36 NOURISHMENT OF PLANTS. 



this means the carbon they contain is converted into 

 carbonic acid. We call the substance into which 

 such organic matters are changed during decompo- 

 sition, vegetable mould or humus, when, as speedily 

 happens, it has assumed a dark color. Humus, 

 when air and moisture can act upon it, is, by grad- 

 ual but unceasing alteration in its constitution, still 

 further decomposed, and therefore continually fur- 

 nishes fresh supplies of carbonic acid to the roots of 

 plants as nutriment. At the same time, also, the 

 azotized and mineral substances which it contains 

 become soluble, often by the aid of this carbonic 

 acid, and capable of being received as food by plants, 

 and are thus in like manner appropriated to their 

 nourishment. The farmer is, therefore, quite correct 

 in attributing to humus an especially beneficial influ- 

 ence upon the growth of plants, and is consequently 

 laboring with all his energies to render his land rich 

 in humus. By its means principally, he makes the 

 soil at once looser, warmer, and better suited to the 

 absorption and distribution of moisture, as well as 

 richer in the power of attracting the nutritive mate- 

 rials existing in the air, as will be more precisely 

 shown in the chapter upon Soil. The farmer, how- 

 ever, must not suppose that this enrichment of the 

 land in humus can be achieved only by directly in- 

 troducing into the ground in large quantities such 

 substances (for example, straw-manure) as have es- 

 pecially the power to produce humus. This end can 

 also be indirectly attained, and frequently with far 



