The Importance of Soil Mineralogy to Plant Nutrient Availability in the Northeastern U.S.

In the northeastern U.S., acid deposition poses a threat to nutrient availability, via leaching of base cations (Ca, Mg, K, Na). While silicate mineral weathering may be the source of most base cations released from soils over millennia, more easily weathered minerals may provide nutrients necessary to meet short term demand resulting from acid deposition and forest harvesting. Through sequential leaching of soils and their parent materials, we can determine whether easily soluble trace minerals are potentially available to plants. Previous studies have shown that apatite, a calcium phosphate mineral present in trace amounts, may provide a significant source of Ca to vegetation at the Hubbard Brook Experimental Forest, New Hampshire. In this study, we explore the regional availability of apatite in soils across the northeastern United States. Soils derived from granitoid and sedimentary rocks were collected from 20 sites across the northeast U.S. and sequentially leached to determine relative availability of base cations. A leach using 1M nitric acid extracted Ca and P from soils developed on crystalline parent materials (0.02 to 0.04 mmol Ca/g soil, 0.01 to 0.03 mmol P/g soil). The Ca:P ratio is 5:3, the stoichiometric ratio of apatite. The presence of apatite in these soils was verified by SEM analysis. The lack of K, Na and Si in this leach suggests that silicate mineral dissolution is not the source of Ca. The Ca and P concentrations indicate that amount of apatite varies in granitoid soil parent materials across the northeastern U.S. Sedimentary rock-derived soils did not contain appreciable amounts of apatite. With the exception of carbonate-derived soils, only small concentrations of Ca (<0.006 mmol/g soil) were leached from sedimentary rocks in 1M nitric acid.