246 CAUSES OF FERTILITY AND STERILITY [chap. 



little below the general level, where the subsoil water 

 comes to the surface. A heavy rainfall may be followed 

 by a rise of alkali, because a connection is then 

 established between the saline subsoil water and the 

 evaporating surface, whereupon a continuous capillary 

 use of salts takes place, followed by their crystallisation 

 at the surface. Per contra, the establishment of a soil 

 mulch, and shading the ground with a crop, so that 

 evaporation only takes place through the leaves, will aid 

 in keeping the alkali down. The composition of the 

 salts varies; as a rule, sodium chloride predominates, 

 with some sulphates of sodium, magnesium, and calcium, 

 in which case the material is known as " white alkali." 

 Under other conditions the material is really alkaline, 

 containing carbonate and bicarbonate of soda ; the 

 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. 16), 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 



