172 

 PART VI. ALKALINITY OF SOILS. 



Descending rain constantly carries clay particles from the surface 

 down to the subsoil, and so enriches the latter with a supply of the 

 available plant food which, as already observed, is generally contained in 

 the finer soil grades, i.e., the silt and clay. On the other hand, surface 

 evaporation, causing an upward movement of the soil water, brings to the 

 upper soil layers a constant supply of readily available plant food. Bearing 

 in mind, then, the references made on a former page* to the different grades 

 of plant food, it may be deduced, that the subsoil is constantly being sup- 

 plied from the surface with plant food constituents of grade II., while the 

 surface soil receives an equally continuous increment from below of the 

 plant food constituents of grade I. The former of these two processes takes 

 place more especially during the wet season, while the latter becomes 

 more marked during the dry, but in arid regions this increase of salts in 

 the surface soil may proceed so far, and result in an accumulation of salts 

 of such a nature, as to cause distinct injury to plant growth. 



The extent to which these salts have accumulated in the surface soil 

 can be definitely estimated only by chemical analysis, f but such analysis 

 is from its very object of totally different nature from that which investi- 

 gates the proportions of plant food or of plant food constituents present, 

 for we are now dealing not with beneficial, but with harmful substances 

 in the soil. 



Thus the agricultural chemical analysis of soils, although as a rule 

 confined to ascertaining whether they contain a sufficiency of plant food, 

 may at times have to proceed beyond this limit, and determine whether 

 there be not an excess of plant poison. This generally resolves itself into 

 an investigation of the brackness or alkalinity of the soil. 



What is generally called brack or alkali in soil consists in the presence 

 of excessive quantities of certain sodium salts. These, under favourable 

 conditions, dissolve in the rain-water and drain out of the soil, but they 

 accumulate and act injuriously where circumstances are unfavourable to 

 their removal by natural drainage. During a rainy season the salts are 

 carried to varying depths, according to the penetrating capacity of the 

 rain-water. The surface soil is thus left free from injurious salts. When 

 dry weather sets in, however, the soil water rises to the surface as oil rises 

 in a wick by capillary attraction, carrying the noxious salts with it from 

 below. The water evaporates from the surface soil, leaving behind the 

 salts as a white incrustation. There is, hence, a constant downward 

 passage of these injurious alkaline salts during rain, and an equally con- 

 stant upward movement during dry weather. 



It will be easily understood that irrigation will tend to increase the 

 accumulation of alkaline salts in the surface layers of a soil, seeing that 

 the irrigation water will probably penetrate to greater depths than the 

 rain, and carry to the surface larger quantities of salts. The danger is 

 naturally augmented when the water used for irrigating is itself alkaline. 



Hence, when adaptability of any tract of country for irrigation has 

 to be pronounced upon, two chemical problems have to be decided: 

 Firstly, does the soil contain any constituent which may render it brack 

 or otherwise unproductive; and secondly, assuming a satisfactory answer 

 to the first point, is the land sufficiently provided with the necessary 

 components of the food of plants to make farming profitable? 



* See page 9. 



t For the present purpose this term may be considered to include the electro- 

 lytic process of the United States Chemists. 



