Soil fertility research on some Ethiopian Vertisols

Desta Beyene

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

Abstract
Introduction
Characteristics
Fertility management
Conclusions
References

Abstract

Vertisols are important agricultural soils in Ethiopia. These soils generally have high clay content and consequently a high moisture storage capacity. The pH is slightly acidic to neutral. These soils have high yield potential, but require proper fertility management. Experimental results from NP fertilizer trials on various field crops showed that grain yields could be substantially improved with the application of N and P fertilizers. A similar response was also observed on forage crops.

Introduction

In Ethiopia, Vertisols cover 12.6 million ha, or about 10% of the country. In addition, there are 2.5 million ha of soils with vertic properties. About 70% of these soils are in the highlands, and about 25% (1.93 million ha) of the highland Vertisols are cropped (Berhanu Debele, 1985).

Vertisols are extensively found in Setit Humera, Gambela, Chilalo and Amibara. These soils occur in the lowlands (<1500 m), at intermediate altitudes (1500-1800 m) and in highland areas (2000 m or higher).

This paper reviews soil fertility studies on Vertisols and indicates future direction for research.

Characteristics

Vertisols are naturally fertile soils, but poor drainage and difficult workability limit nutrient availability. The most important characteristic of Vertisols is their high water-holding capacity (commonly 60-70%), a consequence of the deep profile and high content of montmorillonitic clay. Because of waterlogging, these soils remain unused during part of the rainy season, and many highland crops such as teff, barley, durum wheat, chickpea, lentil, noug and vetch are grown on residual moisture at the end of the rains.

The available information on the chemical properties of Vertisols is very limited. Analytical results from selected sites on Vertisol areas are shown on Table 1. Available P in these soils is generally higher than 20 ppm. Berhanu Debele (1985) reported that in 70% of the cases available P is below 5 ppm. In the surface horizons (0-30 cm) most of the Vertisols contain about 3-10% organic matter. Generally soil organic matter is related to texture, increasing with higher clay contents. Total N contents vary from 0.08 to 0.22% and the C:N ratio is about 11-18. The wide range in C:N ratio is attributed to increased nitrification and loss of N. as the Ca and moisture status are very favourable to increased microbial activity (Krishnamoorthy), 1971). The loss of nitrogen might also be caused by denitrification resulting from poor drainage.

Table 1. Chemical characteristics of Pellic Vertisols at selected sites.

Location	pH	C:N	Total N %	Avail. P (ppm) (Olsen)	Exchangeable cations meq/100 g soil
					K	Ca	Mg	CEC
Holetta	6.0	10	0.16	69.5	1.7	8.0	4.0	22.6
Ginchi	6.4	13	0.15	20.0	1.5	23.5	11.0	62.3
Debre Zeit	6.9	-	0.14	50.0	1.3	24.9	4.5	34.8
Sheno	6.2	11	0.22	53.0	1.2	19.0	8.2	30.0
Melka Werer	8.6	18	0.08	151.0	2.3	29.0	6.2	37.3
Mai Mekden	8.7	11	0.19	33.0	0.45	40.2	2.5	37.3

Source: Desta Beyene (1982).

The pH of Vertisols increases with depth, the topsoil being neutral or weakly acid. According to Berhanu Debele (1985), about 61% of the Vertisols have pH values of 5.5-6.7, 21% have pH values of 6.7-7.3, and 9% have pH values of more than 8. He stated that nearly all of the Vertisols of Ethiopia have CEC of 35-70 meq/100 g soil. In Table 1, however, CEC values range from 22 to 42 meq/100 g soil.

The clay fraction is dominated by smectites. The predominant exchangeable cation, which accounts for up to 80% of the exchange complex, is Ca, followed by Mg: K and Na contribute nearly equal proportions (Berhanu Debele, 1985). In the highlands, base saturation, even in the presence of calcium carbonate nodules, is rarely greater than 80-90%. These nodules are largely crystalline, hard, chemically inactive, and have practically no effect either on the pH or on the base saturation of the soil.

Fertility management

Attempts have been made to improve the productivity of Ethiopian Vertisols through N and P fertilization. Series of experiments have been carried out on Vertisols to study the effects of N and P fertilizers on crop yield.

Response to nitrogen

The response to N at various locations is given in Table 2. There was a marked N response in most of the crops tested (JAR, 1972, 1976 and 1977; Desta Beyene, 1986). Maximum barley yields at Sheno, maximum grain yields for noug, linseed, teff, and bread wheat at Ginchi, and maximum grain yields of wheat, barley and faba bean at Holetta, were all obtained with 90 kg N ha^-1. Similar results were found for teff grown at Debre Zeit, Akaki, Chefe Donsa, and Denkaka (AAU, 1983). Durum wheat grown at Debre Zeit gave maximum grain yield when 46 kg N ha^-1 was applied. Fertilizer trials carried out at Tefki, Inewari and Bichena also showed significant yield increases in bread wheat, durum wheat, teff and faba beans as a result of N fertilizer application (Adugna Haile and Hiruy Belayneh, 1986). For the forage grasses (Guinea and Phalaris) studied at Holetta maximum forage yield was found when 46 kg N ha^-1 was applied.

The high response to N is understandable because total N in most Vertisols is low. Because of rapid nitrification, most of the N added as fertilizer containing NH₄ or NH₂ is subject to leaching or denitrification soon after application. Ammonia fixation also affects fertilizer efficiency in heavy Vertisols (Finck and Venkateswarlu, 1982). Therefore, the application of 90 kg N ha^-1 for most crops may be justified under such conditions since the maximum yield for grain crops was found at this fertilizer level. The efficiency of the N fertilizer applied could be improved through the use of nitrate forms of fertilizer and the deep placement of split application of the ammonium forms of fertilizer.

Response to phosphorus

Responses to P fertilization are given in Table 3. For most crops there was a marked response (AAU, 1983; JAR, 1972, 1976, 1977; Desta Beyene, 1986). At Sheno, barley reached a peak yield of 2057 kg ha^-1 with the application of 13 kg P ha^-1, but higher concentration of fertilizer produced lower yields. Significant P responses were observed for teff and bread wheat at Ginchi. For teff the maximum yield was obtained with 40 kg P ha^-1, and for wheat 20 kg P ha^-1 gave the highest yield. The largest yield increment for bread wheat at Holleta and barley at Sheno was observed with the lowest rate of 13 kg P ha^-1. For faba bean maximum yield was obtained at 26 kg P ha^-1, Trials at Tefki, Inawari and Bichena showed that P was necessary for bread wheat, durum wheat, teff, and faba beans (Adugna Haile and Hiruy Belayney, 1986). Oilseeds and pulses showed little response to P. Two forage grasses (Guinea and Phalaris) at Holetta gave high yields with 40 kg P ha^-1.

Table 2. Response of rainfed field crops to N fertilizer in Vertisols of Ethiopia.

Location	Crop	Applied N (kg ha-1)					Sources
		0	30	46	60	90
		Grain yield (kg ha-1)
Ginchi	Noug	750		860		880	1
"	Linseed	800		960		970	1
"	Teff	720		730		1120	1
"	Bread wheat	1690		2320		2790	1
Holetta	Coloured
	Guineaa	673		1920		1827	2
"	Phalarisa	3794		4216		3630	2
"	Bread wheat	2900	3410		3540	4110	2
"	Barley	3000	2960	3200		3480	2
"	Faba bean	1360	1830		1790	2020	3
Sheno	Barley	1448	1716		2018	2164	4

a. Forage yield

Sources: 1. IAR (1977).
2. IAR (1976).
3. Desta Beyene (1986): IAR (1976).
4. IAR (1972).

Table 3. Response of field crops to P fertilizer in Vertisols of Ethiopia.

Location	Crop	Applied P (kg ha-1)						Sources
		0	13	20	26	40	53
		Grain yield (kg ha-1)
Ginchi	Noug	670		900		920		1
"	Linseed	750		1010		960		1
"	Teff	380		970		1220		1
"	Bread wheat	1690		2590		2250		1
Holetta	Coloured
"	Guineaa	673		1434		2767		2
"	Phalarisa	3794		4610		4570		2
"	Bread wheat	2870		3420		3730		2
"	Faba bean	1500	1690		1910	1890		3
"	Barley	2560	2900		3590	3560		2
Debre Zeit	Chickpea	1910	1470		2120	1930		4
"	Lentil	513	515	472	576			4
Sheno	Barley	1748	2057		1856	1843	1678	5

a. Forage yield

Sources: 1. IAR (1977).
2. IAR (1976).
3. Desta Beyene (1986): IAR (1976).
4. AAU (1983).
5. IAR (1972).

Variation in P response among crops at the same site is mainly due to the complexity of soil P. There are four important soil factors that affect the availability of applied P (Finck and Venkateswarlu, 1982); soil moisture, native available P. nature of the clay, and the amount of clay. Because Ca is the dominant cation in the CEC complex of the Vertisols, added P is usually transformed to calcium phosphate.

Other nutrients

Because K deficiency is uncommon in the country, studies on K fertilizer have not been carried out. Also, little work has been done on secondary nutrients and micronutrients, as these are not considered as factors limiting yield on Vertisols.

Conclusions

Research efforts on Vertisols should concentrate on improving drainage and filth. Once drainage has been improved, these soils are among the most fertile, and can produce high yields. Improved management will not only give higher yields, but will also reduce soil erosion.

Biological nitrogen fixation (BNF) is a field that requires special attention. Since N is the most limiting nutrient on Vertisols, the use of forage and grain legumes should be encouraged. Increased N fixation by these legume crops will lead to increased productivity of cereal crops.

References

Addis Ababa University (AAU). 1983. Debre Zeit Agricultural Research Centre, Annual Report 1977-1982.

Adugna Haile and Hiruy Belayneh. 1986. Influence of fertilizer and improved varieties on the seed yield of cereals, oil crops and pulses in the JAR/ADD sites. Paper presented at the Workshop on Review of Soil Science Research in Ethiopia, Addis Ababa, Ethiopia, 11-14 February 1986.

Berhanu Debele. 1985. The Vertisols of Ethiopia, their characteristics, 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 Report No. 56. FAO (Food and Agriculture Organization), Rome. pp. 31-54.

Desta Beyene. 1982. Diagnosis of phosphorus deficiency in Ethiopian soils. Soil Science Bulletin No. 3. IAR (Institute of Agricultural Research), Addis Ababa, Ethiopia

Desta Beyene. 1986. The response of pulse crops to N and P fertilizers. Paper presented at the Workshop on Review of Soil Science Research in Ethiopia, Addis Ababa, Ethiopia, 11-14 February 1986.

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.

Institute of Agricultural Research (IAR). 1972. Holetta Agricultural Research Station, Progress Report for the period April 1971 to March 1972. JAR, Addis Ababa, Ethiopia.

Institute of Agricultural Research (IAR). 1976. Holetta Agricultural Research Station, Progress Report for the period April 1973 to March 1976. JAR, Addis Ababa, Ethiopia.

Institute of Agricultural Research (IAR). 1977. Holetta Agricultural Research Station, Progress Report for the period April 1975 to March 1976. JAR, Addis Ababa, Ethiopia.

Krishnamoorthy Ch. 1971. In: A review of soil and water research in India in retrospect and prospect. ICAR, New Delhi, India. pp. 170-174.