THE CONTROL OF SOIL-MOISTURE 211 



soluble material, it is generally wise to facilitate the rapidity 

 of its action while checking, if possible, its magnitude. The 

 encouragement of the rate of percolate is spoken of as land 

 drainage, which is the process of removing the excess or 

 superfluous water from the sdil as rapidly as possible. 

 Excess water, by interfering with aeration, sets up unsanitary 

 conditions within the soil. By draining the land many favor- 

 able reactions are promoted. Granulation is encouraged, 

 heaving is checked, while the root zone and water capacity 

 of the soil are markedly increased. By facilitating aeration, 

 favorable chemical and biological changes are encouraged, 

 thus increasing the nutrients available for plants. The sum- 

 total of good drainage is an increase of crop production to 

 such an extent as to meet the investment costs and pay a hand- 

 some profit besides. 



While the drainage of swamps and the reclamation of over- 

 flow areas are urgent, the drainage of lands already under 

 crop is more important. Practical farm drainage is para- 

 mount in almost every community, even in arid regions where 

 irrigation must be practiced. Two types of drainage are 

 feasible — open and closed. Ditch drainage is the usual type 

 of the first group. Ditches have the advantage of large ca- 

 pacity and are able to carry water at a low grade. On the 

 other hand, they waste land, are ineffective and inconvenient, 

 encourage erosion and demand a yearly up-keep expenditure. 

 Wherever possible under-drains should be used. 



Jeffery, J. A., Text-Boole of Land Drainage; New York, 1916. 



Fippin, E. O., Drainage in New York; Cornell Agr. Exp. Sta., Bui. 

 254, 1908. 



Brown, C. F., Farm Drainage; Utah Agr. Exp. Sta., Bui. 123, 

 1913. 



Lynde, H. M., Farm Drainage in North Carolina; N. C. Agr. Exp. 

 Sta., Bui. 234, 1915. 



Yarnell, D. L., Trenching Machinery Used for the Construction of 

 Trenches for Tile Drains; U. S. Dept. Agr., Farmers' Bui. 698, 1915. 



Leidigh, A. H., and Gee, E. C, Tile Drainage; Tex. Agr. Exp. Sta., 

 Bui. 188, 1916. 



Hart, R. A., The Drainage of Irrigated Farms; U. S. Dept. Agr., 

 Farmers' Bui. 805, 1917. 



