384 IRRIGATION PRACTICE 



constituents remain in the soil and accumulate as the 

 years go by. This is one of the characteristic differences 

 between the soils of arid and humid regions. These 

 accumulations often become so large as to be injurious, 

 and then are called alkali. Alkali means the water-soluble 

 materials in the soil, especially when the quantity is so 

 large as to be injurious to plant-growth. 



Much alkali is of early geological origin. In early 

 geological times, as now, large accumulations of soluble 

 soil materials occurred in lakes similar to the Great Salt 

 Lake or the Dead Sea. When these prehistoric interior 

 lakes dried up, there were left behind great deposits of the 

 salts held in full or partial solution in the lake water. In 

 time these layers of saline materials were covered by silt 

 and other materials derived from soils, and thus conserved 

 until the present day. When such lands are irrigated the 

 water dissolves the soluble salts which may become very 

 injurious. 



Ordinarily, alkali appears as an incrustation on the 

 soil surface, even on native soils, where conditions are 

 favorable to the accumulation of soluble matter. As often, 

 however, alkali does not appear as an incrustation, but 

 is held in the concentrated soil solution, with equally 

 injurious effects. 



230. Seepage and alkali. Soils tend, naturally, to 

 retain their soluble matters, especially those of value in 

 plant-growth; but, when a continuous excess of water 

 passes through the soil, the soluble substances are given 

 up and pass into the country drainage. When such seepage 

 water, heavily charged with salts, appears in some low- 

 lying place and is evaporated, the alkali is left behind, 

 either on the surface or in the remaining soil water itself. 

 In either case, the soil becomes increasingly less suitable 



