66 CHEMISTRY OF FARM PRACTICE 



the soil is proportional to the rapidity with which the 

 organic matter decomposes; for carbon dioxide is one of 

 the main products of the decay of organic matter. Decay 

 goes on most rapidly during the warm months of the year, 

 when the crop is being produced. At the time that soil 

 moisture will be most highly charged with carbon dioxide 

 and its solvent powers most increased, plant food is most 

 needed. It has been proved that the plant rests absorb 

 oxygen and give off carbon dioxide, which action has a 

 tendency to increase the solvent power of the soil moisture 

 in the immediate vicinity of the roots. 



The formation of carbonates by the reaction between 

 soil bases and carbonic acid may be beneficial to the soil, 

 as in the case of carbonate of lime, and, in moderate quan- 

 tities, carbonate of magnesium. On the other hand, large 

 quantities of the carbonates of sodium and of potassium 

 are deleterious, as is seen in the alkali lands of the West. 

 There is a tendency for carbonates of sodium and of potas- 

 sium to deflocculate the clay and, consequently, harm- 

 fully affect the tilth of the soil. 



55. Oxygen Must be Present. The oxygen of the air 

 is very important in its effects; the process of decay is in 

 reality oxidation. Some mineral compounds are oxidized, 

 and their solubilities changed. Most vegetable materials 

 are oxidized during the process of decay, the ash elements 

 contained being brought into solution so that they may 

 be made use of by growing plants, and carbon dioxide is 

 liberated, which, in turn, promotes the availability of 

 insoluble plant food from the mineral materials. Oxygen 

 is necessary for the germination of seeds, the growth of 

 plant roots, and in combination with carbon as COa for 

 the formation of the carbohydrates stored up in plant 

 structures. 



56. Factors Affecting Soil Air. The air content of the 

 soil is affected by several factors. In the first place, there 

 is an exchange of air between the air above the soil and 



