250 PLANT SOCIOLOGY 



Adsorptively saturated humus is formed when aeration is good and 

 in the presence of electrolytes, especially calcium ions, which neutralize 

 the acid humus substances. The more lime in the soil the more humus 

 is neutralized. The mild, saturated humus is relatively coarsely 

 dispersed, crumbly, easily penetrated by plant roots, of dark brown to 

 black color. It absorbs large amounts of water, without changing into 

 a gelatinous form. Its favorable soil condition furthers the growth of 

 minute living organisms and thereby the production of carbonic acid 

 with its chain of consequences. This in turn leads, according to 

 Wiegner (1926, p. 31), to a decrease in dispersity. Coagulated, satu- 

 rated humus shows more or less neutral or weakly basic reaction. Its 

 buffering capacity varies greatly; it may be low (mull forms of the 

 Scandinavian forest soils) or very high (Caricetum firmae of the Alps). 

 Adsorptively saturated humus soils support economically good, highly 

 productive meadow and forest communities, such as the Caricetum 

 ferruginei of the Alps with its many luxuriant herbs, the Seslerieto- 

 Semperviretum, and notably the tall-herb communities of the 

 Adenostylion. 



Viewed simply as a soil type, as well as in respect to its effect upon 

 vegetation, the adsorptively unsaturated acid humus is very different. 



Unsaturated humus, as found characteristically in high moors and 

 in the low-shrub heath, is poor in electrolytes, always gives an acid 

 reaction, appears almost maximally dispersed, and, according to Wieg- 

 ner, maintains the mineral dispersoids in a state of high dispersion. 

 The interior surface is very great, involving a large water fixation; 

 hence very acid humus soils are physiologically dry. An outstanding 

 property of the highly dispersed sour humus is its marked ability to 

 prevent the coagulation of ferric hydroxide, aluminum hydroxide, and 

 silicic acid, in the presence of electrolytes (see p. 163). All communi- 

 ties with considerable quantities of Nardus, Deschampsia flexuosa, or 

 Vaccinium myrtillus, such as the Caricion curvulae of the Alps, the 

 Rhodoreto-Vaccinion, etc., are strictly confined to unsaturated humus 



soils. 



For climatic reasons the adsorptively unsaturated, acid soils are 

 limited to regions with low or moderate temperatures and moderate to 

 abundant precipitation where decomposition of humus proceeds slowly 

 (see p. 246). 



Varieties of Adsorptively Unsaturated Humus. — Acid humus occurs 

 in numerous forms, as yet not sharply defined. These forms are due to 

 external conditions, especially to the plant communities, which 

 initiated their formation. Only the more important of these forms 

 are mentioned here. 



