Soil mineral nitrogen concentrations (NO₃⁻–N and NH₄⁺–N) and physicochemical properties (pH, soil organic carbon, total nitrogen, C:N ratio, gravimetric soil moisture) measured at multiple depths and 17 sampling time points across the full measurement period in smallholder maize and potato plots in Uasin Gishu County, Kenya. Soil cores were collected from two points along the two inner rows per plot to 60 cm depth in 20 cm increments (0–20, 20–40, 40–60 cm) at 14, 21, 42, 56, 70, 95, 112, 127, 165, 195, 210, 224, 238, 287, 301, 315, and 329 days after planting (DAP). Fresh soil was extracted with 0.5 M K₂SO₄ (10 g soil: 100 mL solution, 1 hour agitation); NO₃⁻–N and NH₄⁺–N concentrations were determined by UV spectrophotometry (BIOBASE BK-D590) at 419 nm and 655 nm, respectively. Gravimetric soil moisture was determined by oven-drying at 105°C for 24 hours. Data span April 2024 to March 2025 at the University of Eldoret, Uasin Gishu County, Kenya (0°34′45″N, 35°18′22″E; 2,213 m a.s.l.) on Ferralsol soils. In a randomised complete block design with three replicates, five nitrogen fertilisation rates were applied to maize at 25, 50, 75, 100, 125 kg N ha⁻¹, two rates applied to potato at 50 and 100 kg N ha⁻¹ and unfertilised controls (0 kg N ha-1). This dataset is part of a multi-variable collection archived at PANGAEA.