Effects of Low Ionic Strength Solutions on pH of Acid Forested Soils

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Abstract
“Salt effects” on soil pH are not well‐documented with highly acidic soils in contact with solutions of very low ionic strength (< 1 mMc/L). These dilute, acidic conditions are typical of soil solutions in many forest ecosystems. A wide range of acid forested soils were contacted with very dilute salt solutions to determine effects of low electrolyte concentrations on soil and solution pH under a variety of experimental conditions. In many soils, pH was lowered markedly following addition of dilute salt solutions. In fact, salt effects on pH (i.e., dpH) were greatest at low concentrations of added salts (e.g., between 0 and 0.6 mMc/L of added salt solutions). At least 40% of the variation in dpH among the 19 mineral soils was attributable to soil differences in low‐level concentrations of water soluble electrolytes. Leaching experiments that manipulated electrolytes in soil solution demonstrated that pH could be depressed up to 1 pH‐unit by increasing salt concentrations by as little as 0.3 mMc/L. Significant correlations between low ionic strength and dpH indicated that water‐soluble electrolytes influence pHw, even of unfertilized, acid soils that are very low in concentrations of soluble ions. Measuring pH of forest soils with 0.01 M CaCl2 is thus preferable to such measurements with water, to mask variations in soil pH that are caused by variations in relatively low concentrations of water‐soluble electrolytes. Results support the contention that variations in electrolyte concentrations in soil solutions on the order of 0.1 to 0.5 mMc/L (due to natural causes or “acid rain”) affect soil‐solution pH, although probably by relatively minor amounts in many or most soils.
Funding Information
  • Electric Power Research Inst. Partial
  • School of Natural Resources
  • Univ. of Michigan

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