Agronomic Performance of Enhanced Rock Weathering in a Tropical Smallholder System: A Maize Trial in Kenya

Enhanced Rock Weathering (ERW) is a promising negative emission technology that can simultaneously sequester atmospheric CO₂ and improve agricultural productivity. While its efficacy has been demonstrated in temperate climates, a critical knowledge gap exists regarding its application within the complex, low-input smallholder farming systems of sub-Saharan Africa. This study investigates the agronomic impacts of applying a locally sourced volcanic rock powder (nephelinite) on smallholder farms in Kisumu County, Kenya. The trial was conducted with 56 women smallholder maize farmers, with each farm divided into a control plot and a treatment plot. A single application of the rock powder (20 t/ha) was applied at the start of the trial. This resulted in an average maize yield increase of 71.17% in the first year compared to control plots (2159 kg/ha control, 3184 kg/ha treatment). This significant yield benefit was sustained and amplified in the second year, with the average increase growing to 79.24% without any further rock dust application (2540.5 kg/ha control, 4366.5 kg/ha treatment). Post-harvest soil analysis provided strong indication for weathering; while the net change in soil pH between treatment and control plots was not statistically significant, treated plots showed a positive trend toward pH improvement. A similar positive trend was also observed for the soil's cation exchange capacity. The impacts on micronutrients were complex and site-specific: the amendment acted as a direct source of iron in one cluster, increasing its availability by 14.8% (p < 0.05), while dynamics of manganese, zinc, and copper were primarily driven by environmental factors like seasonal flooding and crop uptake. While the soil data presented here is inconclusive regarding possible modes of action, the yield results clearly demonstrate ERW's ability to deliver substantial agronomic benefits in smallholder farming. Further research will be required to test its effects in a wider range of cropping systems and pedoclimatic conditions.