164 



PLANT SOCIOLOGY 



ions displace the cations on adsorbed humus and clay colloids of the 

 rendzina soil according to the following graphic formula: 



Na+K+Ca^ 



H^ 



colloid clay Mg++ + mHCl 



Al+++Fe 



+++F0+++ 



jjH H H Hjj 



colloid clay| + 

 H H H H^ 



^NaCl; MgCla'j 

 /i^ni. TTori-V^ soil 



KCl; FeCl; 



(caCl 



2; AICI3) 



/solution 



The displaced, soluble cations are washed out by rain and melting 

 snow and migrate to greater depths, while the H ions, in spite of 



continuous additions of bases (from 

 decaying plants, drifting dust par- 

 ticles, animal remains), gather in 

 the upper horizon (Fig. 94). 



The exchange of bases here 

 effects a thorough change in physi- 

 cochemical soil properties, which, 

 as described elsewhere (p. 311), cer- 

 tainly reacts upon the vegetation. 

 Gola's Osmotic Theory. — The 

 first application of the results of 

 colloid-chemical research to geo- 

 botanical problems was attempted 

 by Giuseppe Gola, professor at 

 Padua. His work on this subject, 

 dating back to 1905, was sum- 

 marized in 1910 in the important 

 "Saggio di una teoria osmotica dell' edafismo. " 



According to Gola, the prime factor in the relation between the 

 plant and the soil is the osmotic concentration of the soil solutions 

 which come in contact with the root system. The osmotic pressure 

 of these solutions is conditioned by their degree of concentration. The 

 concentration of the dissolved salts in the soil is either stable (eustatic) 

 or rapidly changing (anastatic). The rapidly changing concentration 

 of anastatic solutions exerts upon many plants a particularly harmful 

 influence. 



According to their degree of concentration, soil solutions are divided 

 into slightly and highly concentrated solutions. The pecuHarities 

 of soils with shghtly concentrated salt solutions are due to their 

 colloidal nature. On the other hand, in the soils of highly concentrated 

 solutions the crystalloidal properties predominate. Gola therefore 

 divides soils into two great classes: 



Fig. 94. — Diagram showing hydrion 

 accumulation in the humus horizon 

 (.4), and the leaching of the products of 

 weathering from horizon {B) into the 

 subsoil (C). 



