DRAINAGE 



225 



(f) The removal of the excess of water from the soil increases 

 decomposition and nitrification, processes necessary for the growth 

 of plants. As a general rule, the mosses and grasses of swamps 

 have decomposed to a very slight extent only, because of the excess 

 of moisture which prevents the access of oxygen. Drainage allows 

 aeration and the process of nitrification may then take place. 



(g) "Heaving" of soil or crops on medium- to fine-grained 

 soils is diminished or almost entirely prevented l>y the removal 

 of the water. When a wet soil freezes it expands in the direction 

 of least resistance, which is upward, and the crop, whatever it is, 



Fro. 9R. Pipe heaved nearly inches during winter of lOl.S-lOlfi. 



is pushed along with it. This process heing repeated over and 

 over may " heave" a crop out of the soil entirely, as in the ease of 

 young alfalfa, clover or wheat. If the soil is drained, the expan- 

 sion of the smaller amount of water in free/ing will lie taken care 

 of in the pore spaces of the soil without expanding upward to 

 such great extent. Figure !><; shows the heaving of a gas pipe 

 stake during one winter, and figure !)? shows the heaving of 

 alfalfa in a poorly drained soil. Where tight suhsoils are present 

 the danger of heaving is very great, so that it is praeticallv im- 

 possihle to grow alfalfa and clover. 

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