SOIL AND SUBSOIL. 



125 



creasing to the extent of two to eight times; so that humus 

 stands foremost in this respect among the soil constituents. 

 The density of natural humus is about 1.4, being the lightest 

 of the soil constituents. Hence soils rich in humus are " light " 

 not only in the farmer's sense of being easily tilled when not 

 too wet, but also of light weight for equal volumes when com- 

 pared with clayey and sandy soils. Some data bearing upon 

 these points are given in the table l below, for the substances 

 moderately and uniformly packed : 



VOLUME-WEIGHTS OF 



Humus. 2 Clay. Quartz Sand. 



.3349 1.0108 14485 



When saturated with water, the same substances gave the 

 following figures : 



Air-dry. Saturated Increase, 



with water. % 



Humus 2 3565 1.1024 20 9- 2 



Clay 1-0395 1.6268 55.9 



Quartz sand.. 1.4508 1.8270 25.9 



These data show strikingly the effects produced by the sev- 

 eral physical soil constituents upon some of its physical prop- 

 erties. 



Chemical Nature. While humus artificially produced by the 

 action of caustic alkalies upon sugar or cellulose is free from 

 nitrogen, all naturally occurring humus contains the latter. 



It is not, however, present in the form of ammonia, as it cannot be 

 set free by treatment in the cold with lime or alkalies. When, how- 

 ever, natural humus is boiled with these substances, ammonia is slowly 

 given off, but the process continues indefinitely and it seems to be im- 

 possible to expel all the nitrogen in this manner. This behavior being 

 characteristic of amido-compounds, it is presumable, in view of the 

 slightly acid nature of the humus substances, that natural humus is 

 largely of an amidic constitution. Artificial humic acid, formed by the 



1 Wollny, Zersetzung der Organischen Stoffe, pp. 242,243. 



2 Peat pulverized and extracted with alcohol and ether to remove resinous sub- 

 stances. 



