THE WOBK OF ROOTS 125 



If, on the other hand, the crumbs are too small, the 

 soil becomes impervious to water and wholly unfit for 

 plant-growth: this is the case with the finest clays or 

 puddled clay soils. As already explained, the surface 

 mulch of larger- sized crumbs produced by cultivation 

 conserves the moisture of the soil by preventing 

 surface evaporation. 



2. The air in the soil is kept in slow but constant 

 circulation by the fluctuations of barometric pressure 

 by which it is alternately forced into the soil and 

 sucked out again: this phenomenon is sometimes so 

 pronounced that the air, escaping from the lower strata 

 of soil and rushing up through wells, produces a loud 

 noise, causing them to be known as "whistling wells." 

 But, despite this, it is necessary to stir the soil 

 frequently in order to admit air if we wish to get the 

 best results from crops. The great importance of air 

 as an agent which promotes chemical changes in ' the 

 soil is now becoming better understood: this point will 

 be discussed later. 



The necessity for a supply of air may be strikingly 

 shown by placing a potted plant in a pail of water so 

 that the water-level stands a little above the top of the 

 pot. Note the appearance of the plant from day to 

 day. Alfalfa fields, if flooded for two or three days in 

 summer time, turn yellow and the plants die. Note the 

 effects of flooding on meadows or trees whenever an 

 opportunity occurs. A crust on the surface of the soil 



