Extractant ph and the release of phenolic compounds from soils,plant roots and leaf litter

The influence of pH was examined, over the range from 6 to 14, on the amounts of p-hydroxybenzoic, vanillic, p-coumaric, ferulic and syringic acids, p-hydroxybenzaldehyde and vanillin, extracted from four soils and associated roots or leaf litter. Adjustment of pH was obtained by the addition of graded amounts of Ca(OH)₂ to water or by 2 m NaOH. The roots associated with three of the soils were from permanent pasture, perennial ryegrass and red clover, while the leaf litter associated with the fourth soil was from beech.The amounts of each phenolic compound extracted increased continuously with increasing pH, from a “threshold” value which varied between pH 7.5 and 10.5. The amounts extracted by water alone from the soil under permanent pasture, at pH 5.8, were equivalent to concentrations in the soil solution ranging from 1.4 μm for p-hydroxybenzoic acid to < 10 nm for ferulie acid. Amounts of up to 2000 times greater than these were extracted by 2 m NaOH. Similar effects of extractant pH were found with the other soils.Comparisons of the amounts of the phenolic compounds extracted from the soils, with the amounts extracted from the associated roots or leaf litter, suggested that substantial proportions of the soil phenolic compounds were either derived from organic residues more than 4 yr old or were the result of microbial synthesis.