Medium-Term Effects of Conservation Agriculture on Soil Physical Properties in Smallholder Farmer-Managed Sandy Soils of North-Eastern Namibia

Conservation agriculture (CA) is widely promoted in sub-Saharan Africa as a means of improving soil health, yet evidence from farmer-managed systems remains limited, particularly for coarse-textured soils. We evaluated soil physical properties after at least five years of farmer-managed CA in Kavango East and Zambezi, Namibia, using adjacent conventionally tilled (CON) and natural-vegetation (NAT) reference fields. Measurements during the 2023/2024 and 2024/2025 cropping seasons included penetration resistance (PR), bulk density (BD), soil texture, and mean weight diameter (MWD) as an index of aggregate stability. The soils were predominantly sandy (>85% sand and <11% clay). PR increased with depth and differed significantly by region and land-use system, with the highest values under NAT. BD differed between regions (p = 0.0171) but not among land-use systems and remained below 1.65 g cm−3. Aggregate stability showed the clearest land-use effect: mean MWD was 0.384 mm under both CA and CON, compared with 0.722 mm under NAT. Thus, after at least five years, farmer-managed CA did not produce detectable improvements in the measured soil physical properties relative to conventional tillage. The limited response may reflect the strongly sandy texture and partial implementation of CA principles, particularly limited residue retention and crop diversification. These findings highlight the importance of evaluating CA under real-world smallholder conditions and suggest that longer-term management with greater organic inputs may be required before physical improvements become detectable in these soils.