CHAE. V] THE SOIL 107 



phanerogams and ferns have utilized this property of lower plants, and are 

 thus able to absorb indirectly from humus both carbon and nitrogen ; in 

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

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



Of more general importance than carbon dioxide and nitrogen to most 

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

 centrated form and in a better state of mechanical subdivision than do the 

 purely mineral deeper layers of the soil. The w r ealth of humus in useful 

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

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

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

 state of division, and mix them with humus in their alimentary canals. If 

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

 thoroughly understand its beneficial effects on vegetation. However, as 

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

 form a good substratum for plant-life. 



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

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

 earths insoluble, dark-coloured compounds. Brown humus-substances are 

 comprehended under the collective name ulmic acid, and the blacker ones 

 under that of Jiumic acid. The neutral constituents of humus that are 

 insoluble in alkalis are termed huuius, if they are black, and humiu, if they 

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

 accumulation of acid compounds and hence the formation of acid humus, 

 which, in contrast with mild humus resulting from an abundant supply of 

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

 rich in species. 



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

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

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

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

 development in shady forests, where the earthworms pass it continually 

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

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

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

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

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



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

 contrast with mould, forms a cohesive compact crust, which lies superposed 

 on the mineral layers of the soil, without gradually passing over into them. 

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

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

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



