Ratios of vanillic acid to vanillin [acid to aldehyde ratio, (Ad/Al)v] in latest Permian and earliest Triassic organic matter assemblages from the Vigo Meano section (southern Alps, Italy), providing proxy evidence for pulses of soil acidification (pH <4). For calibration of (Ad/Al)v values to pH values, see the Data Repository (Fig. DR1 [see footnote 1]). VG—Val Gardena Formation; PTB—approximate position of the Permian-Triassic boundary; extinctions—interval of principal marine extinction and floral turnover in southern Alps; δ13C—position of end-Permian negative carbon-isotope shift in southern Alps. Letters at bottom of stratigraphic column, from left to right, correspond to clay, silt, fine sand, medium sand, and coarse sand, respectively.