Soils and Vegetation 



The properties of soils are primarily influenced by their min- 

 eral content, the amount of organic matter present, abundance 

 of water, and degree of aeration. Some other features of soils 

 that reflect the presence and nature of these properties are the 

 soil texture, the proportion of mineral nutrients present, the 

 availability of nutrients, heat-exchange capacity, presence of salts, 

 level of the water table, moisture-retaining capacity, presence 

 of micro-organisms, and so on. The complex interactions among 

 these various factors influence the overall nature of the soil and 

 this in turn, has an important effect on the plant cover present 

 in that soil type. 



In addition to the factors listed above, the nature of the 

 parent rock from which the soil has been derived is also import- 

 ant. In Cahfornia, there are large expanses of granitic rock, ba- 

 salts, serpentine, and even limestone. The nature of the parent 

 material has a strong influence on the type of soil that is ulti- 

 mately derived from these rocks. Climate, too, has an important 

 effect in determining soil types. In very wet climates, the soil 

 that results from the weathering of limestone may be quite dif- 

 ferent from that which develops in an arid climate. Topography 

 is also important; deep soils may never form in steep areas. 



One soil property that has an important effect on plants is 

 degree of salinity. In many areas of the deserts of California and 

 even in the Central Valley, there are tracts of land that are some- 

 times called alkali flats. In such areas, the soil contains high 

 amounts of various salts of potassium and/or sodium, as well as 

 of other soluble minerals. When these soils are dry, the minerals 

 ("alkali") may leave a whitish or greyish crust on the soil sur- 

 face. In small depressions or other low areas where water collects 

 during the rainy seasons, the salt concentration may become so 

 high that growth of most flowering plants is inhibited. The higher 

 the salt concentration, the fewer the plant species that can tol- 

 erate the soil. After a certain concentration has been reached, 

 no vascular plants can grow, resulting in patches of soil devoid 

 of vegetation. These barren areas are sometimes called "alkali 

 scalds" (Plate 3D). In the Central Valley they may be only a few 



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