488 SOILS: PROPERTIES AND MANAGEMENT 



406. Manures. Farm manures, lime, and those 

 amendments that improve the structure of the soil, have 

 for that reason a beneficial action on soil aeration. By 

 their effect on the physical condition of the soil they 

 increase its permeability, and by their action in con- 

 tributing to the production of carbon dioxide they stimu- 

 late diffusion. 



It is chiefly through its effect in increasing the volume 

 of air space in soils that farm manure is injurious in light 

 soils of semiarid regions. It may thus be injurious in- 

 stead of beneficial, if used under certain conditions. 



407. Underdrainage. By lowering the water table, 

 underdrainage by means of tiles removes from the soil 

 the water from all but the small capillary spaces, and 

 leaves free to the air the remainder of the interstitial 

 spaces. There is also a very considerable movement 

 of air through the drains, and a movement of air upward 

 from the drains to the surface of the soil, which serves 

 to aerate to some extent this intervening layer. The 

 aeration of the soil brought about by underdrainage 

 is one of its beneficial features. 



408. Irrigation. The influence of irrigation on the 

 soil is much like that of rainfall. The alternate filling 

 and emptying of the interstitial spaces with water and 

 air causes a very considerable change of air. 



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

 after a crop has been harvested decay and leave channels 

 in the soil through which air penetrates. Below the fur- 

 row 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 absorption of moisture 

 from the soil by roots also causes the air to penetrate, in 

 order to replace the water withdrawn. 



