242 THE PRINCIPLES OF SOIL MANAGEMENT 



creases the available moisture capacity of fine-textured 

 soil. This is accomplished through the better granulation 

 and larger porosity which results. The possibilities in 

 this direction are indicated by the effect of structure 

 on the moisture capacity of the soil. (See page 151.) 

 Field experience has many times shown this result to 

 follow drainage. Instead of plants suffering from lack 

 of moisture, as a result of drainage, it is found that they 

 are not only free from the excesses, but that in dry 

 periods the soil is likely to contain more moisture than 

 the same kind of soil under poor drainage. This is especi- 

 ally true of those soils which are wet only a part of the 

 season. They are subject to great extremes in moisture 

 content. 



113. Improves the aeration. Drainage improves 

 the aeration of the soil in two ways. (1) It removes 

 the gravitational water from the large pores, thereby 

 permitting the admission of air. (2) Through its effect 

 on granulation it permits the soil to hold a larger volume 

 of air and facilitates its circulation. This also is due to 

 two conditions, especially where the drainage is beneath 

 the surface. The larger pores resulting from granulation 

 greatly aid the process. And the underground passages, 

 formed by tile or other media, afford channels for the 

 escape of soil air following rain or reduction in baro- 

 metric pressure, and facilitate its readmission when the 

 opposite conditions prevail. The net result is a much 

 larger total change between the outer air and the soil 

 air. This reacts strongly upon the soil organisms and 

 upon the general chemical activity of the soil. 



114. Raises the average temperature. Drainage 



