Soil Structural Degradation Under Different Mechanised Tillage Systems in Smallholder Agriculture Across Three Nigerian Agroecological Zones: An Original Assessment Framework

This dissertation develops an original multi-indicator soil structural degradation assessment framework and applies it to evaluate soil structural change under contrasting mechanised tillage systems across three Nigerian agroecological zones: Guinea savanna (North Central), Derived savanna (South West), and Humid forest (South South). The increasing penetration of tractor-based mechanisation into Nigerian smallholder farming is occurring without systematic evaluation of its soil structural impacts, creating a growing but poorly documented soil compaction problem that threatens the long-term productivity of mechanised smallholder plots. This study establishes 12 paired experimental plots comparing conventional disc ploughing, chisel ploughing, strip tillage, and no-till across the three zones, monitored over four cropping seasons. Soil structural quality is characterised by a comprehensive framework encompassing compaction metrics (bulk density, cone penetration resistance), structural stability (water stable aggregates, macro-aggregate hierarchy), pore architecture (computed tomography at 68 micrometre resolution), gas exchange capacity (air permeability, diffusivity), and biological quality (earthworm abundance, microbial biomass). Zone-specific threshold values for structural quality indicators are established from undisturbed reference profiles. The dissertation proposes the Nigerian Tillage Structural Quality Framework (NTSQF) as an original assessment contribution, integrating the 12 structural indicators into a composite index sensitive to tillage management differences. Key findings demonstrate that conventional disc ploughing produces critical compaction below 20 cm in all three zones within two seasons, while strip tillage maintains acceptable structural quality. Zone-specific tillage recommendations are derived from NTSQF results. Keywords: soil structural degradation, mechanised tillage, assessment framework, agroecological zones, Nigeria.