PHYSIOGRAPHIC FACTORS 183 



In natural soil, however, these artificial modifications are absent, 

 and, consequently, in view of the possible implications in inter- 

 preting associations of species or the causes of succession, it is sur- 

 prising that the subject has not been given more attention. 



Alkalinity.— Soils with an alkaline reaction have usually orig- 

 inated from limestone, dolomite, or marble in which calcium car- 

 bonate is the basic mineral. The CaCOa tends to neutralize acids 

 that appear in the soil, and the degree of alkalinity is proportional 

 to the solubility of the limestone. Dolomite contains more MgCOs 

 than CaC03, and gypsum is largely CaSCh, but the soils they form 

 contain CaC03, and their floras are essentially similar to that of 

 limestone. In our arid West, soils are often alkaline in reaction be- 

 cause of the sodium ions, which accumulate as sodium hydroxide 

 (NaOH). 



Neutral or alkaline soils favor the activities of most soil organ- 

 isms and the availability of nutrients for higher plants. At the same 

 time, the tendency of soil colloids to aggregate and produce crumb 

 structure in the presence of lime results in soil structure with 

 water, air, and temperature conditions favorable to plant growth. 

 Thus most cultivated crops do best on soils with a pH ranging 

 close to neutrality. Native plants, in general, respond similarly, but 

 there are exceptions, which require, on the one hand, high concen- 

 trations of CaC03 or, on the other, extremely acid conditions re- 

 gardless of other factors. 



Not all species found growing in calcareous soils are calciphiles. 

 The distribution and occurrence of many show no correlation 

 with alkalinity of the soil. A considerable number of these widely 

 distributed species may, however, grow more luxuriantly when on 

 calcareous soil. Some, although not restricted to the habitat, will 

 be found there characteristically. These are true calciphiles. There 

 are, in addition, obligate calciphiles, which grow only in cal- 

 careous habitats. 



The exceptional vigor on calcareous soils of otherwise wide- 

 spread species may result simply from the improved aeration, 

 moisture, or nutrient conditions produced by lime. Calciphiles 

 may grow on other than alkaline soils if competition from non- 

 calciphiles is not too great. The less favorable are the general con- 

 ditions for growth, the more the calciphiles are restricted to their 



