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Data from 199 plots in the semi-arid Karoo showed that relationships between soil infiltrability and plant cover/species richness, as depicted by boundary lines, yielded ecological insights not evident if only commonly measured soil properties such as pH, electrical conductivity and the content of clay, silt, sand, nitrogen and carbon were considered. For example, the common grass Stipagrostis obtusa, herb Lepidium africanum and shrub Pentzia incana showed potentially maximal cover at high, low and intermediate infiltrability, respectively (r (2) > 0.65 for boundary lines derived by segmented quantile regression), but did not show distinct boundary lines for sand, silt or clay content data (r (2) < 0.5). Potentially maximal species richness was revealed under soil conditions of low infiltrability, high nitrogen content and low pH. Distinct boundary lines suggested that the drivers of species richness at any particular point in the Karoo landscape may operate in opposing directions simultaneously.