Relationships between species richness per plant life form and the soil properties: infiltrability, clay plus silt content, salinity, and pH were investigated at eighteen study sites located in western South Africa and Namibia. Plant species data were categorized into five life forms: phanerophytes, chamaephytes, hemicryptophytes, therophytes, and geophytes. A boundary line approach was used to determine ranges of soil properties across which plant richness was constrained or potentially maximal. Ranges of soil properties at which richness was potentially maximal differed between life forms as follows: total plant richness (similar to 50-200 mm h(-1) infiltrability, similar to 5-20\% clay \& silt, <100 mS m(-1) EC and 6.5-8.5 pH); phanerophytes (similar to 200-500 mm h(-1), <5\%, <100 mS m(-1), 5.5-7.0); hemicryptophytes (similar to 100-250 mm h(-1), 5-18\%, <200 mS m(-1), 6.0-8.0), therophytes (similar to 50-250 mm h(-1), 5-10\%, >50 mS m(-1), 6.5-8.5), chamaephytes (<250 mm h(-1), 5-25\%, >50 mS m(-1), 7.0-8.0), and geophytes (<200 mm h(-1), 5-20\%, similar to 50-100 mS m(-1), 6.5-8.0). Soil infiltrability, clay plus silt content, EC and pH seem to be useful properties to consider within plant richness investigations. Depending on physiological adaptations of plants these properties may constrain species distribution. Causality cannot be demonstrated from the relationships; however they can lead to plausible hypotheses as to the ecological processes governing plant distribution patterns. (C) 2010 Elsevier Ltd. All rights reserved.