OF AGRICULTURE. 13 



through their leaves. When vegetable matter in the 

 soil undergoes decay, this gas is freely generated, and, 

 being absorbed by the water in the soil, is conveyed 

 abundantly to the roots of growing plants. As rain 

 decends through the air, it absorbs a considerable 

 quantity of carbonic acid gas, and conveys it to the 

 soil. Thus, we find both the atmosphere and the soil 

 to contain carbonaceous food for plants. Whether 

 the carbonic acid is absorbed by the roots or the 

 leaves of the plant, it circulates through the plant in 

 solution in the sap ; and under the influence of light it 

 is decomposed, the plant retaining the carbon and 

 throwing off the oxygen through its leaves. This action 

 goes on more rapidly under the direct rays of the 

 sun. 



Hydrogen and oxygen are supplied to plants in the 

 form of water. All vegetable compounds contain 

 hydrogen and oxygen in the same proportion as they 

 unite in forming water. The leaves of plants can absorb 

 water from the air ; the roots always absorb it from 

 the soil. 



Nitrogen is not so abundant in plants as the other 

 three organic elements ; but it is no less important, 

 and even essential, to their growth. Ammonia is, no 

 doubt, the chief source from which plants get their 

 nitrogen. Nitric acid, in the nitrates, and other 

 nitrogen compounds, are doubtless sources from which 

 this important element is often derived. Ammonia 

 is the great source of nitrogen in the vegetable world. 

 Hence the great value of Peruvian guano. 



Ammonia is found both in the atmosphere and the 

 soil. From the atmosphere it is carried down by the 

 rain and snow. In the soil it is formed by the decay 



