454 SOILS. 



the dry season, in untilled irrigated land, practically the entire 

 mass of salts may be within six or eight inches of the surface. 



(3) The direct injury to vegetation 1 is caused largely 

 within a few inches of the surface, by the corrosion of the bark, 

 usually near the root crown. This corrosion is strongest when 

 carbonate of soda (salsoda) forms a large proportion of the 

 salts; the soda then also dissolves the vegetable mold and 

 causes blackish spots in the soil, popularly known as black 

 alkali. 



(4) The injury caused by carbonate of soda is aggravated 

 by its action in puddling the soil so as to cause it to lose its 

 crumbly or flaky condition, rendering it almost or quite un- 

 tillable and impervious. It also tends to form in the depths of 

 the soil-layer a tough, impervious hardpan, which yields neither 

 to plow, pick, nor crowbar. Its presence is easily ascertained 

 by means of a pointed steel sounding-rod. 



(5) While alkali lands share with other soils of the arid 

 region the advantage of unusually high percentages of plant- 

 food in the insoluble form, they also contain, alongside of the 

 noxious salts, considerable amounts of water-soluble plant- 

 food. When, therefore, the action of the noxious salts is done 

 away with, they should be profusely and lastingly productive ; 

 particularly as they are always naturally somewhat moist in 

 consequence of the attraction of moisture by the salts, and are 

 therefore less liable to injury from drought than the same soils 

 when free from alkali. 



1 For a general statement and discussion of the physiological effects of saline 

 solutions on plants, see chapter 26. 



