Drainage Ventilates the Soil 421 



both from the surface and through the tile drains, 

 because the removal of the soil water by the roots 

 leaves an open space, which must be filled with air 

 so far as capillarity fails to do it with water, and 

 hence deep root feeding means deep soil ventilation. 



Then, again, when heavy rains fall which move 

 downward through the soil, they displace both the 

 air and the water previously there, crowding them 

 forward into the drains, and then draw in after them 

 a fresh supply from above. But only on well- 

 drained soils is this action marked and helpful. 



A word should be said here regarding the value 

 of clover and alfalfa as soil ventilators, for by their 

 thicker, stronger roots they set the soil aside more than 

 most other cultivated crops do, and when these roots 

 decay the soil is left better aerated and better 

 drained. Further than this, the roots of these legu- 

 minous plants remove from the soil both free oxygen 

 and free nitrogen, and in so far as they do this with- 

 out returning an equal volume of another gas, their 

 action tends to develop a vacuum which must be 

 filled by bringing in a fresh supply from without. 



TOO THOROUGH AERATION OF THE SOIL 



There may be too strong and rapid changes of 

 soil- air, just as there may be too rapid and complete 

 drainage. If the air enters a rich, damp soil too 

 rapidly, there is so strong a development of nitrates 

 that the humus and other organic nitrogen are quickly 

 changed into the soluble forms, and rapidly leach 



