Field-Extracted Spodosol Solutions and Soils: Aluminum, Organic Carbon, and pH Interrelationships

Using the syringe-pressure technique, we field-extracted soil solution from Spodosol horizon samples in a high-elevation forested watershed. Dissolved organic carbon (DOC) was negatively correlated with solution pH. Solution reactive Al (catechol-violet labile) was strongly correlated with total solution Al (graphite furnace), averaging 71% of the total. Except in extremely acidic, low-Al Oa horizons, both forms of solution Al were positively correlated with DOC. Exchangeable Al, measured in subsamples of the same horizons from which soil solutions were extracted, correlated with total and reactive solution Al. The Oa horizons that were relatively low in solution and exchangeable Al were also low in total, acid-digestible Al. Soil pH in water was generally higher than soil solution pH while soil pH in 2.5 mM CaCl 2 was always lower, consistent with the well-documented decrease in most soils' pH under increasing ionic strength (the salt effect). Organic C, Al, and H + are interrelated in these soils. These interrelationships support current hypotheses that organics control the concentration of solution Al.