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The relationship between plant diversity and productivity has been hotly debated over the last two decades. What makes the relationship complex is, in part, the interplay of several biotic and abiotic effects, which include rainfall variability, soil characteristics and grazing. We investigated the influence of grazing intensity on the diversity-productivity relationship in a wide range of soil pH along a rainfall gradient in semi-arid grassland. Vegetation and soil surveys were conducted in high grazing (HG) and low grazing (LG) grasslands around Bethlehem (716 mm mean annual rainfall, moderately acidic soil), Bloemfontein (543 mm, slightly acidic/neutral soil) and Kimberley (467 mm, neutral/slightly alkaline soil) in South Africa. Plant species occurring in the study area were recorded, and aboveground biomass was estimated by clippings. Soils sampled were analysed for chemical properties. Species richness increased with increasing biomass and decreasing pH in Kimberley and Bloemfontein and subsequently declined with increasing biomass and decreasing pH in Bethlehem. The relationship between species richness and biomass was hump-shaped across the study area but when we differentiated between the degrees of grazing, we found linear positive relationships at both LG and HG sites. This suggests that the diversity-productivity relationship needs to be carefully examined in grazing land.