If soil sulfur is derived consistently from one source, the water-soluble and insoluble organic S fractions acquire similar isotopic compositions. In the case that there are two or more sources and/or if the isotopic composition of atmospheric or groundwater sulfate fluctuates, there may not be sufficient time for isotopic homogenization among the various forms of sulfur. The primary form of sulfur in soil is sulfate, which is transported upwards through the root system with minimal δS fractionation by 1–2‰. In contrast to higher canopy plants reflecting atmospheric δS, protected understory plants tend to reflect soil sulfur.