MANURES. 



exactly in proportion to the amount of carbonic acid 

 which it contains. We should, therefore, strive t- 

 have as much carbonic acid as possible in the v 

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

 admit to the soil the largest possible quantity of at- 

 mospheric air, which contains this I 



The condition of soil necessary lor this, is the Minn- 

 as is required for the deposit of ammonia by 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 rusting some of the constit- 

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

 plants. 



2. It unites with the j>rofoxide of iron, and 

 changes it to the peroxide. 



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

 sour 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, 



