A granular, calcareous seaweed soil product, “Mermin”, was applied to an established grass sward at rates of 317 and 633 kg ha−1annually for four years from October 1983 and compared with hydrated lime at the same rates and an untreated control. Soil pH rose in response to both lime and “Mermin” treatments and “Mermin” significantly increased magnesium and phosphorus levels in the soil. Earthworms were sampled on five separate occasions from March 1984 to March 1987. There was no evidence of a substantial effect of either lime or “Mermin” on worm numbers or biomass. Silage cut in May 1987 indicated that the four years of treatment had had a similarly slight effect on herbage yields. There appears to be no evidence supporting claims that “Mermin” causes increases in earthworm numbers.