428 Irrigation and Drainage 



soils, and hence land drainage may be beneficial to 

 crop growth in this manner. 



CONDITIONS UNDER WHICH LAND DRAINAGE 

 BECOMES DESIRABLE 



It must be kept ever in mind that all lands, of 

 whatever kind, require draining, but it is extremely 

 fortunate that for most lands this is done by the 

 natural methods of percolation and underflow of 

 ground water. 



The cases in which it becomes desirable to supple- 

 ment the methods of natural drainage fall into five 

 classes : first, those comparatively flat lands or basins 

 upon which the surface waters from surrounding 

 higher land frequently collect ; second, areas border- 

 ing higher lands, whose structure is such as to permit 

 the underflow of the ground water from the adjacent 

 regions to rise from beneath, thus keeping the soil 

 too wet ; third, lands regularly inundated by the rise 

 of the tides, or which would be if not shut off by 

 dykes ; fourth, those extremely flat lands which are 

 underlaid by considerable thicknesses of close, heavy 

 beds of clay, through which water does not readily 

 percolate, and which lie very close to the surface, so 

 that the clays become the subsoil of the fields, and 

 fifth, lands like rice -fields, water-meadows and cran- 

 berry marshes, to which water is applied by irrigation 

 in excessive quantities. It may also be found desir- 

 able on some irrigated lands to introduce drainage to 

 remove injurious salts, as described under alkalies. 



