Open access publications in the SLU publication database

Abstract

The plant-available pools of calcium, magnesium and potassium are assumed to be stored in the soil
as exchangeable cations adsorbed on the cation exchange complex. In numerous forest ecosystems,
despite very low plant-available pools, elevated forest productivities are sustained. We hypothesize
that trees access nutrient sources in the soil that are currently unaccounted by conventional soil
analysis methods. We carried out an isotopic dilution assay to quantify the plant-available pools
of calcium, magnesium and potassium and trace the soil phases that support these pools in 143
individual soil samples covering 3 climatic zones and 5 different soil types. For 81%, 87% and 90%
of the soil samples (respectively for Ca, Mg and K), the plant-available pools measured by isotopic
dilution were greater than the conventional exchangeable pool. This additional pool is most likely
supported by secondary non-crystalline mineral phases in interaction with soil organic matter and
represents in many cases (respectively 43%, 27% and 47% of the soil samples) a substantial amount of
plant-available nutrient cations (50% greater than the conventional exchangeable pools) that is likely
to play an essential role in the biogeochemical functioning of forest ecosystems, in particular when the
resources of Ca, Mg and K are low.