Take up a handful of soil, examine it and compare it with samples further up slope. If there are shell fragments in the soil it will be alkaline. Some plants prefer alkaline conditions like the Melkhout . Others, like Blombos , seem to be able to tolerate any conditions and will be found growing in both acid and alkaline soils. Alkaline soils are more fertile.
Drainage is another important characteristic.
In sandy soil drainage would be rapid. However clay soils may sometimes become waterlogged in winter. In these conditions there would be no air for plant roots. In some areas the water table may be at the surface during winter, creating waterlogged conditions and would drop a metre or two during summer. All these conditions will determine what kind of plant can grow there.