Relationships Among Iron, Aluminum, Carbon, and Sulfate in a Variety of Forest Soils

Among several soil properties tested, percent Fe c (i.e., Fe by citrate‐dithionite minus oxalate extraction) was the single parameter most closely related to SO 2‐ 4 adsorption properties in a variety of forest soils. There were exceptions to this general relationship, however, and a combination of percent C, citrate‐dithionite, and oxalate extractions for both Fe and Al appear most promising in predicting sulfate adsorption. Percent clay, pH, and pyrophosphate‐extractable Fe + Al were either insignificantly or inconsistently related to SO 2‐ 4 adsorption. Because organic matter had a decidedly negative influence upon SO 2‐ 4 adsorption, surface soils and B horizons of Spodosols (and highly podzolized soils) had relatively poor SO 2‐ 4 adsorption properties, even when their dithionite‐extractable Fe values were high. Organic matter also reduced Fe crystallinity (i.e., increased Fe o /Fe d , or the ratio of oxalate to dithionite Fe), and the results of this study suggest that crystalline rather than amorphous, inorganic Fe (i.e., oxalate minus pyrophosphate Fe) is most highly correlated with adsorbed, water‐insoluble SO 2‐ 4 .