CHAPTER VI 

 EDAPHIC OR SOIL FACTORS: CHEMICAL 



While man must accept the cHmate and surface contours of the 

 earth as they are and must confess himself almost powerless to alter 

 them, it is within his power to influence the conditions of the soil 

 considerably, transforming them to fit his needs. Soil problems, there- 

 fore, have engaged not only research workers in botany but also 

 farmers, foresters, and gardeners. Indeed, there is no other field where 

 so many different practical and purely theoretical interests coincide. 

 The soil presents a tangled complex of factors. The individual factors 

 are constantly interacting and in their reactions they may more or 

 less displace or even eliminate one another. 



The soil problem, therefore, cannot be resolved into simple formu- 

 lae, as was formerly supposed. In the search for such formulae the 

 soil experts of the nineteenth century have spent a vast amount of 

 effort and labor with relatively little results. 



Chemical Soil Theory.— A. P. de Candolle, of Geneva, refused to 

 admit that the chemical composition of the soil had any considerable 

 influence upon the occurrence and distribution of plants. He ascribed 

 prime importance to the presence or absence of organic substances in 

 the soil (1832, p. 1245). linger, however, placed great emphasis on the 

 dependence of plants upon the chemical nature of the soil. The North 

 Tirolean Alps revealed to him the sharp contrasts which exist between 

 the silicicolous flora of the interior mountain chains and the calcico- 

 lous flora of the outer ranges. In his famous prize essay "Uber den 

 Einfluss des Bodens auf die Verteilung der Gewachse" (Vienna, 1836) 

 on the influence of the soil upon the distribution of plants, he demon- 

 strated that, besides those plants which thrive in any variety of soil, 

 there are many others which are strictly confined to certain kinds of 

 soil. On this basis he distinguished between plants which are, with 

 respect to soil, constant, preferential, and indifferent. 



The constant and preferential species he considered chiefly con- 

 ditioned by the chemistry of the soil and divided these "soil indicators" 

 into two large groups: lime plants and clay-slate or silicicolous plants. 

 For both groups he cited numerous examples. Shortly afterward 

 (1838) Ruehle published an extensive index of indifferent and soil- 



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