ALKALI SOILS 329 



tial. It is estimated that 13 per cent, of the irrigated land of 

 the United States contains sufficient soluble salts seriously to 

 interfere with crop growth. This alone amounts to nine mil- 

 lion acres and does not include the millions of acres not under 

 the ditch that are affected to a marked degree by alkali. Sim- 

 ilar figures are available from other continents and, since 

 alkali conditions can be alleviated and controlled to a certain 

 extent, the importance of the subject becomes apparent. 



Entirely aside from the economic aspects, alkali is of great 

 interest scientifically, offering a research field of such range 

 and complexity as to involve many sciences. A greater por- 

 tion of the practical information regarding alkali and its con- 

 trol has arisen from the purely scientific interest that has 

 been directed towards this peculiar soil condition. 



173. Composition of alkali. — It has been emphasized pre- 

 viously that the solution of a normal humid-region soil is of 

 such dilution as to be largely ionic in character except in 

 periods of low moisture content. In a soil affected with 

 alkali it is obvious that the molecular state is dominant and 

 that certain salts may exist and function as definite entities. 

 Thus the following bases may be expected to be present — 

 sodium, potassium, magnesium, calcium, and sometimes am- 

 monium. The common acid radicals are chlorides, sulphates, 

 carbonates, bicarbonates, phosphates, and nitrates. The salts 

 that are present and their proportion not only in the soil solu- 

 tion but as a precipitant will vary with conditions. 



The following table indicates not only the salts that may be 

 present but the composition of the alkali as reported by a 

 number of different investigators. (See table LXXV, p. 330.) 



174. White and black alkali. — Sulfates and chlorides of 

 the alkalies, when concentrated on the surface of the soil, 

 produce a white incrustation, which is very common in alkali 

 regions during a dry period as a result of the evaporation of 

 moisture. Incrustations of this character are called white 

 alkali. 



