Phosphorus Lateral Movement through Subsoil to Subsurface Tile Drains

Little research has focused on lateral P transport through subsoil to subsurface tile drains. We investigated P lateral movement through 3.0 m of an Iowa subsoil (1.6 g kg−1 Olsen P [OP], 157 g kg−1 clay) between two 1.07‐m deep field trenches and a 1.0‐m deep tile line. Well water was added to the trenches, and once the inflow rate equaled tile water outflow, a solution containing 8.7 mg L−1 P and 98 mg L−1 Br– tracer was added and maintained at a 0.6‐m depth for 25 d. Most Br– tracer reached the tile, as the highest concentrations were 83 to 97 mg L−1 Br–, but little P reached the tile since the highest concentrations were 0.035 to 0.13 mg L−1 P. After draining the trenches, we measured subsoil OP and Bray‐P1 (BP) from 0.60‐ to 0.75‐, 0.75‐ to 0.90‐, 0.90‐ to 1.05‐, and 1.05‐ to 1.20‐m depths of 22 vertical cores located between each trench and the tile line, and five 5‐cm sections of three horizontal cores (0–0.25 m from the trench wall) taken from a 0.69‐m depth. Subsoil P saturation was measured in selected samples. Subsoil P concentration and P saturation increased the most near the trench walls and decreased asymptotically to background levels at a 1‐ to 1.5‐m distance from the trench wall. For instance, at a depth of 0.60 to 1.20 m, OP was 33 mg kg−1 and decreased to 2 mg kg−1. A typical P‐deficient Iowa subsoil can limit the lateral movement of P from concentrated solutions toward tile drains, although continued P application can increase subsoil P levels and decrease P sorption.