DRAINAGE AND "HEAVING" 247 



texture holds more water and is most likely to contain 

 an excess of water. Drainage reduces this type of injury 

 in two ways: (1) By reducing the amount of water 

 present to freeze. (2) The larger volume of free pore 

 srace, due to the removal of part of the water and to the 

 better granulation, permits the expansion due to freezing 

 to be taken up within the mass of the soil, rather than 

 produce a lifting of the surface. Serious "heaving" 

 is always dependent upon an excess of soil water. 



119. Removes injurious salts from alkali soils. 

 Drainage in conjunction with heavy irrigation is the 

 most effective means of removing "alkali salts" from arid 

 soils. These salts are dissolved in the irrigation water 

 as it passes through the soil, and are then removed 

 in the drainage system beyond any possibility of further 

 injury. By this practice it is possible to reclaim the most 

 pronounced areas of alkali soils to the growth of the most 

 sensitive crops. 



120. Reduces erosion. Drainage reduces erosion 

 due to water. This type of injury results from the flow 

 of water over the surface. Drainage reduces this process: 

 (1) By increasing the absorption of water. (2) By 

 affording channels in which it may be removed without 

 injury, due to a less fall, or in conduits not subject to 

 erosion, such as tile drains. 



121. Increases crop yields and improves sanitary 

 conditions. The direct practical result of all of the 

 above effects is larger and more reliable crop-yields, 

 together with greater ease in all cultural and harvesting 

 operations. 



Coupled with the direct economic effect of drainage, 



