ALKALI SOILS. 



429 



ping when the moisture in the soil is exhausted. We thus not 

 infrequently find that after an unusually heavy rainfall there 

 follows a heavier accumulation of alkali salts at the surface, 

 while a light shower produces no perceptible permanent effect. 

 We are thus taught that, within certain limits, the more water 

 evaporates during the season the heavier will be the rise of the 

 alkali ; provided that the water is not so abundant as to leach 

 the salts through the soil and subsoil into the subdrainage. 



Leaky Irrigation ditches. Worst of all, however, is the 

 effect of irrigation ditches laid in sandy lands (such as are 

 naturally predominant in arid regions), without proper pro- 

 vision against seepage. The ditch water then gradually fills 

 up the entire substrata so far as they are permeable, and the 

 water-table rises from below until it reaches nearly to the ditch 

 level; shallowing the subsoil, drowning out the deep roots of 

 all vegetation, and bringing close to the surface the entire mass 

 of alkali salts previously diffused through many feet of sub- 

 strata. 



Surface and Substrata of Alkali Lands. Aside from the 

 desert proper, in the greater portion of the alkali country 

 " alkali spots," /. c. ground covered with saline efflorescences 

 and showing little or no vegetation, are interspersed with larger 

 areas apparently free from salts and covered with the ordinary 

 vegetation of the region. A view of such country is given in 

 a plate on a previous page. The alkali spots are usually some- 

 what depressed below the surrounding lands, and after rains 

 remain covered with water for some time ; the water frequently 

 assuming a brown or blackish tint after standing. 



When a pointed steel probe is pushed down within such an 

 alkali spot, it will usually be found that, although the soil may 

 appear quite sandy, it is penetrated with some difficulty ; while 

 outside of the spots, the probe does not encounter serious re- 

 sistance until it reaches the depth of two or three feet, when it 

 frequently becomes impossible to penetrate farther without 

 the aid of a hammer. On the margin of the spots, the transi- 

 tion from utter barrenness to a luxuriant vegetation of native 

 weeds is mostly quite sudden ; as is shown in the figure, p. 425. 



Vertical Distribution of the Salts in Alkali Land. The re- 

 sults of a comparative examination of such land before and 



