Mineral sources of Calcium and Phosphorus in soils of the Northeastern United States

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Apatite is ubiquitous in igneous, metamorphic, and sedimentary rocks, although usually in trace amounts. Apatite is the primary source of P and, due to its relatively rapid dissolution rate, can be an important Ca source in noncarbonate soils. We investigated the distribution of apatite using a 1 mol L–1 HNO3 extraction of glacial till soils at 31 sites across the northeastern United States. Parent materials formed from crystalline silicate and clastic sedimentary rocks contained 0.2 to 41 mmol Ca kg–1 soil and 1 to 27 mmol P kg–1 soil; at most of these sites, the P/Ca ratio was approximately 3:5, indicating the dominance of apatite. Parent materials underlain by carbonate rocks had 3 to 16 mmol P kg–1 soil, similar to the noncarbonate groups, but had large concentrations of easily weathered Ca (56–1890 mmol kg–1 soil) due to the extraction of carbonates. The same extraction procedure applied to samples from the upper 30 cm of mineral soil at the same sites showed lower concentrations of both Ca and P than in the C horizon, except for a few sites where the upper soil layers probably developed in different parent materials than the current C horizon. We also measured neutral-salt-exchangeable Ca concentrations in the 10- to 20-cm depth increment. Exchangeable Ca concentrations in soils underlain by carbonate bedrock were an order of magnitude higher than in soils underlain by crystalline silicate and clastic sedimentary rocks. For this reason, the exchangeable Ca concentration in the upper soil was correlated to the concentration of 1 mol L–1 HNO3 extractable Ca in the underlying soil parent material. To predict concentrations of apatite in parent materials and the concentrations of exchangeable Ca in overlying horizons in greater detail would require more specific characterization of bedrock sources.

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