EDAPIIIC OR SOIL FACTORS: CHEMICAL 165 



1. Geloid soils, characterized by low salt content, slightly concentrated soliv 

 tions, with colloid properties dominant; dissolved crystalloids less than 0.5 part 

 per 1,000. 



2. Haloid soils, marked by rich salt content, highly concentrated solutions 

 with more than 0.5 part per 1,000 of dissolved crystalloids; the concentration, 

 as a rule, rapidly changing, i.e., anastatic. 



Plants and plant communities are divided into two major groups 

 according to the following system : 



1. Plants of geloid soils: 



(I. Pergelicole species. Growing in soils which contain less than 0.2 part per 

 1,000 of dissolved crystalloids; saprophytes, epiphytes, humus plants. 



b. Gelicole species. In soils with from 0.2 to 0.5 part per 1,000 of dissolved 

 crystalloids; including the species which avoid lime, characteristic of most of 

 the raw silicate soils. 



2. Plants of haloid soils: 



c. Halicole species. In soils with dissolved crystalloids from 0.5 to 2 parts per 

 1,000; plants of soils rich in lime, and denizens of fence rows and roadsides. 



d. Perhalicole species. Growing in soils which have more than 2 parts per 

 1,000 of dissolved crystalloids; nitrophilous ruderal plants, species of the 

 saline soils of the seaboard and of arid lands. 



This classification of soils according to the concentration of nutritive 

 solutions suggests a division according to dispersion and acid content. 

 The geloid soils are more or less identical with those which are acid 

 and highly dispersed; the haloid, with the basic and less dispersed. 



B. Soil Acidity 



The more modern doctrine of acidity rests upon the electrolytic 

 dissociation theory of the Swedish investigator Svante Arrhenius. 

 The words of Arrhenius are: "Entirely independent of an electric 

 current every electrolyte in solution is wholly or partially split into 

 electrically charged atoms or groups of atoms, which are called ions. " 



To this must be added the following considerations: Pure water, 

 H2O, will conduct an electric current to a very slight degree only, and 

 the fluid gas hydrochloric acid, HCl, insulates completely. It might 

 therefore be expected that pure H2O and HCl mixed together would be 

 equally non-conductive. Strangely enough, however, this mixture 

 proves to be a good conductor of electricity. The HCl gas in aqueous 

 solution really enters into a new state or condition. The solvent, H2O, 

 splits the neutral molecules into their electrically charged positive and 

 negative ions; i.e., it effects their dissociation. The fraction of a salt 

 which is divided into ions is called the "dissociation gradient" and is 

 measured in the percentage of ionization. The ions, as the smallest 



