Placement of Plant Root Simulator (PRS^®) probes (ion-exchange membranes in a plastic support) may strongly influence nutrient supply measurements and their relationship to nutrient loss to overlying water due to gradients in ion activity and redox potential with depth. A laboratory study was conducted with two soils contrasting in potential nutrient loss (manured vs. unamended control) to determine the impact of probe placement (vertical, horizontal, and flat on the soil surface) on nutrient supply rate. The supply rates of the redox-sensitive nutrients Mn and Fe were generally 1–2 orders of magnitude lower for PRS probes placed on the soil surface than buried vertically. In contrast, the supply rate of P and K varied by 1–2 orders of magnitude between soils, but placement impacts were modest or absent. The ratio between manured and control soils in water P concentration was identical to that of soil P supply rate determined with PRS probes placed flat on the soil surface. All placements were effective in demonstrating the increased potential for loss of P and K from the manured soil, but only measurements from PRS probes placed on the soil surface were closely related to loss of the redox-sensitive nutrients Mn and Fe.