DRAINAGE 



223 



thawing is also another means for producing granulation, it' t lie- 

 right amount of water is present. If there is an excess of water the 

 effect of freezing and thawing is to break down the granules. In- 

 stead of producing good tilth a " runny," puddled condition results. 

 The soil of a pond where water has stood during the winter will 

 be run together very badly by spring and become quite compact. 



(c) It seems almost paradoxical that the removal of the excess 

 of water should increase the available moisture for plants, yet it is 

 true. Lowering the water table to a depth of three or four feet 

 enables plant roots to develop in a larger area than i*j otherwise 



Fio. 9. r >. The difference in germination and growth on undrained soil (Al and drained 

 (B) soil. Same kind of soil and the same kind and number of seeds were planted, (t'niversity 

 of Illinois.) 



possible, since plant roots do not penetrate a waterlogged soil. 

 This will give them a chance to secure the water from a depth of 

 three feet or more where otherwise they would lie limited to one 

 or two feet. Capillary water, only, is used by plants, and drainage 

 increases the volume of soil that contains this form of moisture. 



(d) The removal of the excess of water aids af ration, since the 

 water is replaced by air. About 50 per cent of the volume of the 

 soil as it ordinarily exists is pore space, and about one-half of this 

 should be occupied by air under ordinary conditions. This, in a 

 waterlogged soil, would be filled with water. The optimum con- 

 dition for plant growth is sufficient moisture for the use of the 



