148 SOILS AND FERTILIZERS 



eluded from this that an artificial supply of carbon dioxide 

 will always be beneficial, but it does indicate that carbon 

 dioxide assists in making the plant nutrients more available, 

 although in many soils the natural supply is sufficient for its 

 maximum effect. 



191. Control of the volume and movement of soil air. — 

 It will be gathered from the preceding paragraphs that a 

 good supply of air in soil with opportunity for its exchange 

 with atmospheric air is desirable for a number of reasons. 

 These conditions can be controlled by man to some extent. 

 In fact those operations that usually promote tilth serve at 

 the same time to effect a desirable condition of the soil with 

 respect to air. The operations by which man may control 

 soil air are as follows : 



1. Tillage of all kinds, when properly done and at the 

 right time, increases the volume of air in most soils by help- 

 ing to form the crumbly structure, and by disposing of 

 excess water. 



2. Both farm manure and lime cause an increase in the 

 carbon dioxide content of soil air, the former by contribut- 

 ing organic matter that finally decomposes, the latter by 

 hastening decomposition processes. 



3. Underdrainage by removing water from the pore 

 spaces increases the volume of air and causes its movement. 



4. Cropping produces channels through the soil where 

 roots have decayed, and these openings, on account of their 

 large number and ramifications through the soil, aid greatly 

 in increasing the volume of soil air. 



192. Soil temperature. — The temperature of the soil 

 may influence plant growth both directly and indirectly. 

 The direct effect is to be found in the plant itself, the roots 

 of which require a certain degree of heat before they begin 

 to function. A temperature somewhat above the freezing 

 point is necessary for this purpose, some common plants 



