The Root and the Soil. 69 



of clpds, making the latter permeable to air and root- 

 lets, or flakes off new fragments of t rock, thus unlock- 

 ing new supplies of mineral fertility. 



92. The Importance of Organic Matter in the Soil. 

 Crops secure a large part of their nitrogen, as well as 

 of other food substances, from dead organic matter, 

 i e., animal or vegetable materials. The application 

 of such matter to the soil is, therefore, of great im- 

 portance, where large crops are expected. Stable and 

 barn-yard manure, the offal from slaughter-houses, tan- 

 neries, breweries, etc., are all valuable for this purpose, 

 when wisely used. Stable manure is further beneficial 

 by absorbing moisture, oxygen, ammonia and carbonic 

 acid from the air as well as much solar heat. Not only 

 does organic matter in the soil furnish plant food, but 

 while in a partially decomposed state (humus), it ren- 

 ders the soil porous and greatly increases its water- 

 holding power. 



93. The Soil Needs Ventilation. The roots of grow- 

 ing plants and the decomposition of organic maliter in 

 the soil tend constantly to exhaust the latter of its free 

 oxygen, and to replace this with carbonic acid, which 

 is not used by the roots. Hence, without some inter- 

 change between the contents of the soil cavities and the 

 atmosphere above, the roots sooner or later become 

 smothered and perish. In sufficiently porous soil, 

 changes in temperature and in atmospheric pressure, 

 aided by wind and rain, furnish the needed soil ventila- 

 tion, but in poorly-drained soils, and soils not thor- 

 oughly tilled, the roots of plants often suffer from in- 

 sufficient oxygen. A puddled crust on the surface of 



