PLANT ROOTS AND SOIL AERATION 447 



aeration of the soil brought about by underclrainage 

 is one of its beneficial features. 



313. Irrigation. The influence of irrigation upon 

 the soil is much like that of rainfall. The alternate 

 filing and emptying of the interstitial spaces with water 

 and air causes a very considerable change of air. 



314. Cropping. The roots of plants left in the soil 

 after a crop has heen harvested decay and leave channels 

 in the soil through which the air penetrates. Below 

 the furrow slice, where the soil is not stirred and where 

 it is usually more dense than at the surface, this affords 

 an important means of aeration. The growth of legumi- 

 nous plants and other deep-rooted crops is in this way, 

 among others, beneficial to the soil. The absorption of 

 moisture from the soil by roots also causes the air to 

 penetrate, in order to replace the water withdrawn. 



