214 MOVEMENTS OF SALTS IN THE SOIL 



They are chiefly made up of salts of sodium, but occasionally 

 salts of potassium and magnesium are present in considerable 

 quantities. Chloride of sodium, though always present, very 

 seldom preponderates as in the case of soils which owe their 

 salinity to sea water. The most characteristic salts are 

 sodium sulphate and sodium carbonate. When the former 

 is the main constituent, the salt is termed in America ' white 

 alkali ' ; when the sodium carbonate is present in considerable 

 quantity the term * black alkali ' is made use of. The latter 

 term is naturally suggested by the black colour of the soil 

 in the spots where this salt is concentrated ; the colour is due 

 to the solution of the humus of the soil by the sodium car- 

 bonate. 



The white alkali is far less injurious than the black. Of 

 the three salts named, plants appear to be most tolerant of 

 the sodium sulphate, while the carbonate is much the most 

 pernicious, causing the bark of the plant stems to rot as soon 

 as it appears on the surface. The sodium carbonate has also 

 a very injurious effect upon the physical condition of the soil 

 when the soil contains clay, causing it to shrink in bulk, 

 become extremely sticky, and dry into a stony mass ; the 

 cause of this action we have already noticed (pp. 32, 34). 

 The presence of -08 per cent, of sodium carbonate in a heavy 

 soil is sufficient to make it quite untillable. The very in- 

 jurious action of sodium carbonate on the humus of the soil 

 has been already referred to. 



The origin of the sodium carbonate is in some cases due 

 to the weathering of mineral silicates, as for example soda 

 felspar, in the soil. Hilgard, however, believes that its most 

 usual source is to be traced to a reaction between sodium 

 sulphate and the calcium or magnesium carbonate in the soil. 



