208 WALL'S MANUAL 



carbon which plants contain, and which forms the 

 bases, as it were of their vegetable substance, is 

 derived from the air and not from the soil. Plants 

 can grow in soils absolutely devoid of vegetable 

 matter, provided the soils contains mineral ingredients 

 required by the plant, in a soluble state, or at least 

 in a condition available to rootlets of plant. 



This condition we do not often find fulfilled in- soils 

 in their natural state. It has been before stated that 

 only a part of the nutritive ingredients present in 

 the soil is usually available, the greater part being 

 ''locked up" in the undecomposed, or partially de- 

 composed minerals, and but very gradually set free 

 by the action of the atmosphere. This very action 

 is due chiefly to the carbonic acid contained in the 

 air, and will be the more powerful and rapid, the more 

 carbonic acid is present. Now, carbonic acid is con- 

 tinually formed in the process of decay of vegetable 

 matter ; hence the importance of this vegetable 

 matter in the soil. It is true that in some measure 

 it acts as an exhauster of the mineral ingredients in 

 the soil, and in that respect would be injurious if we 

 do not supply annually ashes, phosphate of lime, 

 soda, or stable and barn yard manures which contain 

 hem all. 



But the " hnmus " or vegetable mold has other 

 highly important offices in vegetable economy. One 

 of these its renjentiveness of moisture it soaks up 

 moisture Jike a sponge ; it serves as a correction for 

 light, sandy soils; while on the other hand it renders 

 clay soils less compact and stiff. Yet there is another 

 virtue possessed by "humus" more important than 

 either of the others its power of absorbing ammonia 



