CHAPTER X 



SOIL AIR AND SOIL TEMPERATURE 



The volume of soil air depends on the volume of pore space 

 that is not filled with water. It is, therefore, evident that 

 ordinarily the non-capillary or larger spaces are the ones 

 that contain air. It will be remembered that the most im- 

 portant conditions that favor a large pore space in soils are : 

 (1) granular structure, (2) presence of organic matter. In 

 any soil the pore space may change from time to time with 

 the structure and the application of organic matter. 



179. Soil air contained largely in non-capillary spaces. — 

 The removal of water allows more space to be filled with 

 air. Immediately after a heavy rain much of the pore space 

 of the surface soil is filled with water. After this has had 

 time to drain away only the capillary spaces remain filled, 

 but capillary water is lost much more slowly. It is the non- 

 capillary pore space that, during the greater part of the time, 

 constitutes the air space of the soil. As a compact condition 

 of soil results in smaller pore spaces and consequently in 

 more capillary spaces, it causes a decrease in the volume of 

 air. 



180. There may be too much or too little soil air. — Soil 

 air is a necessary constituent of a productive soil, as will be 

 explained later, but it is not always the case that the more 

 air space in a soil the better it is for crop production. Very 

 large air spaces, like those found in a cloddy soil, allow the soil 

 to dry out too readily. Up to a certain limit a good supply 



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