G2 PRINCIPLES OF PLANT CULTURE 



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 importance, 

 where large crops are expected. Stable and barn-yard 

 manure, the offal from slaughter-houses, tanneries, 

 breweries, and so on, 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, it renders the soil 

 porous and greatly increases its water-holding power. 



93. Soil ventilation. —(The soil needs ventilation. 

 The roots of growing plants and the decomposition of 

 organic matter 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 interchange between the contents of the soil cavities 

 and the atmosphere above, the roots sooner or later be- 

 come 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 thoroughly 

 tilled, the roots of plants often suffer from insufficient 

 oxygen. A puddled crust on the surface of clayey soil, 

 due to the compacting influence of rain, is a great hin- 

 drance to its ventilation. Earthworms and other animals 

 that burrow in the soil aid in aerating it. 



94. Ventilation of hotbeds. — Hotbeds require especial 

 care in ventilation (365), since they usually contain large 

 quantities of decomposing organic matter (manure), 

 which rapidly absorbs oxygen from the soil, replacing it 

 with carbonic acid. 



