BDAPHIC OR SOIL FACTORS: OHEMICAL Idl 



geographers into two hostile camps and gave a characteristic stamp to 

 the geobotany of the period. Much later it was recognized that both 

 theories have their justification, indeed, that they are frequently 

 complementary, but that neither the one nor the other in its original 

 form does justice to all the evidence recorded. A special chapter 

 would be required to deal with all the arguments which the followers of 

 the two theories advanced in support of the correctness of their views. 

 Famous names — Lecoq, Sendtner, Kerner von Marilaun, A. P. de 

 Candolle, Christ, Contejean, Fliche and others — are connected with 

 these discussions. Today the discussion is of historic interest only 

 although it certainly stimulated geobotanical research for several 

 decades. Advances in chemistry have made the original hypotheses 

 obsolete. 



The development of colloidal chemistry and the electrolytic dissoci- 

 ation theory of acidity gave a decisive turn to the soil question. With 

 the discussion of these new aspects of the subject we pass from the 

 older to the modern phases of soil science. 



A. Colloid-chemical Viewpoint 



Colloid or, better, dispersoid-chemistry, the link between physical 

 and chemical soil research, aims to discover the laws of the relation 

 between the degree of dispersion of substances and their physical and 

 chemical properties. 



According to Wiegner (1926), who was the first to base the investi- 

 gation of soils wholly upon dispersoid chemistry, the soil as a whole is 

 conceived to be a colloidal system. In this system the grains of the 

 soil are the dispersed phase; the soil atmosphere and soil water repre- 

 sent the dispersion medium. The dispersed phase shows an extremely 

 large surface and is the seat of a special form of energy (surface 

 energy). Upon this fact rests the common distinction between 

 light (coarsely divided) and heavy (finely divided) soils. According to 

 their degree of division, their dispersity, substances can be divided into 

 two large groups differing in their physical state and general properties : 

 crystalloidal and colloidal. 



Crystalloids are substances whose external, regular form stands in 

 definite relation to the special physical and chemical properties of the 

 substance. As the name indicates, they can be obtained in crystal 

 form. Furthermore, they are capable of passing through thin mem- 

 branes (parchment) out of a solution into pure water: they dialyze. 

 Colloids (Gr. colla, glue) are characterized by their amorphous con- 

 sistency, their great amount of surface and hence their surface 

 phenomena, their poor diffusion and low osmotic pressure, and the 



