192 



THE SOIL: INORGANIC MATTER 



pulverulent all the way down. Weathering is uniform and 

 production of water-soluble material is uniform. The sub- 

 soil will raise just as good crops as the soil. In many places 

 material thrown out in excavating for the cellar of a house 

 makes just as good a garden as the top soil. 



(d) Alkali Soils. — In certain parts of the arid west, as 

 well as of other arid regions of the world, there exist patches 

 of so-called alkali soil (Fig. 44). They are usually barren 

 of vegetation and are covered with white or black incrusta- 

 tions of soluble salts. The white salts are called " white 

 alkali" and the black salts "black alkali." Chemically 



Fig. 44.— Alkali soil, showing patches of white alkali. Agronomy Depart- 

 ment, California Station. 



the white salts are not alkaline in character but consist 

 largely of sodium chloride and sodium sulphate with some 

 chlorides and sulphates of calcium and magnesium. "Black 

 alkali" is composed largely of sodium carbonate, the solution 

 of which dissolves from the soil through which it has passed 

 some of the humus, thus coloring the evaporated salts black.. 

 As was noticed in Section 144 (a), the soluble salts formed 

 by decomposition of the minerals are not leached out of arid 

 soils, and under normal conditions are spread throughout 

 the soil and subsoil in amounts not at all injurious to plants; 

 but when there arise conditions which permit the accumula- 



