Layered basal phosphorus application combined with drip fertigation enhances cotton water productivity and yield by optimizing soil phosphorus distribution

Promising phosphorus management is critical for improving water and nutrient use in drip-irrigated cotton. This study evaluated layered phosphorus placement under different basal-to-topdressing ratios and placement depths. A two-year field experiment was conducted in 2021–2022. Three basal-to-topdressing ratios (P25: 25:75, P50: 50:50, P75: 75:25) and three placement depths (L1: 5 + 15 cm, L2: 15 + 25 cm, L3: 5 + 15 + 25 cm) were compared with conventional uniform application (CK). We hypothesized that optimized basal-to-topdressing ratios combined with layered placement would improve soil phosphorus distribution, which could stimulate root growth in phosphorus-rich zones and enhance crop phosphorus and water uptake. Results showed that compared with CK, optimized layered placement significantly increased soil available phosphorus (8.72–72.76%) and the phosphorus activation coefficient (13.41–71.25%) in the 20–40 cm soil layer. Root growth significantly increased by 17.89–95.69% at this depth. Seed cotton yield significantly increased by 5.35–18.61%. Water productivity significantly improved by 3.45–12.74%, and phosphorus use efficiency by 11.99–50.60%. Among all treatments, P50L2 provided the most consistent and largest benefits. These results indicate that layered phosphorus placement with a 50:50 basal-to-topdressing ratio at 15 and 25 cm depths aligns soil phosphorus availability with root distribution. This enhances nutrient and water acquisition and improves cotton productivity. Overall, layered phosphorus placement offers a practical strategy for sustainable phosphorus management in arid cotton-growing regions. • Layered phosphorus (p) placement optimized the vertical distribution of soil phosphorus. • Phosphorus placement at appropriate depths increased soil P availability in the 20–40 cm layer. • Improved P availability promoted root growth and enhanced water and P uptake. • Optimized P placement increased water productivity, P use efficiency, and seed cotton yield.