SOIL ORGANISMS 



249 



ically. These materials may be characterized by their common origin 

 and by their similar state of dispersion. 



All humus materials are colloid complexes; all of them have certain 

 properties which are based upon their colloidal nature (see p. 161). 

 Therefore from an ecological point of view one is justified in treating 

 them collectively. 



Humus substances absorb water with consequent swelHng; they 

 hold the water tenaciously. The absorbed water is removed only 

 partially by evaporation. This notable ability to retain water and the 



7J 20 



Depth 



Wcm 



Fig. 125. — Parallelism between decreasing pH, humus content, and hygroscopic 

 water (air dry) with soil depth in the climax Curvuletum at Schlern. (.4//er Braun- 

 Blanquet and Jenny, 192G.) 



loss of this ability with decrease of humus content in the soil are shown 

 in Fig. 125. 



The chemical transformations in humus obey the laws of surface 

 reactions and are considerably dependent upon the concentration of the 

 solutions. Humus substances precipitate electrically charged colloids, 

 such as ferric hydroxide, in a characteristic manner (Van Bemmelen, 

 cited in Ramann, 1911). Humus samples, treated with hydrochloric 

 acid and alkali, show a characteristic dark-brown coloration, due to 

 certain compounds (humic acid, hymatomelanic acid, and sulphonic 

 acids). 



Adsorptively Saturated and Unsaturated Humus.— Vv'ith respect to 

 vegetation, humus substances arc to be segregated into two qualita- 

 tively different groups: the neutral, mild, saturated; and the adsorp- 

 tively unsaturated humus. 



