Il8 THE SOIL SOLUTION 



it takes place a few inches below, owing to the fact that under 

 conditions of rapid evaporation, there is ordinarily a discon- 

 tinuance in the capillary columns or the film water at a point 

 below the surface of the soil, the water diffusing thence into the 

 above-surface atmosphere as the vapor phase. 



The composition of alkali is varied. In the vast majority of 

 cases, the world over, the predominating compound is sodium 

 chloride. When calcium carbonate is a conspicuous component 

 of the soil, as a hard-pan or otherwise, sodium carbonate or 

 black alkali is also generally present, or apt to appear when the 

 land is irrigated. When calcium sulphate or gypsum is likewise 

 present, there is less probability of appreciable amounts of black 

 alkali, and where gypsum predominates or the calcium carbo- 

 nate is present in relatively inappreciable amounts, black alkali 

 is generally absent, and sodium sulphate is an important con- 

 stituent of the alkali. Relative rates of diffusion, selective ab- 

 sorption, and sometimes other factors are prominent, however, 

 and the character of the alkali in different spots within a few 

 yards of one another may differ greatly. One of the most in- 

 teresting manifestations of alkali is the occasional occurrence of 

 a predominating amount of calcium chloride which, as a result 

 of its unusually high hygroscopicity, renders the soil damper, 

 and therefore darker in color than the surrounding soil, and 

 frequently causes even experts to suspect the presence of black 

 alkali. Its true nature can, of course, be determined by a sim- 

 ple chemical examination. 



The effect of alkali on the physical properties of the soil is 

 often very marked, aside from the cementing action or hard-pan 

 formation by the carbonate or sulphate of lime. Black alkali, 

 by dissolving and segregating the organic matter at the surface, 

 removes from the lower soil layers the "humus" compounds 

 which are of enormous importance to the maintenance of a soil 

 structure favorable to plant growth. Moreover, black alkali 

 is one of the best of deflocculating agents, and consequently 

 soils where it is a noticeable component, frequently puddle with 

 great readiness and are reclaimed with the utmost difficulty. 

 Most of the other constituents of alkali, however, are flocculat- 



