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-P-1 (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.