SOIL CHARACTERIZATION, GENESIS, CLASSIFICATION AND LAND SUITABILITY EVALUATION OF KULUMSA SUBWATERSHED, ARSI, ETHIOPIA

Abstract

Environmental factors such as topography, climate, geology, biotic factors and land-use activities are largely responsible for creating local differences in soil characteristics. These in turn create variations in the agricultural potential and management options of the soils that need detailed study in order to have adequate and comprehensive information and be able to solve site specific problems related to their sustainable use. However, there exists still a dearth of data in this aspect in Ethiopia in general and in Arsi Zone in particular. Cognizant of this fact, the present study was carried out in Kulumsa Subwatershed (KSW) as part of the Arsi-Highlands with the main objective to generate information through environmental and soil morphological description, soil characterization, genesis and classification as well as land suitability evaluation for rain fed production of the major food crops of KSW (teff, bread wheat, barley and faba bean) at an intermediate level of inputs and management. Soils of KSW were surveyed and 19 representative profile locations identified and sampled based on 1:50,000 scale topographic map using free survey along altitudinal and latitudinal transects; 103 bulk soil samples, 75 core soil samples and 8 rock samples were collected and analyzed. The sampled pedons were categorized into 10 Reference Soil Groups (RSG) based on FAO-World Reference Base and delineated into 8 soil mapping units (SMU) and 7 land mapping units (LMU). The study area was characterized by a strong physiographic diversity, so that areas with a rugged topography and sloping to steep major landforms comprised altogether about 75% of the total area. The proportion of relatively level land was only about 21% and 4% was occupied by a mixture of dissected plain and depression that had also certain ruggedness. The great altitudinal variation (1810-4000 masl and 2190 m elevation range) and topographic ruggedness were the main causes of the differences in micro-climate, landforms, vegetation types, land-use patterns and thereby the soils. Several soil characteristics such as effective soil depth, horizonation, soil color, internal drainage, average clay content, etc. seemed to be affected mainly by the slope; whereas solum thickness appeared to be linked to annual rainfall amount and slope. The clay content increased with depth up to the B horizon, and about 90% of the profiles had clayey texture in their subsoils or throughout even though almost all had well-drained profiles except few with hydromorphic properties, which may be attributed mainly to the aggregating effect of Ca that dominated the exchange complex. The trends observed in the FC and PWP values conformed to that of clay content by increasing with increasing clay content, whereas available water and drainable porosity records had the reverse trend. There was a linear correlation between altitude and several chemical soil characteristics. Accordingly, a direct relationship was detected between the mean topsoil SOM, TN, Olsen-P (mg/kg) and mean subsoil variable negative charge values (cmolc/kg and as percent of CEC7) and altitude; and an inverse relationship between mean subsoil pH, CEC7, sum of exchangeable bases, base saturation (PBS) and %CaCO3 and altitude. The degree of leaching assessed in terms of PBS increased also with altitude with the least leached profile at 2050 masl and most leached one at 3800 masl; whereby exchangeable K and Mg values that implied their deficiency, were also encountered mainly in pedons of higher elevation areas above 2600 masl. Even though there were acidic soils with pH records as low as 5.15 within their profile and PBS below 50%, the problem of exchangeable Al was not observed, most likely due to the chelating effect of SOM. Olsen-extracted P values in mg/kg were rated as low (6-10) to very low (<5) almost in all pedons with higher values in the Ap or Ah horizons, the highest record (22) being in an Ap horizon of a crop land pedon (KS14) under cultivation for more than 30 years, probably due to gradual P accumulation from P-fertilizer application. Values of DTPA-extracted cationic micronutrients showed a decreasing tendency with depth, probably as a result of redistribution through vegetation. Zn and Cu contents in mg/kg were rated as low (< 2.5 and <4.0 respectively) to very low (< 0.2); Fe contents as high (5.1-220) and Mn as medium (1.0-20) to very high (>50) in most and as low (0.3-0.9) to very low (<0.2) in some pedons, implicating generally the inevitability of Zn and Cu deficiency within the KSW. The petrographic analysis results showed the prevalence of pyroclastic rocks and deposits as parent materials in areas below 2300 masl altitude and mainly extrusive volcanic rocks such as andesites xx in areas above 2300 masl. High-activity clays such as smectites were estimated to be predominant in areas below 2550 masl; whereas in the higher areas the occurrence of mixed-type clay mineralogy with distinct proportions of the low-activity clays such as kaolinite, but also with the predominance of high-activity clays, were estimated to exist. The acidic profile condition of these high altitude areas seemed not to be in a position to cause an advanced weathering of the high-activity clay minerals, so that they still dominated the clay fraction. The agro-climate related pedogenetic assessment and soil classification results indicated that the least leached Kastanozems and Hypereutric Regosols in the Lower-Woina-Dega (1800-2100 masl), Chernozems and Luvisols (Hypereutric) in the Proper-Woina-Dega (2100-2250 masl), Cambisols, Planosols and Vertisols in the Upper-Woina-Dega (2250-2400) to Lower-Dega (2400-2550 masl), Retisols and Regosols (Orthodystric) in the Proper-Dega (2550-3000 masl), Umbric Alisols in the Upper-Dega (3000-3200 masl) and Hyperdystric Umbrisols in the High-Wurch (>3700 masl), were prevalent. The physical land suitability assessment revealed that all (7) the land qualities (LQ) selected as relevant had some kind of limitation, though there were differences both in the degree and type of limitation among the LMUs and the land utilization types (LUT). However, only four LQs, namely temperature regime (too high elevation), moisture availability (too much growing season rainfall), erosion hazard (steep slopes) and oxygen availability (water logging), were with severe limitations (n). Agro-climate related land suitability assessment results showed that the LMUs within Proper-Woina-Dega up to Upper-Woina-Dega were the most suitable (S1 to S2) for bread wheat and the other LUTs. Except for the steepness of the landscape-topography, barley seemed to be most favored in the Proper-Dega agro-climatic belt (M4eII). The LMU within Upper-Dega up to High-Wurch agro-climate (M5fI) had an N2 class for all LUTs mainly due to very severe (n2) limitations of temperature regime and erosion hazard. In inference, the predominance of topographic ruggedness within the KSW implicates that for the sustainable agricultural use of most LMUs, stringent soil and water conservation measures are the prerequisite. Moreover, integrated suitability assessment that may address also various rotation crops and the existing mixed farming system is required. Generally, in the E.2.03 type Rainfall Pattern Region of Arsi Zone, the results of the soil characterization, pedogenetic assessment, soil classification and the land suitability evaluation, indicated that the differences observed by Ethiopian farmers since long in their altitudinally varied environments in terms of their land-use practices, is found to be valid. However, for more reliable conclusion, this needs more detailed investigations that include the other Rainfall Pattern Regions of Arsi and of the country as well.