SOIL AIR AND SOIL TEMPERATURE 147 



and as a means of promoting in the plant the processes 

 necessary to its growth. Roots of most crops must have 

 access to a supply of oxygen. 



(2) Decomposition of plant residues and other organic 

 matter in soils requires the presence of oxygen, and without 

 decomposition these materials would accumulate in the soil 

 to the exclusion of higher plant life. Decomposition is 

 also of use in the production of carbon dioxide, the function 

 of which will be discussed later, and in the formation of 

 compounds of organic matter with mineral matter, decom- 

 position serves to increase the availability of mineral sub- 

 stances (see § 118). 



(3) The process by which the nitrogen of organic matter is 

 converted into nitrates can proceed only in the presence of 

 oxygen. 



189. Nitrogen. — Although not so essential as oxygen, 

 there is at least one important service that is rendered by 

 the nitrogen of soil air. This is to furnish the nitrogen-fixing 

 organisms with a supply on which they may draw to produce 

 the nitrogenous compounds that become incorporated in 

 leguminous plants, or that are formed directly in the soil by 

 the free-living nitrogen fixers. 



190. Carbon dioxide. — The principal service that carbon 

 dioxide renders is in acting as a solvent for the mineral matter 

 of the soil. For this purpose it is itself first dissolved in 

 soil water, in which condition it is a weak acid, but although 

 weak, its universal presence and constant action make 

 it an effective solvent. It dissolves from the soil more or 

 less of all the nutrient substance's required by plants in dis- 

 tinctly greater quantities than does pure water. 



A number of experiments in which carbon dioxide was 

 artificially brought in contact with soil on which plants were 

 growing have resulted in producing larger crop yields than 

 were obtained from soil not so treated. It cannot be con- 



