Field-capacity water extracts from serpentine soils

Soils derived from serpentinites and other ultramafic rocks have long been called ‘serpentine soils’ by biologists1. They often bear a distinctive vegetation with poor cover or rare species or both and have attracted botanical interest for centuries2,3. The chemical causes of the unusual vegetation have often been investigated using soil analyses of exchangeable and total quantities of ions or elements. These have shown that serpentine soils are rich in magnesium and have relatively high concentrations of nickel but low ones of calcium and other nutrients. Although these factors have been judged to be important in many serpentine soils, there have been few analyses of their soil solutions4–6, which would give a better assessment of likely soil toxicities and deficiencies. Moreover the soil solution can be effectively simulated in water–culture experiments to investigate directly the effect of each possible factor on plant growth. We report here (Table 1) the results of soil-solution analyses for a range of Scottish and Zimbabwean serpentine soils which bear unusual vegetation. These analyses have substantially altered previous conclusions concerning the chemical causes of serpentine vegetation7.