Soil fertility and maize response to subsoil deep tillage and termite mound amendments in strongly weathered plinthic soils

Plinthite, an iron-rich and humus-poor clay horizon that hardens irreversibly, poses major constraints to agriculture in the Lubumbashi region. Termite mound materials, which are enriched in basic cations, combined with subsoiling to fragment and remove plinthite, may improve soil fertility. This study evaluated the combined effects of subsoiling and termite mound amendments on Plinthosols under maize across ten blocks covering 660 ha. Surface soils (0–10 cm) were sampled, soil profiles were described, and maize yields were measured over two growing seasons. Spatial patterns of soil properties and yield were mapped using the Spline With Barriers method, and stepwise regression was applied to identify key variables controlling yield. Soil thickness ranged from <9 cm in areas requiring secondary subsoiling to >73 cm in blocks B2, B5, B6, and B9. Soil pH (KCl) ranged from 4.1 to 7.8, while pH in water ranged from 4.9 to 8.7, with stronger acidity observed in blocks B8–B10. Total organic carbon (TOC) was generally low (0.4–2.5%). Nutrient contents were highly heterogeneous: P ranged from 5.1 to 145.5 mg kg−1; Ca from 1360 to 18,268 mg kg−1; K from 130.2 to 942.0 mg kg−1; and Mg from 238.8 to 2987 mg kg−1. Available Al (44–293 mg kg−1), Fe (28.1–351.7 mg kg−1), Mn (4.4–669 mg kg−1) and Cu (1.9–25.8 mg kg−1) also showed strong spatial variability. Bulk density decreased with depth, and although Ksat remained low, water retention was improved in the surface layer. Maize grain yield ranged from 2.3 to 11.1 t ha−1, with seasonal means of 7.1–8.2 t ha−1. Regression models identified soil pH, Ca, and TOC as the main positive determinants of maize yield, whereas high concentrations of Fe, Cu, and Mn were associated with reduced yields.