Identifying the bioavailability of soil P pools and quantifying the amount of potential P uptake is important for defining long-term ecosystem productivity and understanding P-acquiring mechanisms of plants. My research investigates how red maple (Acer rubum) and loblolly pine (Pinus teada L.) take up P from soil clay (60-100 cm) and saprolite (450-500 cm) of the Calhoun Critical Zone Observatory in the Piedmont of South Carolina USA. Locally sourced seeds were germinated and grown in pairs of maple, pine, or maple and pine. Seedlings were fertilized with a P-free fertilizer mix twice per week to induce P-limitation and grown for one year. I measured total P in plants relative to changes in soil P pools defined by Mehlich P, Hedley P, or Total P fractions, and indexed the extent of Fe reduction as a potential mechanism of P release. I found resin Pi and NaOH Pi contributed most to plant growth, and stable P pools (concentrated HCl P and residual P) buffer losses from these P pools.