Agro-Climate Based Pedogenetic Assessment of Soils in Kulumsa Subwatershed, Arsi, Ethiopia

Any particular soil results from the integrated action of soil-forming factors that occur in different kinds and degrees of expression that can be combined in many different ways; so that each combination produces a different soil with its unique properties resulting in the existence of thousands of different soils on earth. Soil genesis brings order to this overwhelming variety of soils aiding their classification. Soils of Kulumsa sub-watershed were surveyed and 19 representative profile locations identified, described, and sampled based on observable site and soil characteristics such as slope, topographic position, vegetation, topsoil color, texture, etc. using a topographic map of 1:50,000 scale as a base map through free survey along altitudinal and latitudinal transects. The sampled 19 pedons were categorized into 10 Reference Soil Groups through pedogenetic assessment; and the relationship of the overall pedogenesis and spatial distribution of the soil groups to altitude and the traditional agro-climates were examined. The pedogenetic assessment of the soils revealed that the degree of weathering and leaching increased with increasing altitude. Accordingly, the least leached and most alkaline Kastanozems in the Lower-Woina-Dega (1800-2100 masl) within the “rift valley” and its border escarpment areas, and the most leached and most acidic Umbrisol in the High-Wurch (>3700 masl) agro-climatic belt on Mount Chillallo, were identified. Even though there were acidic soils with pH as low as 5.15 and very strongly leached profile, the chemical weathering processes appeared to be not so intensive when evaluated in terms of their clay mineralogy. The acidic soil 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 of the soils. In relation to the traditional agro-climates, from the lowest towards the highest elevation within the study area, the least leached Kastanozems in the Lower-Woina-Dega, Chernozems and Luvisols in the Proper-Woina-Dega (2100-2250 masl), Planosols and Vertisols in the Upper-Woina-Dega (2250-2400) to Lower-Dega (2400-2550 masl),Vertisols and Luvisols in Lower-Dega, Retisols in the Dega-Proper (2550-3000 masl), Umbric Alisols in the Upper-Dega (3000-3200 masl) and Hyperdystric Umbrisols in the High-Wurch (>3700 masl) agro-climatic belt, were the dominant soil groups. In inference, the results of the pedogenetic assessment denoted that the trend in pedogenesis and the spatial distribution of the soil groups was generally in concurrence with the traditional agro-climatic classification used by the Ethiopian farmers in the E.2.03-type Rainfall Pattern Region of the Arsi-Highlands. However, this needs more detailed investigations that include the other Rainfall Pattern Regions of Arsi and of the country as well for more reliable conclusion.

任何特定土壤都是土壤形成因素综合作用的结果，这些因素以不同类型和程度表现，且可通过多种方式组合；因此，每种组合都会产生具有独特属性的不同土壤，导致地球上存在数千种不同的土壤。土壤发生学（soil genesis）为这种纷繁复杂的土壤多样性带来了秩序，有助于其分类。研究人员以1:50000比例尺地形图为底图，通过沿海拔和纬度样带的自由调查，对库鲁姆萨子流域的土壤进行了调查，并基于坡度、地形位置、植被、表土颜色、质地等可观测的立地和土壤特征，确定、描述并采样了19个代表性剖面点。通过发生学评估，将采样的19个单个土体（pedon）划分为10个参比土壤组（Reference Soil Group）；并分析了整体土壤发生过程和土壤组空间分布与海拔及传统农业气候带的关系。土壤发生学评估显示，风化和淋溶程度随海拔升高而增加。据此，在“裂谷”及其边缘陡坡区域的下沃伊纳德加（Lower-Woina-Dega，1800-2100 masl）识别出淋溶最少、碱性最强的栗钙土（Kastanozem），在奇拉洛山（Mount Chillallo）高乌尔奇（High-Wurch，>3700 masl）农业气候带识别出淋溶最多、酸性最强的暗沃土（Umbrisol）。尽管存在pH值低至5.15的酸性土壤和强淋溶剖面，但从粘土矿物学角度评估时，化学风化过程似乎并不强烈。这些高海拔地区的酸性土壤剖面条件似乎不足以导致高活性粘土矿物的深度风化，因此它们仍主导着土壤的粘土组分。就传统农业气候带而言，从研究区最低海拔到最高海拔，各农业气候带的优势土壤组如下：下沃伊纳德加的栗钙土（淋溶最少）、正沃伊纳德加（Proper-Woina-Dega，2100-2250 masl）的黑钙土（Chernozem）和淋溶土（Luvisol）、上沃伊纳德加（Upper-Woina-Dega，2250-2400 masl）至下德加（Lower-Dega，2400-2550 masl）的潜育土（Planosol）和变性土（Vertisol）、下德加的变性土和淋溶土、正德加（Dega-Proper，2550-3000 masl）的网纹土（Retisol）、上德加（Upper-Dega，3000-3200 masl）的暗沃淋溶土（Umbric Alisol）以及高乌尔奇（>3700 masl）农业气候带的高淋溶暗沃土（Hyperdystric Umbrisol）。综上，发生学评估结果表明，土壤发生趋势和土壤组空间分布总体上与埃塞俄比亚农民在阿尔西高原（Arsi-Highlands）E.2.03型降雨模式区使用的传统农业气候分类一致。然而，这需要更详细的调查，包括阿尔西及全国其他降雨模式区，以得出更可靠的结论。