Effect of triple superphosphate (TSP) fertilizer rates on cowpea growth and yield under irrigation: a case of lowland area, Sokoto state, Nigeria

Abstract

Generic fertilizer recommendations often fail to reflect the spatial heterogeneity of soils and the varietal specificity of crop nutrient requirements, thereby limiting yield potential and nutrient-use efficiency. This study evaluated the response of two improved cowpea varieties to different rates of triple superphosphate (TSP) fertilizer under irrigated lowland conditions in order to develop site- and variety-specific phosphorus management recommendations. Field experiments were conducted during the 2025 dry season across three lowland environments using a split-plot arrangement within a randomized complete block design with three replications. Treatments consisted of different TSP Fertilizer rates (0, 20, 40, 60, and 80 kg ha?¹) applied as triple superphosphate. While uniform basal applications of nitrogen and potassium were supplied to isolate phosphorus effects at 20 kg N ha^-1 and 25 kg K?O ha^-1 respectively. Data on crop growth, yield components, grain yield, and soil properties were collected and analyzed. Results showed that varietal performance and yield response to phosphorus varied across locations. One variety (SAMPEA-18) consistently exhibited stronger vegetative growth and higher grain yield than the other (SAMPEA 20-T). Moderate phosphorus application (20–40 kg P ha?¹) generally produced the best yield performance, while both phosphorus deficiency and excessive application reduced productivity. SAMPEA 18 is the recommended variety for irrigated cowpea production at all P levels in this region, and a sustainable P management framework for this agroecological zone must integrate pre-season soil testing, in-season P budgeting calibrated to residual soil P, and multi-season evaluation of legacy P effects to maximize crop yield and economic return.