Conservation agriculture shows limited soil gains on Namibian smallholder farms

Conservation agriculture has been promoted across sub-Saharan Africa as a way to rebuild degraded soils, boost yields, and buffer smallholder farmers against a changing climate. Yet a new field study from northeastern Namibia suggests that, under real farming conditions, the promised chemical improvements in the soil may be far harder to achieve than controlled experiments have implied. Researchers from the University of Namibia examined farmer-managed plots in the Kavango East and Zambezi regions and found that soils under conservation agriculture differed only modestly from those under conventional tillage, with outcomes that swung dramatically depending on the region, the season, and the depth of the soil being measured.

The research, published in the journal Discover Agriculture, was designed to answer a persistent question in African agronomy: what do farmers actually get from conservation agriculture as they practice it, rather than as researchers deploy it on experimental stations? Conservation agriculture rests on three pillars: minimum soil disturbance, permanent soil cover through crop residues, and crop rotation or diversification. In theory, together these principles reduce erosion, conserve moisture, and slowly build organic matter and nutrient reserves. In the Namibian study sites, however, the implementation was incomplete. Crop residues were routinely removed to feed livestock, and rotation was rarely practiced in cereal-dominated systems, leaving reduced tillage and intercropping as the main distinguishing features of the conservation fields.

The team established a factorial experiment across the two regions, working with smallholder fields that had been managed under conservation agriculture for more than three years. Each participating farmer’s land was divided into adjacent one-hectare plots representing three systems: the conservation agriculture system, a conventional system based on annual mouldboard or disc ploughing with residue removal and monocropping, and an uncultivated natural reference area. Soil cores were taken with an auger at five depth intervals, from the top ten centimetres down to one metre, over two consecutive cropping seasons spanning 2023 to 2025. Samples were air-dried, sieved, and analysed in the laboratory for pH, electrical conductivity, soil organic carbon, Olsen-extractable phosphorus, ammonium, nitrate and nitrite, and the exchangeable cations calcium, magnesium, potassium, and sodium.

The analytical protocols were rigorous. pH and electrical conductivity were measured with a multiparameter meter after shaking soil in deionised water. Phosphorus was extracted with alkaline sodium bicarbonate using the Olsen method and quantified colorimetrically with the ascorbic acid technique at 880 nanometres. Organic carbon was determined by the classic Walkley–Black dichromate oxidation and titration, corrected with a recovery factor of 1.3. Cations were displaced with neutral ammonium acetate and measured by inductively coupled plasma spectrophotometry, while mineral nitrogen species were extracted with potassium sulphate and read by colorimetric assays. All results were then subjected to three-way analysis of variance to disentangle the effects of region, tillage system, and depth, along with their interactions.

The results revealed a patchy and often contradictory picture. In Kavango East, conservation agriculture plots were slightly more alkaline, with pH values between 7.60 and 7.67, compared with 7.17 to 7.24 under conventional tillage, and they maintained far lower electrical conductivity, ranging from about 115 to 120 microsiemens per centimetre against 209 to 379 under ploughing. But in the wetter Zambezi region, pH and conductivity were statistically indistinguishable between the two managed systems. Soil organic carbon told an equally sobering story. Conventional plots actually averaged higher organic carbon than conservation plots in both regions, at 0.62 percent versus 0.39 percent in Kavango East and 0.33 percent versus 0.31 percent in Zambezi, a significant difference that runs counter to the core expectation of conservation farming.

Nutrient dynamics were similarly ambivalent. Olsen phosphorus under conservation agriculture peaked at 2.84 milligrams per kilogram in Kavango East during the first year but fell behind conventional plots in the second, while in Zambezi the conservation fields held more phosphorus, 3.91 against 2.77 milligrams per kilogram. Ammonium levels were higher under conventional tillage in most comparisons, a pattern the authors attribute to reduced microbial immobilisation being outweighed by greater mineralisation under disturbed soil. Nitrate and nitrite showed no significant response to tillage at all, consistent with the extreme mobility of nitrate, which moves with water through the profile largely regardless of what happens at the surface. Potassium and sodium were higher under conservation agriculture in Kavango East but greater under conventional tillage in Zambezi, and calcium and magnesium displayed strong regional and depth-dependent fluctuations, including a striking spike to 398 milligrams per kilogram of calcium in uncultivated Zambezi soil in the second year.

To integrate these disparate measurements into a single verdict, the researchers built a soil quality index using principal component analysis. Electrical conductivity, calcium, and sodium emerged as the most informative indicators, together explaining nearly 99 percent of the variance. Because conductivity and sodium reflect salinity risk, they were scored with a less-is-better function, while calcium was scored as more-is-better. The resulting index ranked the regions and systems in unexpected ways. In Kavango East, conventional tillage achieved the highest average index at 0.502, ahead of conservation agriculture at 0.396, with natural land lowest at 0.301. In Zambezi the ranking reversed, with conservation agriculture leading at 0.548 while conventional and natural land trailed at 0.391 and 0.389 respectively. The authors caution that the conventional system’s advantage in Kavango may reflect short-term nutrient pulses from disturbance rather than genuine long-term sustainability.

The deeper lesson of the study is contextual. Kavango East is dominated by deep Kalahari sands classified as Arenosols, with low nutrient reserves, poor water-holding capacity, and high leaching potential, whereas Zambezi sits on more fertile Cambisols and Fluvisols enriched by floodplain alluvium. Rainfall also diverged sharply, with Zambezi receiving up to 676 millimetres in a season compared with 423 to 656 in Kavango East. These environmental gradients, the data show, frequently shaped soil chemistry more powerfully than tillage practice did. Significant region-by-tillage and region-by-depth interactions appeared for nearly every parameter, meaning the same management system produced different chemical signatures in different agroecological zones. Soil depth mattered too: organic carbon and phosphorus declined steeply with depth, and management-related improvements were largely confined to the surface layers.

The findings align with a broader and increasingly candid debate in African agricultural science. Studies in South Africa, Kenya, Zambia, India, and Brazil have reported genuine gains in organic carbon, phosphorus availability, and overall soil quality under conservation agriculture, but most of that evidence comes from on-station trials with complete residue retention, deliberate rotations, and careful agronomic management. Since conservation agriculture first arrived in Namibia in 2005 through the Conservation Tillage project, adoption has been driven by the country’s acute climate vulnerability, yet farmers often cannot retain residues in landscapes where crop leftovers are critical livestock fodder. The Namibian authors echo the well-known ‘heretics’ critique of conservation agriculture, arguing that partial implementation of a three-pronged system inevitably dilutes its effects, and that soil chemistry responds slowly, over years to decades, to changes in carbon inputs.

The authors conclude that farmer-managed conservation agriculture in northeastern Namibia does not yet deliver chemical soil benefits that clearly separate it from conventional practice, and that nutrient responses are strongly site-specific. Their prescription is not abandonment but completion: farmers should be supported to implement all three principles, including residue retention and rotation, so that reduced tillage is accompanied by the ground cover and diversification needed to accumulate organic matter in semi-arid conditions. They also call for strengthened farmer-led implementation, training, and residue management strategies, noting that minimum tillage alone cannot guarantee success. For the millions of smallholders betting their futures on conservation agriculture across dryland Africa, the message is that the practice remains promising but unproven on their own fields, and that the gap between the textbook version and the farm-gate version of the technology is where its benefits are currently being lost.

Subject of Research: Effects of farmer-managed conservation agriculture on soil chemical properties in northeastern Namibia

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

Article References: Aipanda, E., Petrus, A., Ikechukwu, M. K., Handura, B., Siyambango-Mulisa, N., & Mupambwa, H. A. (2026). Soil chemical properties show limited improvements under smallholder farmer-managed conservation agriculture systems in Northeastern Namibia. Discover Agriculture, 4(1), Article 286. https://doi.org/10.1007/s44279-026-00759-1

Image Credits: AI Generated

DOI: 10.1007/s44279-026-00759-1

Keywords: conservation agriculture, soil chemical properties, smallholder farmers, Namibia, soil organic carbon, soil quality index, tillage systems, semi-arid agriculture, Kavango East, Zambezi region, phosphorus availability, soil fertility