Manganese, an essential nutrient critical for photosynthesis and a toxic element in excess, impacts forest metabolism, carbon storage, and ecosystem productivity. Given the significant role Mn can play, it is important to understand how soil geochemistry controls Mn uptake. We conducted a greenhouse pot experiment to quantify Mn uptake by plants based on controlled geochemical constraints. Specifically, we investigated whether Mn uptake was limited by the supply of Mn to soil solution or by biological controls within the plants. Tree pots containing soil-only or soil and red maple saplings were supplied with either dissolved Mn, Mn-oxides, or crushed shale containing Mn-bearing pyrite. We predict that Mn uptake would be higher in systems with dissolved Mn because it is not limited by mineral weathering, and that Mn uptake would be higher when the system is supplied with fast-weathering substrates (pyrite in the shale) than slow-weathering substrates (Mn-oxides). We analyzed the chemical composition of leaf tissue to quantify Mn uptake and soil leachate to quantify Mn losses. Leaf chemistry varied on orders of magnitude, with Mn uptake being the highest in the dissolved Mn treatment and lower in the Mn-oxide and shale treatments. Conductivity data indicates major solute loss in leached water from the dissolved Mn and shale treatment groups. The leached water from the shale group was extremely acidic, suggesting rapid dissolution of pyrite. Leachate chemistry indicates that Mn loss in the dissolved Mn treatment groups was two to 10 times higher than the other treatment groups. We conclude that vegetation stored Mn and reduced leaching rates in all treatments, and that Mn dissolution rates influenced plant uptake. Ongoing analyses include constructing mass balance models to quantify Mn uptake and leaching, microscale imaging techniques to examine root-soil associations and mineral transformation, and community analysis of Mn-oxidizing or reducing microorganisms.