284 CAUSES OF FERTILITY AND STERILITY [chap. 



Alkali Soils. 



In arid climates the rainfall is often insufficient to 

 produce percolation through the soil and subsoil into the 

 underground water system ; in consequence, the salts 

 produced by the weathering of the rocks tend to 

 accumulate in the subsoil, and may be brought to the 

 surface by capillary rise so as to cause almost entire 

 sterility. Such bad lands are known in America as 

 " alkali soils," but they are well known in India and in 

 Egypt, and indeed are common to all countries possess- 

 ing a small rainfall and great evaporation. In its most 

 aggravated form alkali land, particularly at the end of 

 the dry season, shows an actual white efflorescence of 

 salts at the surface ; all vegetation is destroyed, except 

 one or two plants which seem tolerant of large quantities 

 of saline matter, such as " greasewood," Sarcobatus sp., 

 or the Australian " saltbushes," Atriplex semibaccatum, 

 etc. In some cases the alkali is chiefly located at a 

 slight depth in the soil, and only effloresces on spots a 

 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 



