KINDS OF SOIL L93 



tion of a large quantity of soluble salts in one spot, then the 

 "rise of alkali," as it is called, begins. For example, steady 

 drainage of soluble salts from a higher region to a lower, 

 with insufficient water to completely remove the salts in the 

 drainage water, results in the accumulation of salts in the 

 lower area. Where irrigation is practiced and there is used 

 water heavily charged with salts, but not enough water to 

 remove the salts into the country drainage, an accumula- 

 tion is apt to occur. And where excessive amounts of water 

 are used for irrigation in lands not properly provided with 

 underdrainage, the rising water table dissolves out the soluble 

 salts in the subsoil and brings them within capillary reach 

 of the surface. 



In any case the salts in solution in the soil water rise to 

 the surface by capillarity and are left there by the evaporation 

 of the water. A rain dissolves them and carries them into 

 the soil for a greater or less distance depending on the 

 amount of rainfall, and they return to the surface when 

 surface tension is again established. 



Many different kinds of compounds are made soluble by 

 the decomposition of rocks, but in passing through con- 

 siderable portions of soil, chemical absorption plays an 

 important selective part. As was seen in Section 133, 

 potassium and magnesium are retained more than sodium 

 and calcium. Phosphates are easily retained in an insoluble 

 form, carbonates, sulphates, chlorides less easily. As a 

 result it is to be expected that chlorides, sulphates, and 

 carbonates of not easily fixed bases will predominate in 

 alkali soils, and the chloride, sulphate, and carbonate of 

 sodium, sulphates of calcium and magnesium, and some 

 chlorides of calcium and magnesium prevail in such soils. 



White alkali is not so injurious to vegetation as 

 black alkali, sodium sulphate being the least injurious, but 

 both kinds are very troublesome to farmers in some arid 

 regions. In addition to the harmful effects on crops, black 

 alkali or sodium carbonate has the puddling effect on soils 

 common to most alkaline solutions. Limiting strengths of 

 these salts in sandy loam soil for cereals have been found to 

 be about 0.1 per cent, of sodium carbonate, 0.25 per cent. 

 13 



