244 THE PRINCIPLES OF SOIL MANAGEMENT 



inches it was 1.7 warmer. King reports the frequent 

 observation of a difference of 12 between the tempera- 

 ture at the surface of drained and undrained land. 



115. Influences the growth of soil organisms. 

 Drainage promotes the development of the desirable 

 forms of organisms, and hinders the development of the 

 undesirable forms. As will be shown (page 399), the 

 soil organisms may be divided into two groups, one of 

 which requires free oxygen for their growth, the other 

 does not. These two groups are concerned with different 

 types of chemical change, the one producing decay 

 the other putrefaction. In proportion as the air is ex- 

 cluded by an excess of water, normal decay is inhibited 

 and putrefaction promoted. The one is beneficial, the 

 other is likely to be injurious. Further, the products of 

 the organisms accumulate in the excess of soil water and 

 sooner or later may kill most of the forms; as is exempli- 

 fied in peat bogs, which owe their origin chiefly to this 

 fact. Not only is the decomposition of organic matter 

 retarded, but the chemical changes in the mineral 

 portion of the soil resulting from these processes are 

 correspondingly reduced by lack of drainage. And most 

 important of all is the stimulation to the formation of 

 nitrates which results from good drainage. The supply 

 of nitrates is often the controlling factor in plant growth, 

 and consequently, in so far as drainage increases this 

 supply, it is directly beneficial. 



116. Increases the food-supply. Drainage increases 

 the available food-supply of the soil in three direct ways: 

 (1) By holding in the soil a larger proportion of avail- 

 able moisture which favors a larger chemical activity 



