Toposequence Effects on Selected Physical and Chemical Properties of Arable Soils in Ikono and Oruk Anam L. G. A, Akwa Ibom State, Nigeria

Abstract

Understanding the spatial variability of soils across landscape positions is critical for sustainable agricultural planning in the humid tropics. This study was conducted to assess the effect of toposequence on selected physical and chemical properties of arable soils in Mbiabong Ekpip (Ikono) and Obio Akpa (Oruk Anam) Local Government Areas, Akwa Ibom State. Two toposequences comprising crest, middle slope, and valley bottom positions were investigated. Six profile pits were excavated, and twenty-two composite soil samples were collected from genetic horizons for laboratory analysis using standard procedures. The results revealed that surface horizons were predominantly sandy loam, while subsoils were sandy clay loam, indicating clay illuviation. Sand content ranged from 58.0% to 81.8%, while clay content varied from 12.8% to 38.0%. Soil pH ranged from 4.6 to 5.0, reflecting strongly to moderately acidic conditions. Organic carbon (0.20% to 1.92%) and total nitrogen (0.028% to 0.168%) were low in surface horizons and valley bottom positions. Effective cation exchange capacity (ECEC) ranged from 5.54 to 8.37 cmolkg⁻¹, with base saturation between 56.99% and 79.68%. Slope position influenced clay content, organic carbon, total nitrogen, and ECEC (p \le 0.05). Positive correlations were observed between clay content and ECEC (r = 0.91** for Ikono; r = 0.88** for Obio Akpa, n = 22), and between organic carbon and total nitrogen. The results demonstrate strong topographic control on soil variability and highlight the need for slope-specific management strategies including liming, organic amendments, and erosion control to enhance sustainable agricultural productivity in the study area.