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54 Mwendwa Et Al., 2020 Tropical and Subtropical Agroecosystems 23 (2020): #54 Mwendwa et al., 2020 A GEOPEDOLOGICAL APPROACH TO SOIL CLASSIFICATION TO CHARACTERIZE SOILS OF UPPER KABETE CAMPUS FIELD, UNIVERSITY OF NAIROBI, KENYA† [UN ENFOQUE GEOPEDOLÓGICO DE LA CLASIFICACIÓN DEL SUELO PARA CARACTERIZAR LOS SUELOS DEL CAMPO DEL CAMPUS KABETE SUPERIOR, UNIVERSIDAD DE NAIROBI, KENIA] S.M. Mwendwa*, J.P. Mbuvi, G. Kironchi and C.K.K. Gachene Department of Land Resource Management and Agricultural Technology, College of Agriculture and Veterinary Sciences, University of Nairobi. P. O. Box 29053-00625 Kangemi, Nairobi, Kenya. Email: [email protected] *corresponding author SUMMARY Background. The study area exhibits a first class catena having homogenous parent material and forming a spatial continuum. Functionally and taxonomically distinct soils result from differences in drainage and lateral movement of materials in the soil. Objective. To classify the soils using a geopedological approach which involves a strong relationship between pedology and geomorphology. Methodology. The area was delineated into Soil Mapping Units (SMUs) through augering into soils defined by different macro-relief. Mapping units were demarcated according to slope categories namely 0 to 5%, 5 to 8%, 8 to 16%, 16 to 30% and >30% connoted as flat to gently undulating (AB), undulating (C), rolling (D), moderately steep (E) and steep (F) respectively. Profile pits were dug in the five identified mapping units using Stratified Random Sampling technique. Identified SMUs include UmIr/F, UmIr/E, UxIr/D, UxIr/C and UxIr/AB in the order of decreasing slope gradient. The first entry represents the physiographic unit (Uplands, U), followed by physiographic position (lower middle uplands, m or uplands, undifferentiated levels, x), geology (I), color (r) and slope class respectively. A soil map with a legend describing the mapping units was produced using a scale of 1:10000. Topographic influence on soil properties was presented by Pearson’s correlation coefficient (r) with p-value included where the influence was significant. Statistical analysis was done using IBM SPSS 25th edition and MS Excel. Results. All the mapping units are well drained and deep to very deep (>80 cm). The color of the upper B horizon is predominantly dark reddish brown. The texture of top horizon is clay in UmIr/F and UmIr/E and is clay loam to clay, sandy clay loam to clay and loam to clay loam in UxIr/D, UxIr/C and UxIr/AB respectively, lucidly exposing the influence of topography on the depth of clay illuviation (clay: r = 0.724; p ≤ 0.01). The structure is predominantly subangular blocky throughout the profiles with the top horizon of cultivated areas having predominantly granular structure. Saturated hydraulic conductivity (Ksat) generally decreases with increasing clay content down the profiles and the bulk density ranges from 0.9 to 1.2gcm-3. Means of soil reaction of top horizons generally slightly decrease with decreasing gradient (r = 0.231) having lower values in cultivated areas. Percent organic carbon regularly decreases down the profiles with higher values in uncultivated, steeper areas (r = 0.521; p ≤ 0.05). In the top horizon: Total nitrogen is predominantly medium across the study area ranging from 0.2 to 0.56% (r = 0.185) and follows the organic carbon trend; Available phosphorus is deficient (<20 ppm) in the study area. Bases are sufficiently to richly supplied while micronutrients are richly supplied. All soils are non-saline and non-sodic; the cation exchange capacity (CEC) soil is predominantly medium across the profiles ranging from 15 to 27.6 cmol(+)/kg with values increasing slightly with increasing slope (r = 0.320). Based on data collected from description of the profiles and physicochemical data of the soils and according to IUSS Working Group WRB (2014) soil classification legend, the soils were classified as Mollic Nitisols. Implications. The soils are generally fertile for crop production but organic manure is recommended to buffer the acidic soil reaction, improve nitrogen and phosphorus sources. Conclusion. Soil characterization, land evaluation and precise input application are encouraged. Keywords: First class catena; Soil Mapping Units; Stratified Random Sampling; Soil classification. RESUMEN Antecedentes. El área de estudio exhibe una catena de primera clase que tiene material parental homogéneo y forma un continuo espacial. Los suelos funcional y taxonómicamente distintos resultan de las diferencias en el drenaje y el movimiento lateral de los materiales en el suelo. Objetivo. Clasificar los suelos utilizando un enfoque geopedológico que implica una fuerte relación entre pedología y geomorfología. Metodología. El área se delineó en Unidades de † Submitted February 13, 2019 – Accepted March 20, 2020. This work is licensed under a CC-BY 4.0 International License. ISSN: 1870-0462. 1 Tropical and Subtropical Agroecosystems 23 (2020): #54 Mwendwa et al., 2020 Mapeo de Suelos (SMU) a través de la perforación en suelos definidos por diferentes macro-relieves. Las unidades de mapeo se demarcaron de acuerdo con las categorías de pendientes, a saber, del 0 al 5%, del 5 al 8%, del 8 al 16%, del 16 al 30% y> 30% connotadas como planas a onduladas suaves (AB), onduladas (C), onduladas (D), moderadamente empinada (E) y empinada (F) respectivamente. Se cavaron pozos de perfil en las cinco unidades de mapeo identificadas utilizando la técnica de muestreo aleatorio estratificado. Las SMU identificadas incluyen UmIr / F, UmIr / E, UxIr / D, UxIr / C y UxIr / AB en el orden de gradiente de pendiente decreciente. La primera entrada representa la unidad fisiográfica (Uplands, U), seguida de la posición fisiográfica (tierras altas medias bajas, m o tierras altas, niveles indiferenciados, x), geología (I), color (r) y clase de pendiente respectivamente. Se produjo un mapa de suelo con una leyenda que describe las unidades de mapeo utilizando una escala de 1: 10000. La influencia topográfica en las propiedades del suelo fue presentada por el coeficiente de correlación de Pearson (r) con el valor p incluido donde la influencia fue significativa. El análisis estadístico se realizó utilizando SPSS 25th edition y MS Excel. Resultados. Todas las unidades de mapeo están bien drenadas y de profundas a muy profundas (> 80 cm). El color del horizonte B superior es predominantemente marrón rojizo oscuro. La textura del horizonte superior es arcilla en UmIr / F y UmIr / E y es franco arcilloso a arcilloso, franco arcilloso arenoso a arcilloso y franco arcilloso a franco arcilloso en UxIr / D, UxIr / C y UxIr / AB respectivamente, exponiendo lúcidamente la influencia de topografía sobre la profundidad de la iluviación de arcilla (arcilla: r = 0.724; p ≤ 0.01). La estructura es predominantemente en bloques subangulares en todos los perfiles con el horizonte superior de las áreas cultivadas que tienen una estructura predominantemente granular. La conductividad hidráulica saturada (Ksat) generalmente disminuye al aumentar el contenido de arcilla en los perfiles y la densidad aparente oscila entre 0.9 y 1.2 gcm-3. Los medios de reacción del suelo de los horizontes superiores generalmente disminuyen ligeramente con el gradiente decreciente (r = 0.231) que tiene valores más bajos en las áreas cultivadas. El porcentaje de carbono orgánico disminuye regularmente en los perfiles con valores más altos en áreas no cultivadas y más empinadas (r = 0.521; p ≤ 0.05). En el horizonte superior: el nitrógeno total es predominantemente medio en el área de estudio que varía de 0.2 a 0.56% (r = 0.185) y sigue la tendencia del carbono orgánico; El fósforo disponible es deficiente (<20 ppm) en el área de estudio. Las bases se suministran de forma suficiente a abundante, mientras que los micronutrientes se suministran de forma abundante. Todos los suelos son no salinos y no sódicos; El suelo con capacidad de intercambio catiónico (CEC) es predominantemente medio en los perfiles que van desde 15 a 27.6 cmol (+) / kg con valores que aumentan ligeramente con el aumento de la pendiente (r = 0,320). Con base en los datos recopilados de la descripción de los perfiles y los datos fisicoquímicos de los suelos y de acuerdo con la leyenda de clasificación de suelos del Grupo de Trabajo IUSS WRB (2014), los suelos se clasificaron como Nitoles Mollic. Implicaciones. Los suelos son generalmente fértiles para la producción de cultivos, pero se recomienda el abono orgánico para amortiguar la reacción ácida del suelo, mejorar las fuentes de nitrógeno y fósforo. Conclusión. Se recomienda la caracterización del suelo, la evaluación de la tierra y la aplicación precisa de insumos. Palabras clave: catena de primera clase; Unidades de mapeo de suelos; Muestreo aleatorio estratificado; Clasificación de suelos. INTRODUCTION topography and time. Where all state factors are the same, the soil is homogeneous but when any of these Soil characterization helps to predict the behavior of factors change, the soil changes. This concept was different soils and present the results in a common echoed by Hartemink and Bockheim (2013). language understandable by scientists worldwide Topography can accelerate or retard the effect of (Brevik et al., 2016; Hartemink, 2015). This helps to climate on soils as it influences the chemical, relate the physicochemical properties of the soil to the morphological and physical characteristics of the soil, climate, landscape position, petrography, vegetation, same parent material notwithstanding (Esu et al., time and human influence
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