Dry semi-natural grasslands are highly species-rich, and therefore often protected in cultural landscapes of Central Europe. However, reserves are typically small and may be subjected to edge effects. Here, we ask how grassland biodiversity changes from edges neighbouring arable fields into grassland interior. We compared plant diversity of protected grasslands adjacent to organically versus conventionally managed arable fields. We surveyed the vegetation along 50 m transects from the arable field border into the grasslands, and compared site characteristics via Ellenberg indicator values and measured soil parameters. Averaged linear mixed-effects models showed an increase of diversity with increasing distance from the adjacent field, from a mean observed richness of 25 species per 4 m²-plot at the border to more than 30 species in the interior. For organic sites, models consistently showed 13 species more per plot than for conventional sites. Diversity for the two land use types remained different over the entire transect length. Thus, the effect of adjacent field farming extends far into the interior of the protected grasslands, yet overall organic farming is less detrimental. We assume that this is related to a combination of differences in local farm management, such as pesticide application, and the wider landscape context associated with the farm type. Buffer zones with at least organic-like management could improve the conservation value of nature reserves.