Atmospheric deposition of sulphate is the primary link between the atmosphere and acidification of soils and aquatic ecosystems1. The flux of sulphate to forest soils can be measured in the water that drips from trees following the interception of rain to form stemflow and throughfall (the sum of which is designated here as TF). Enrichment of sulphur in TF over that found in rain is widely reported2–4; sulphur sources include the wash-off of previously dry-deposited sulphate particles and SO2 and the leaching of internal plant sulphur from foliage (termed foliar leaching)5. To quantify foliar leaching, we labelled mature trees in the field with radiosulphur and measured atmospheric sulphur concentrations and fluxes. Here we report that dry deposition provides >85% of the enrichment of sulphate in the TF flux to soils below three different tree species at low-elevation sites in the southeastern United States. This supports evidence from several forests that total atmospheric deposition of sulphate is strongly reflected in the measured flux in TF.