122 SOILS. 



Ulmin Substances; Sour Humus (Germ. Rohhumus). In 

 the presence of so much moisture or liquid water as will mater- 

 ially impede the access of air, and with the concurrence of 

 reasonably low temperatures, the organisms that at first take 

 the chief role in the transformation of the vegetable tissues into 

 humus-like substances are bacteria. But the antiseptic nature 

 of the compounds thus formed 1 soon puts an end to their ac- 

 tivity, and thereafter the process seems to be a purely chemical 

 one, and very slow. In peat bogs, the transition from the fresh, 

 dead stems and roots to brown peat is easily followed down- 

 ward, white cellulose fibers remaining apparently unchanged 

 to some depth ; so that such fiber has been used for tissues and 

 paper. The solid decomposition-products are brown substances, 

 partly soluble in water and imparting to it a brown or coffee 

 color (frequently seen in the drains of marshes) and an acid 

 reaction; the latter due to ulmic (as well as apocrenic) acid, 

 readily soluble in caustic and carbonated alkalies, and forming 

 insoluble salts with the earths and metals; while another por- 

 tion, ulmin, is insoluble in the same, but gradually becomes 

 soluble by oxidation. 



The gaseous products formed under these conditions are 

 carbonic dioxid and "marsh gas" (methan, CH 4 ), the 

 former predominating in the early stages; while later, the car- 

 buretted hydrogen predominates, rendering the gas readily in- 

 flammable. 



Sour Soils. The " sour " soils thus produced in nature in 

 presence of excess of water bear only " sour " growth, such 

 as sedges and rushes, of little agricultural value; they usually 

 require reclamation processes before becoming adapted to ordi- 

 nary crops. In old forests of northern climates a peaty and 

 more or less acid layer is sometimes formed on the surface, 

 above the black woods-earth, and retards somewhat the full 

 production of such land when taken into cultivation. 2 



Marshes and swamps, both fresh and salt, as above stated 

 usually show coffee-colored waters, which are also characteristic 

 of the streams that drain them, until by intermixture with 



1 The antiseptic properties of sour humus are well exemplified in the perfect 

 state of preservation in which the remains of animals, wood implements, etc., are 

 found in bogs into which they have sunk in prehistoric times. 



2 See Muller, Natiirliche Humusformen. 



