MANURES. 173 



exactly in proportion to tlie amount of carbonic acid 

 which it contains. We slionld, therefore, strive to 

 have as much carbonic acid as possible in the water 

 in the soil. One way, in which to elfect this, is to 

 admit to the soil the largest possible quantity of at' 

 mospheric air, which contains this gas. 



The condition of soil necessary for this, is the same 

 as is required for the deposit of ammonia b}' the same 

 circulation of air. 



OXYGEN. 



Oxygen^ though not taken up by plants as food 

 in its pure form, may justly be classed among ma- 

 nures, if we consider its effects both chemical and 

 mechanical in the soil. 



1. By oxidizing or rxisting some of the constit- 

 uents of the soil, it prepares them for the uses of 

 plants. 



2. It unites with the protoxide of iron, and 

 changes it to the j^t-roxide. 



3. If there are acids in the soil, which make it 

 60ur and unfertile, it may be opened to the circula- 

 tion of the air, and the oxygen will prepare some of 

 the mineral matters contained in the soil to unite 

 with the acids and neutralize them. 



4. Oxygen combines with the carbon of organic 

 matters in the soil, and causes them to decay. The 

 combination produces carbonic acid. 



5. It undoubtedly affects in some way the matter 

 which is thrown out from the roots of plants. This, 



