:HAr. V] THE SOIL 107 



)hanerogams and ferns have utilized this property of lower plants, and are 

 hus able to absorb indirectlj- from humus both carbon and nitrogen ; in 

 feneral, however, these substances are taken, not from humus, but from 

 he carbon dioxide of the air and from the nitrates of the soil. 



Of more general importance than carbon dioxide and nitrogen to most 

 slants are the ash-constituents, which humus contains in a more con- 

 :entrated form antl in a better state of meclianical subdivision than do the 

 )urel}' mineral deeper layers of the soil. The wealth of humus in useful 

 ish-constituents depends partly on the amount of such matter in the 

 lecomposing plant-parts, and partly on the activity of earthworms, which 

 )ring up from below the constituents of the soil, reduce them to a fine 

 tate of division, and mi.x them with humus in their alimentary canals. If 

 the above properties of humus be added the power of absorption, we can 

 horoughly understand its beneficial effects on vegetation. However, as 

 >ill be shown later on, not all kinds of humus possess these properties, or 

 orm a good substratum for plant-life. 

 The organic constituents of humus are as yet imperfectly known. Some 

 ave an acid character, and form with alkalis soluble, and with alkaline 

 arths insoluble, dark-coloured compounds. Brown humus-substances arc 

 omprehended under the collective name ithiiic acid, and the blacker ones 

 nder that of Iiuinic acid. The neutral constituents of humus that are 

 isoluble in alkalis are termed /iimius, if they are black, and hiiuiin, if they 

 re brown. A deficient supply of oxygen favours the formation and 

 ccumulation of acid compounds and hence the formation of acid hiiuius, 

 hich, in contrast with mild hiiiiuis resulting from an abundant supply of 

 xygen, does not permit the development of a luxuriant vegetation that is 

 ch in species. 



Mild Inniius is usually loose in texture and is then termed mould. It is 



itimately mixed with mineral constituents, and by the gradual increase of 



lese it insensibly passes over into the purely mineral subsoil. Mould 



curs on moderately damp fresh soil only, and attains its most complete 



velopment in shady forests, where the earthworms pass it continually 



irough their alimentary canal and eject it in the form of separate pellets 



ose in texture. The mould in forests actually consists of worm- 



■ccrements, and its excellent qualities are due to this circumstance. The 



ch aeration of mould leads to the formation of highly oxidized neutral 



ibstances ; acids form only about a sixteenth of its organic substance. 



Acid hiiunis is usually matured in the form of peat. The latter, in 



)ntrast with mould, forms a cohesive compact crust, which lies superposed 



1 the mineral laj-ers of the soil, without gradually passing over into them. 



nly the abundant humous acids that are soluble in water penetrate the 



ineral soil and give to it a dark colour. In opposition to mould, which in 



iny weather rapidly becomes saturated, peat is only slightly permeable, so 



