Soil chemical properties show limited improvements under smallholder farmer-managed conservation agriculture systems in Northeastern Namibia

Conservation agriculture (CA) has been extensively promoted to improve soil fertility and resilience; however, its effectiveness in smallholder farming systems is uncertain. This study assessed the medium-term effects of farmer-managed CA tillage systems on selected soil chemical properties in the Kavango East and Zambezi regions of Namibia. A factorial experimental design was applied, consisting of three tillage systems (conservation agriculture (CA)-based system, conventional-based tillage (Con) system, and natural/ undisturbed (Nat) system) and five soil depths (0–10 cm to 50–100 cm taken at 10 cm intervals). The results revealed minor differences between conventional based tillage (Con) and conservation agriculture (CA). In Kavango East, CA recorded a slightly higher pH (7.60–7.67) than Con (7.24–7.17) and lower electrical conductivity (EC) (114.73–120.44 µS/cm vs. 208.70–379.25 µS/cm) (p < 0.05). In Zambezi, the pH and EC values were not significantly different between CA and Con. Soil organic carbon (SOC) under Con was higher in Kavango East, with an average of 0.62%, than CA, with an average of 0. 39% while in Zambezi, Con had an average of 0. 33% slightly exceeded CA with an average of 0.31% (p < 0.05). Olsen extractable Phosphorus (P) in Kavango East was higher under CA in Year 1 (2.84 mg/kg) but con surpassed it in Year 2 (2.26 mg/kg vs. 2.14 mg/kg) (P = 0.0497). In Zambezi, the P content was 2.77 mg/kg under Con and 3.91 mg/kg under CA. Ammonium (NH₄⁺) was greater under Con in Kavango East (17.56 and 20.60 mg/kg) than CA (12.01 and 22.09 mg/kg), and in Zambezi, Con (14.31 and 20.60 mg/kg) slightly exceeded that of CA (14.57 and 19.43 mg/kg) (P = 0,028). Potassium (K) and sodium (Na) were higher under CA in Kavango East (K: 5.90 − 7.33 mg/kg; Na: 2.06–3.52 mg/kg) but were greater under Con in Zambezi (K:10.00 mg/kg). Kavango (CA) had the lowest average soil quality index of 0.396, compared to Con with the highest average SQI of 0.502. In Zambezi, conservation CA had the highest average SQI (0.548), Con and Nat had nearly similar indices (0.391; 0.389). Overall, these findings suggest that CA under farmer-managed systems does not result in significantly different soil chemical properties compared to Con, and nutrient responses to CA are site-specific. Farmers are encouraged to implement all three principles of CA, including residue retention and rotation, to enhance soil fertility and sustain long-term productivity under semi-arid conditions, such as those in the present study.