Measuring Plant-Available Mg, Ca, and K Pools in the Soil—An Isotopic Dilution Assay

In many forest ecosystems, plant-available pools of Mg, Ca, and K are assumed to be stored in the soil as exchangeable cations adsorbed on the cation exchange complex (exchangeable pools). However, between soil minerals and exchangeable cations exists a gradient of Mg, Ca, and K storage forms that have not been fully characterized and may play an important role in plant nutrition and biogeochemical cycles. We hypothesize that sources of Mg, Ca, and K in the soil other than the conventionally measured exchangeable pools are plant-available on very short time scales (<1 day). In the present study, we developed and applied an isotopic dilution technique using the stable isotopes 26Mg, 44Ca, and 41K to trace and quantify the pools of Mg, Ca, and K (isotopically exchangeable pools) in the soil of a hardwood forest that contribute directly to equilibrium processes between the soil water and the soil. We characterize the equilibrium between the soil and soil solution using both a batch approach and a flow-throug...