418 Irrigation and Drainage 



THE DEMANDS FOR AIR IN THE SOIL 



It must ever be kept in mind that an abundance of 

 free oxygen in the soil is as indispensable to the life 

 of the plant as it is to that of an animal. The 

 germinating seeds must have it, or they rot in the 

 soil ; the roots of plants must have it to enable 

 them to do their work ; and the vast army of 

 soil bacteria, which change the nitrogen of decaying 

 organic matter into nitric acid, which is the chief 

 nitrogen supply for most higher plants, must have 

 it or they cannot thrive. Again, those very impor- 

 tant germs which live on the roots of clover and 

 other allied plants, and which are the chief source 

 of the organic nitrogen of the world, must have an 

 ample supply of both free oxygen and free nitrogen 

 in the soil, or they are unable to accomplish their 

 task. 



Again, there lives in all fertile soils a class of 

 germs which have the power of breaking down 

 nitrates, or even organic matter, to supply them- 

 selves with oxygen whenever the conditions are such 

 that the soil does not contain enough to meet their 

 needs. But when these germs are forced to do this, 

 as happens in a water -logged or poorly drained soil, 

 the nitrogen of the soil nitrates and of organic 

 matter is liberated in the form of free nitrogen 

 gas, and hence the soil may thus be depleted of 

 this most expensive ingredient of plant -food wherever 

 proper drainage does not exist. 



Finally, many purely chemical changes taking 



