IX.] ALKALI SOILS 285 



saline solution then dissolves some of the humus present 

 in the soil, and also causes the resolution of the clay 

 material into its finest particles, so that the soil forms 

 an intensely hard black pan when dry, which is known 

 as " black alkali." The carbonates are far more 

 injurious to vegetation than the neutral salts ; few 

 plants can bear as much as o-i per cent, of sodium 

 carbonate, but are tolerant of 0-5 to 1 per cent, of 

 the other salts. 



Though the alkali salts are sometimes chiefly 

 sulphates, more commonly sodium chloride is the 

 main constituent, together with the products of its 

 action in mass upon calcium carbonate and sulphate. 

 The diagram (Fig. 17), due to Hilgard, shows the dis- 

 tribution with depth of alkali salts in this type of soil 

 at Tulare, California ; the greatest accumulation of salts 

 takes place at a depth of 30 inches, the point to which 

 the annual rainfall penetrates. One of the most difficult 

 features presented by the cultivation of land in arid 

 regions where alkali occurs in the soil, comes from the 

 tendency of the sterile spots to spread and the alkali to 

 be brought to the surface as soon as irrigation water is 

 employed, for without irrigation agriculture is hardly 

 possible. Many districts, which at first carried good 

 crops and were even laid down in fruit or vines, have been 

 ruined through the rise of alkali to the surface brought 

 about by irrigation ; in fact, in all these arid regions it 

 becomes exceedingly dangerous to raise the water table 

 in the land anywhere near the surface, because capillarity 

 then causes a rise of the salt-changed water, and evapora- 

 tion concentrates it on the top. Just as some of the worst 

 alkali land occurs where rain falling upon the surround- 

 ing mountains finds its way by seepage through the 

 subsoil rich in salts and then rises to the surface in the 

 dry basin areas below, so the introduction of irrigation 



