34 ' THE SOILS AND CHOPS OF THE FAKM. 



Tegetable Matter in ISoils. — Plants of a low 

 order, as lichens and mosses, are able to live on rocks 

 the surface of which has been very slightly acted on 

 by the air and moisture. These plants have the pow- 

 er, probably through the agency of the acid sap, of 

 partly or wholly burying their roots in the rock, cling- 

 ing so closely to it that it is sometimes necessary to 

 chip off pieces of the rock to secure specimens of the 

 plants. Minute furrows are often left in the rock sur- 

 face after the plants have decayed. The decay of 

 these plants adds a trifle of vegetable matter to the 

 mineral elements of the soil, thus fitting for a more 

 abundant growth or for plants of a higher order. At 

 last there is a considerable supply of humus or decayed 

 vegetable matter mingled with the mineral matter. 

 The plants have aided in the work of disintegrating 

 the rock in several ways, both while living and when 

 decaying, and the humus they leave in the soil is a 

 most important element of fertility, being a chief source 

 of the nitrogen of the soil. 



The decayed and decaying vegetable matter in soils 

 is often spoken of as "organic matter," and the same 

 term is applied to aninaal matter in the soil. 



Chemical Composition of Noils. — A soil cap- 

 able of sustaining plants must contain all the min- 

 eral, ash, or incombustible substances found in plants, 

 for the plants can obtain these from no other source. 

 It will also contain alumina. Such a soil will also 

 always contain, in some form, sometimes in several 

 combinations, each of the elements which pass into 

 the air when a plant is burned ; but as the air is the 

 source from which the soil obtains these substances, 



