Assessment of Soil Acidity Status and Magnitude in High Rainfall Areas of South-Western Ethiopia

Received 28 Oct, 2020

Accepted 30 Mar, 2021

Published 04 Jun, 2021

ABSTRACTBackground and Objectives: In south western part of Ethiopia the soil productivity is highly in challenge due to soil acidity. This study is therefore initiated to know the extent of the soil acidity in intensively cultivated lands of kafa, Sheka and Bench-sheko zones and produce area-specific current information to motivate stakeholders to plan soil acidity amendment. Materials and Methods: Chena, Sheybench and Masha woredas were selected for respective zones. Composite samples were collected from five rural kebeles of each woreda from 15 to 20 spots at 0-20 cm depth and analyzed for pH (water), pH (kcl), N, OM, CEC, avP, exchangeable Al, exchangeable acidity and soil texture. Results: Accordingly, in Sheybench woreda, 20% of the areas showed very strong acidic (pH 4.0-5.0), while 80% showed strongly acidic (pH 5.1-5.5). Similarly, in Masha woreda, 40% of the area showed strong acidic (pH 5.1-5.5) and 60% moderately acid (pH 5.6-6.0) while Chena woreda has 20% strong acidity (pH 5.1-5.5) and 80% moderate acidity (pH 5.6-6.0). Aluminium toxicity was observed in about 40% of area. Conclusions: Therefore, lime application trials should be made and the optimum amount of lime should be recommended in the areas. In addition, crop management options should be selected to amend and improve the entire fertility of soils.

INTRODUCTION

The primary soil forming factors giving rise to increasing soil acidity in Ethiopia involve climatic factors such as rainfall, temperature, topographic factors, morphological factors and severe soil erosion¹. In Ethiopia, huge surface areas of highlands located at almost all regional states of the country are affected by soil acidity. Presently soil acidity has grown in scope and magnitude across different regions of Ethiopia in general and in southwest Ethiopia in particular. There is the currently increasing problem of soil acidity on agricultural activities in Ethiopian highlands (cultivated lands) is explained². In the humid tropics, soils such as Nitisols become acidic naturally due to leaching of basic cations under high rainfall conditions³. The detrimental effect of acidity on plant growth depend on the H⁺ and Al³⁺ ions activities in the soil solution, which may be related to the exchangeable contents of calcium (Ca²⁺), magnesium (Mg²⁺⁾ and potassium (K⁺) cations, of orthophosphate (H₂PO₄^-), nitrate (NO₃) and sulphate (SO₄^2-) anions, and with organic matter (MO) content⁴. With the neutralization of part of the soil total acidity by lime application, negative charges of the soil exchange complex are released, and then occupied by Ca²⁺, Mg²⁺ and K⁺ improving the soil fertility and the conditions for agricultural production⁵. According to another study Golla² around half of the world’s arable soils are acidic and may be exposed to the impact of Aluminium (Al) toxicity of which the tropics and subtropics account for 60% of the acid soils in the world. Soil acidity affects the growth of crops because acidic soil contains toxic levels of Aluminium and manganese and characterized by deficiency of essential plant nutrients such as P, N, K, Ca, Mg, and Mo⁶. The cause of soil acidity could be the type of parent materials from which the soil are formed, leaching of the base forming cations, continuous use of acid-forming fertilizers such as Urea and DAP⁷. In the tropics the soil acidity is aggravated by leaching or/and continuous removal of basic cations through crop harvest. At pH below 5, Aluminium is soluble in water and becomes the dominant ion in the soil solution. In acid soils, excess Aluminium primarily inhibits root elongation¹. The poor root growth leads to reduced water and nutrient uptake, and consequently crops grown on acid soils are confronted with poor nutrients and water availability. The net effect of which is reduced growth and yield of crops⁶. Liming acid soils is general practice to reduce Aluminium toxicity and is considered by many scientists as the first step towards providing a balanced nutrition for cultivated plants^8,9. Yields of the major cereal crops, particularly barley, are as low as 0.5mg/ha partly as a result of soil acidity⁸. Improving the soil pH by the application of lime, nitrogen and phosphorous has increased yield by three folds¹⁰. Generally agricultural soil is severely affected by acidity in central highlands of, Ethiopia¹¹ the western parts of the country¹². Specially, the soil fertility degradation is a serious problem in southwestern part of the Country¹³. Despite this, no well-recorded documents are available describing the extent and magnitude of soil acidity in the south-western part of Ethiopia. For acidic soil management, lime has been supplied by Government. However, because of less awareness, farmers were resistant to use the lime properly (e.g. dose, rate, time of application) which was also supported by Bikila¹. Some farmers for instance, consider lime as a substitute to artificial fertilizers. Absence of clear and problem solving information on soil acidity level and extent is the added constraint¹⁴. Therefore, this study was initiated to know to the level of acidity and its extent as well as the nutrient availability and fertility level in the soil of study area so as to generate sustainable soil management strategy for crop productivity improvement. The result could also direct the land manageress and government to plan the action forward.

MATERIALS AND METHODS

Description of the study area: The assessment was done in kafa, Sheka and Bench Maji zones of western Ethiopia where highly and potentially acidic areas exist due to high rainfall distribution and acidic characteristics of Nitisols in the area¹⁵. The selection of rural kebele for soil sample collection was carried out in collaboration with experts in were da Agricultural office.

The area geographical located at 6.22 to 7.76N and 35.09 to 36.51E. Generally three zones have the average temperature range of 15.1^oC to 27.5^oC, 10.1^oC to 27.5^oC and 15.1^oC to 25^oC; rainfall of 400 to 2000 mm, 1001 to 2200 mm and 1800 to 2200 mm, and elevation of 500 to 2500 masl, 501 to 3500 masl and 1001 to 3000 masl at Bench Manji, Kefa and Sheka respectively. Agro-ecologically the areas are classified as moist qolla, woyina-Dega, Dega and high Dega. The area, Southwestern part of the SNNPR, is endowed by natural resources particularly comprise most of the remaining natural forests of the country, which have economically and ecologically important different plant species. The forests of the area are recognized and designated by UNESCO in June 2010 as a biosphere reserve - pursuing to be balance development and conservation for the good life of the communities in the area. Mixed cropping (coffee-based) is the dominant farming system in the areas of Bench-Maji, Kafa, and Sheka zones. Kafa, an Ethiopian highland region that contains 50 per cent of the country’s remaining Afromontane evergreen forest ecosystems and the origin of Coffee arabica, was added to the network, in the country’s southwest. The forest in Sheka, which is also part of the southwest highlands forests of Ethiopia, is important for the conservation of Afromontane forest vegetation types. Coffee holds more than half of their lands, and on the remaining cultivated land they grow cereals mostly maize and tef while Enset and different vegetables are grown around the homestead. In the semi-arid low land wored as of the three zones, sorghum is the dominant crop in terms of area cultured. Maize sorghum and Tef are generally the major crops with some roots and tuber crops¹⁴.

Soil Sampling: From each of three zones, one woreda and five Kebeles from respective woreda were selected totaling to fifteen Kebeles for oil sampling collection focusing on intensively cultivated land. Soil sample from 20 sampling points was collected from homogenous land use with a sampling depth of 0–20 cm and composited to make one representative composite sample based on the uniformity based on soil type/colour, slope, and management.

Analysis of soil chemical properties: Soil samples of the soil was air-dried, sieved to pass 2 mm and analysed for Soil pH (water, KCl), Organic matter, Cation exchange capacity, Exchangeable acidity, soil texture, available Phosphorus, Total Nitrogen. The pH of the soils was measured in water suspension in a 1:2.5 (soil-water) potentio-metrically using a glass-calomel combination electrode¹⁶. The wet digestion method¹⁷ was used to determine soil carbon content and percent soil OM was obtained by multiplying percent soil OC by a factor of 1.724 following the assumptions that OM is composed of 58% carbon. Total N was analyzed using the Kjeldahl digestion, distillation and titration method as described by Rowel¹⁸. Available soil P was analyzed according to the standard procedure of Recena et al.¹⁹ extraction method. Cation exchange capacity (CEC) was determined after extracting the soil samples by ammonium acetate (1N NH4OAc) at pH 7.0. Cation exchange capacity was there after estimated titrimetrically by distillation of ammonium that was displaced by sodium from NaCl solution²⁰. Exchangeable acidity was determined by saturating the soil samples with potassium chloride solution and titrated with sodium hydroxide as described by McLean²¹. Particle size distribution (soil texture) was determined by the hydrometer method²². Soil pH was determined in a 1:2.5 soil to water suspension as outlined by Alemayehu et al.²³. The total soil exchangeable acidity (H⁺ and Al³⁺) was extracted with 1.0MKCl and then titrated by 0.01M NaOH to pH 7.0²⁴. The exchangeable Al³⁺ was the difference between exchangeable acidity and exchangeable H^{+ 22}.

RESULTS AND DISCUSSIONS

Soil texture of the sampling site: Soil texture is one of the physical soil characteristics that influence land use and management especially when soil is acidic. According to Table 1 below, all of the sampled areas were dominated by the clay content of the soil though there are some Silty and loam. The highest clay content was due to the exposure of top soil to soil erosion by water which ultimately exposes the subsoil which is also naturally high in clay content in Nitisols which is the type of soils in southwest Ethiopia. Clayey soils require more tillage practice than silty clay soils for improving aeration and acidic lands.

Table 1: Soil textural classes of selected five kebeles in each woreda
Soil texture by woreda
Sheybench	Masha	Chena
Heavy clay	Clay	Clay
Silty clay	Clay	Silty clay loam
Clay	Silty Clay	Clay
Silty clay	Clay	Silty clay
Clay	Heavy clay	Clay

Status of soil acidity and nutrients in Sheybench: From Table 2, it was observed that Shey bench has a pH range from 4.5-5.5 with an average of 5.2. Similarly, average CEC, exchangeable Al and exchangeable acidity were 4.2, 1.1 and 4.1 respectively. When exchangeable acidity is concentrated in appreciable amounts in the soils with a pH range of 4-5 and lower, it produces strongly acidic soil condition^21,25. The amount of OM, TN and avP ranged from 5.1 to 8.7; 0.3 to 0.4 and 11.8 to 12.7 respectively. It can be interpreted according to Leon et al.²⁶ acidity level ratings that 40% of the area is very strong acidic (4.5-5.0) and the remaining 60% is in a strongly acidic level (5.1-5.5). The result of high OM (5.0-8.7) content within the range of high rating (5-10) according to Hailu et al.²⁷ the area seems to a high content of clay soils. According to the ratings of the same author, the content of TN is also high in this area. Available P in the area (12.8 ppm) approached to the low level (in range of 10-25 ppm). The area is characterized by soils with very low CEC (<6) according to McGrath²⁹ ratings. This affects the buffering capacity of the soil to be less.

Table 2: Status of acidity and nutrients in the soils of Sheybench woreda
		Sheybench
RK	pH (H20)	pH (kcl)	avP (ppm)	% OM	% TN	CEC (meq/100 g)	Exch Al (meq/100 g)	Exch. Acidty (meq/100 g)
1	5.00	4.60	11.99	6.00	0.29	3.68	0.8	1.6
2	5.40	4.80	12.38	7.34	0.37	4.62	0.86	5.2
3	5.50	4.55	12.65	8.74	0.44	4.80	1.08	9.2
4	5.40	4.30	12.32	7.45	0.37	4.78	2.2	2.8
5	4.50	3.20	11.79	5.07	0.25	2.9	0.6	1.8
Av.	5.2	4.3	12.2	6.9	0.3	4.2	1.1	4.1
Max	5.50	4.8	12.7	8.7	0.4	4.8	2.2	9.2
Min	4.5	3.2	11.8	5.1	0.3	2.9	0.6	1.6
NB: RK 1: Bata; 2: Shapagoid; 3: Ziyagin 4: Sheybench zuriya and 5: Kaminan

Status of soil acidity and nutrients in Masha: Table 3 shows that Masha has a pH range from 5.1-5.8 with an average of 5.5. Similarly, average CEC, exchangeable Al and exchangeable acidity were 5.3, 1.2 and 12.1 respectively. The amount of OM, TN and av.P ranged from 5.8 to 10.1; 0.3 to 0.5 and 12.1 to 13 respectively. It can be interpreted according to Leon et al.²⁶ acidity level ratings that 40% of the area is under strong acidic (5.1-5.5) condition while 60% area under moderate acidic status (5.6-5.9). The result of high OM (5.8-10.1) was obtained in the area which could be also due to high clay soils. The ratings also showed high content of TN in the Masha area (0.3-0.5%). Available P in the area (12.7 ppm) approached to low level (from range of 10-25 ppm as a medium. 80% of the area is characterized by soils with very low CEC (<6) according to McGrath²⁸ ratings which can affect the buffering capacity of the soil.

Table 3: Status of soil acidity and nutrients in the in Masha woreda
Masha
RK	pH (H20)	pH (kcl)	avP (ppm)	% OM	% TN	CEC (meq/100 g)	Exch Al (meq/100 g)	Exch. Acidty (meq/100 g)
1	5.10	3.80	12.12	5.79	0.29	3.89	1.6	11.6
2	5.50	4.40	12.4	8.43	0.42	5.09	0.6	2.4
3	5.60	4.40	12.79	8.90	0.44	5.52	0.92	8
4	5.70	4.90	12.92	10.14	0.51	5.9	0.6	18.4
5	5.80	5.10	13.02	9.95	0.49	6.09	2.2	20
Av.	5.5	4.5	12.7	8.6	0.4	5.3	1.2	12.1
Max	5.8	5.1	13.0	10.1	0.5	6.1	2.2	20.0
Min	5.1	3.8	12.1	5.8	0.3	3.9	0.6	2.4
NB: RK 1: Ateso; 2: Yina; 3: Chago 4: Uwa and 5: Ginbeka

Status of soil acidity and nutrients in Chena: Table 4 shows that Chena has a pH range from 5.2-5.9 with an average of 5.6. Similarly, average CEC, exchangeable Al and exchangeable acidity were 5.4, 1.0 and 13.4 respectively. The amount of OM, TN and av.P ranged from 6.7 to 10.7; 0.3 to 0.5 and 12.1 to 13.2 respectively. It can be interpreted according to Leon et al.²⁶ acidity level ratings that 20% of the area is under strong acidic (5.1-5.5) condition while 80% area under moderately acidic status (5.6-5.9). The result of high OM (6.7 to 10.7) was obtained in the area which could be also due to high clay soils. The ratings also showed high content of TN in the Chena area (0.3-0.5%). Available P in the area (12.8 ppm) approached to low level (from range of 10-25 ppm as a medium). 80% of the area is characterized by soils with very low CEC (<6) according to McGrath²⁸ ratings which can affect the buffering capacity of the soil. From the results it can be deduced that the area is at risk of Aluminium toxicity and need great attention to manage it. At pH below 5, Aluminium is soluble in water and becomes the dominant ion in the soil solution. In acid soils, excess Aluminium primarily injures the root apex and inhibits root elongation¹. The poor root growth leads to reduced water and nutrient uptake, and consequently crops grown on acid soils are confronted with poor nutrients and water availability. The net effect of which is reduced growth and yield of crops⁶. It is explained that; in some barley and wheat growing areas of central and southern Ethiopia, farmers have shifted to producing oats which is more tolerant to soil acidity than wheat and barley³. The increasing trend of soil acidity and exchangeable Al in arable and abandoned lands are attributed to intensive cultivation and continuous use of acid-forming inorganic fertilizers². As the assessment sites of this report were intensively cultivated lands the increase in acidity may be continuous cultivation and fertilizer inputs. According to Lelago and Buraka²⁹; Baligar³⁰ intensive cultivation and application of inorganic fertilizers lead to the higher exchangeable acidity content under the crop field.

Table 4: Status of soil acidity and nutrients in the Chena
	Chena
RK	pH (H20)	pH (kcl)	avP (ppm)	% OM	% TN	CEC (meq/100 g)	Exch Al (meq/100 g)	Exch. Acidty (meq/100 g)
1	5.20	4.10	12.08	6.69	0.33	4.11	0.2	9.60
2	5.90	5.00	13.19	10.67	0.53	6.14	1.0	5.60
3	5.60	4.20	12.88	9.10	0.45	5.49	2.2	16.6
4	5.60	4.00	12.88	8.83	0.44	5.61	0.6	16.8
5	5.70	4.50	12.90	9.86	0.49	5.74	0.8	18.2
Av.	5.6	4.4	12.8	9.0	0.4	5.4	1.0	13.4
Max	5.9	5.0	13.2	10.7	0.5	6.1	2.2	18.2
Min	5.2	4.0	12.1	6.7	0.3	4.1	0.2	5.6
NB: RK 1: Agaro; 2: Beko; 3: Kutashoray 4: Kulish and 5: Bobabala

CONCLUSION

Soil acidity in all areas of assessment is very high and need attention to solve the problem to boost the productivity of crops. Very strongly acidic (pH 4.5-5.0) was observed in 40% of Sheybench while, strongly acidic soil condition obtained in 60, 40 and 20% areas of Sheybench, Masha and Chena respectively. The remaining 60 and 80% of Masha and Chena woredas respectively got moderately acidic level (5.6-5.9). Generally from the three sites considered, a strong acidity level with pH of 4.5-5.0 was obtained in 100, 40, and 20% areas of Shey bench, Masha and Chena respectively. In all cases, soil organic matter and total nitrogen content of the soil is high and all the sites have low CEC which indicates the low buffering capacity of the soil. Three sites have soil texture with more clay property as well as more than enough OM and TN, and low available phosphorous (about 12.72 ppm). Regarding CEC, it is at the level of very low content (below 6 meq/100 g) in all areas. Under normal condition, soils with high clay content and OM could not have less CEC. However in this study it is observed that soil acidity had a strong influence on CEC and buffering capacity since high clay content and OM as well as TN, is observed having strong acidic soils. Since the fertility potential of the area is observed with high nitrogen content, organic matter, as well as clay dominance, the optimum crop yield could be obtained if the priority is given to soil acidity management and frequent tillage is on clay dominated soils.

ACKNOWLEDGEMENT

The author is thankful to the supporters who contributed to research and paper preparation such as the Department of Natural Resource Research and Tepi Soil Laboratory Center for its facilities during soil chemical analysis and others.

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