Document 3, Addis Ababa.

Finck A and Venkateswarlu J. 1982. Chemical properties and fertility management of Vertisols. In: Vertisols and rice soils of the tropics. Twelfth International Congress of Soil Science, New Delhi, India, 8-16 February 1982. Indian Society of Soil Science, New Delhi, India. pp. 61-79.

IAR (Institute of Agricultural Research). 1979. Summary of research activities at Sheno substation 1968-1978. JAR, Addis Ababa. 35 pp.

Jutzi S and Mesfin Abebe. 1986. Improved agricultural utilization of Vertisols in the Ethiopian highlands. An inter-institutional approach to research and development. Paper presented at the First IBSRAM (International Board on Soil Research and Management) Networkshop in Africa on Improved Vertisol Management, Nairobi, Kenya, 1-6 December 1986. 10 pp.

Morton W H. 1977. Geological notes for the field excursions. In: Second Meeting of the Eastern African Sub-Committee for Soil Correlation and Land Evaluation, Addis Ababa, Ethiopia, 25-30 October 1976. World Soil Resources Reports, No. 47. FAO (Food and Agriculture Organization), Rome. pp. 96-126.

Murthy R S. Bhattacharjee J C, Landey R J and Pofali R M. 1982. Distribution, characteristics and classification of Vertisols. In: Vertisols and rice soils of the tropics. Twelfth International Congress of Soil Science, New Delhi, India, 8-16 February 1982. Indian Society of Soil Science, New Delhi, India. pp. 3-22.

Virmani S M, Sahrawat K L and Burford J R. 1982. Physical and chemical properties of Vertisols and their management. In: Vertisols and rice soils of the tropics. Twelfth International Congress of Soil Science, New Delhi, India, 8-16 February 1982. Indian Society of Soil Science, New Delhi, India. pp. 80-93.

Willcocks T and Browning J. 1986. Vertisols: (black) cracking clays. A bibliography with some abstracts, extracts, content analyses and comments. Overseas Division, AFRC Institute of Engineering Research (formerly NIAE), Beds, UK. 134 pp.