Soils differ in their ability to supply the nutrients necessary to sustain forest productivity. Nutrients are added through natural processes such as weathering of primary and secondary soil minerals, mineralization of soil organic matter including the forest floor layer, fixation of nitrogen primarily through symbiotic microorganisms, and natural or induced atmospheric deposition. Nutrients become unavailable for plant uptake through immobilization by soil microorganisms and through chemical and mineralogical reactions including precipitation and adsorption reactions and ionic fixation within lattice structures of clay minerals. Losses of nutrients can take place through soil leaching and erosional processes. Nutrients can also be added or lost through human activities such as fertilization and harvesting. Nutrient supply continually shifts with the rate and direction dependent on the prevailing processes in the soil system, but subject to overriding human influence. Over relatively short periods of time, the soil nutrient supply can be subject to seasonal fluctuations. Factors affecting long-term nutrient availability are functions of soil mineralogy, the rate of mineralization of the organic matter of the soil and forest floor layer, and plant-soil relationships of the species occupying the site (deciduous vs. coniferous species, deep vs. shallow rooting, symbiotic nitrogen fixation). The long-term stability of the soil nutrient supply is of increasing concern in the face of a diminished forest land base, increased demand for forest products, and reluctance to apply nutrients to many forest areas because of environmental or economic constraints. There are questions to consider in evaluating the nutrient sustainability of forest areas if we expect to maintain the long-term nutrient stability of natural and managed forest ecosystems.
