roots when magnesium is present in excessive quantities. The 

 result of this imbalance may be stunted growth of plants due to 

 a combination of calcium deficiency and magnesium toxicity. By 

 addition of calcium salts, serpentine soils may be rendered rela- 

 tively amenable to agricultural use. 



2. Serpentine soils also frequently have a high nickel and 

 chromium content. These mineral elements are not only unneces- 

 sary for plant growth, but are toxic to plants even in small quan- 

 tities. 



3. Serpentine soils are low in the content of nutrients, such 



as nitrogen, that are required in relatively large quantities by most 

 plants for adequate growth. In addition, they are deficient in 

 many other nutrients, such as molybdenum, which are equally 

 essential to plants but are needed in smaller quantities. 



4. Serpentine soils often are waterlogged in the winter and 

 excessively dry in the summer season. The transitional periods 

 between being very wet and being very dry may be quite short, 

 with the result that a perennial plant growing on serpentine soil 

 must be able to tolerate very wet soil during some seasons of the 

 year and very dry soil during other seasons. Such extremes in 

 water content also may be characteristic of other soil types in 

 California, but few other soils offer as well the peculiar set of 

 characteristics of mineral composition. 



Serpentine soils have a very high proportion of endemic plant 

 species restricted to them. For example, Sargent Cypress (Cupres- 

 sus sargentii, Cupressaceae) is found only on serpentine deposits 

 in the Coast Ranges from Mendocino County southward to 

 Santa Barbara County. It is therefore termed an obligate serpen- 

 tinophile. Many species of jewel flower {Streptanthus spp., 

 Cruciferae) likewise occur only on serpentine, and some of these 

 species are known only from a single serpentine outcrop. These 

 species, too, are obligate serpentine endemics. There are all 

 gradations of fidelity to serpentine, ranging from 100 percent to 

 nil. Some species, such as Leather Oak (Quercus durata; 

 Fagaceae) of the North Coast Ranges, commonly are found on 

 serpentine, but populations also occur on volcanic soils in Napa 



27 



