Environmental context Calcium silicate was added to a forest watershed in New Hampshire, USA, to accelerate its recovery from acid rain. The acid–base status of soil and stream quality improved over the 12-year study, with the most pronounced response in the upper elevation and the upper soil of the watershed. A total of 95% of the added calcium and 87% of the added silica were retained in the watershed over the study period. Abstract In October 1999, 3450kgha–1 of wollastonite (CaSiO3) was applied to Watershed 1 at the Hubbard Brook Experimental Forest in New Hampshire, USA, with the objective of restoring calcium that had been depleted from soil-exchange sites by chronic inputs of acid deposition. After the treatment, the concentrations and fluxes of calcium and dissolved silica significantly increased in both soil solution and stream water throughout Watershed 1, as did the acid-neutralising capacity. The concentrations and fluxes of inorganic monomeric aluminium significantly decreased. The treatment improved the acid–base status and decreased the potential for aluminium toxicity in stream water, especially in the lower reaches of the watershed. Approximately 4.7% of the added calcium and 17% of the added silica from the wollastonite treatment was exported from Watershed 1 in stream water by the end of 2010. Meanwhile, ~1825mmolm–2 of the added calcium and 2125mmolm–2 of the added silica were either transported to lower mineral soil horizons – as particulate wollastonite, or as dissolved solutes (calcium 77.6mmolm–2; silica 592.2mmolm–2), thus contributing to increases in soil pools – or were taken up by vegetation and incorporated into internal calcium and silica cycles of the watershed ecosystem. This experimental wollastonite addition was an effective tool for mitigating the acidification of the ecosystem and restoring the calcium status and forest health of this base-poor watershed.