SOIL CHARACTERIZATION AND POTASSIUM NUTRITION FOR WHEAT (Triticum aestivumL.) PRODUCTION ALONG THE TOPOSEQUENCE OF QENBERENAWETI SUB-WATERSHED, CENTRAL HIGHLANDS OF ETHIOPIA

Abstract

Topography critically affects soil characteristics through its influence on pedogenesis, nutrient distribution, and crop productivity. Ethiopian agricultural production is constrained by soil degradation (erosion and fertility depletion) as well as by poor fertilization practices (suboptimal and unbalanced) and management (non-integrated and non-sustainable). Despite the nationwide decline in soil potassium (K) reserves (32 kg ha⁻¹ yr⁻¹), its use remains neglected due to a historical assumption of soil sufficiency that led to K deficiency, reducing wheat (Triticum aestivum L.) yield far below 5 tons ha⁻¹ in the highlands. This study addresses critical gaps in soil fertility management and K dynamics inEthiopia's highland agriculture, focusing on the Qenberenaweti sub-watershed in the North Shewa Zone of the Amhara Region. The study area exemplifies these challenges: it lacks detailed soil characterization at local topographic scales, underutilizes cattle dung ash (a viable K-rich amendment), and relies on blanket K recommendations (50 kg K ha⁻¹) without considering soil and crop-specific factors. Cattle manure ash, though abundant as a byproduct of fuel use, is discarded by ignoring its potential to improve CEC, reclaim acidity, and supply nutrients (K, Ca, Mg). Therefore, this study was initiated with principal aims to characterize, classify, and map soils along the toposequence; to assess K forms (labile vs. non-labile), adsorption-desorption kinetics, and equilibrium dynamics; to determine optimum externaland internal K requirements of wheat across soil types; as well as to evaluate K-containing organic (cattle manure ash, CMA) and inorganic (muriate of potash, MOP) amendments on soil chemical properties and productivity of wheat. The soil sampling followed a topographic approach throughout 317 ha of the watershed (2,808–2,960 m.a.s.l). The research methodology included: the World Reference Base (WRB) system of soil characterization and classification, K fractionation into its diverse forms; mathematical models in the K adsorption-desorption experiment; pot trial to establish critical K concentration for optimum internal and external requirements of wheat; and incubation trials comparing CMA and MOP (0–100% substitution) on soil properties and wheat responses. The findings revealed that topographic positions and slope features have directly shaped the extent of soil variability along the toposequence by determining water movement (drainage and percolation). All the K forms showed significant correlations (p < 0.01) among themselves and with soil pH, clay content, and CEC, influencing their availability. The Freundlich isotherm model (qe = aCe b/a ) performed well for the K-adsorption behavior of the entire soils that are characterized by non-linear increases in the K equilibrium concentration (Ce) and adsorbed amount (qe) due to the rising initial K (Ci) levels. The Power function model best fitted the desorption kinetics, which occurred in three distinct release phases over time: rapid (within 12 hours), gradual decline (72-168 hours), and stabilization (after 288 hours). The addition of K nutrient significantly (P < 0.001) improved the growth and yield of wheat, with the quadratic plateau and linear regression models estimating the 2 optimum external and internal K requirements as 24.48-30.75 mg K L -1 and 1.19-1.30%, respectively. The combined application of CMA:MOP at 37.5 to 62.5% ratios achieved the maximum wheat yield, with better N, P, and K harvest indices, owing to their balanced effects on overall soil properties, including pH, OC, exchangeable bases, and CEC. In conclusion, this work articulated the need for site-specific agricultural planning that addresses the localized heterogeneity of soils along the toposequence to sustainably enhance their fertility and productivity. For instance, it is necessary to prioritize CMA in acidic and low-OC soils and KCl blended ratios in high-buffer soils. However, further research on formulating precise fertilization of other essential elements, alongside vital agronomic and soil management practices (develop split fertilization schedules, water requirements, and crop rotation strategies), is needed to maximize profitable wheatproduction at the experimental site.