210 Physics of the Soil 



250. Rolling and Harrowing For Soil Ventilation. It fre- 

 quently happens, especially with small grains in the spring, 

 when the season has been unusually wet and evaporation 

 large, that a crust forms upon the surface, partly by shrink- 

 age, partly by the crumb-structure breaking down and 

 partly by the deposit of soluble salts between the soil grains, 

 thus closing up the pores and greatly impeding the en- 

 trance of air. Under such conditions the harrowing or 

 rolling of small grains after they are up owes its advan- 

 tages in part to the better soil breathing it secures, by 

 .breaking the crust. 



But it will sometimes happen, when small grains are 

 rolled immediately after seeding, if the ground chances to 

 be a little too moist, that soil ventilation will be so much 

 hindered by the packing as to result in defective germina- 

 tion and sickly plants. In one case a crop of barley was 

 so much affected in this way that a serious reduction of 

 yield was the result and the plants, even when mature, 

 were so evidently influenced, that the rolled strip, between 

 two adjacent areas not rolled, but in other respects the 

 same, showed in strong contrast on account of the smaller 

 plants. 



251. Underdraining For Soil Ventilation. When heavy 

 soils are underdrained they are so much more deeply and 

 better aerated that this is one of the chief advantages of 

 that method of land improvement. In such cases the roots 

 of plants penetrate the subsoil so much farther, and earth- 

 worms and ants burrow so much deeper, that with the 

 decay of the roots the more or less vertical galleries formed 

 by these agencies permit much freer and deeper soil ven- 

 tilation. 



Then when the under clays dry out, as they do after 

 draining, great numbers of shrinkage checks form and in- 

 to these both the roots of plants and the free soil-air pene- 

 trate and are brought together. 



After this last stage of soil improvement has taken place 

 the bringing in of carbonic acid with the air leads, through 



