96 SOILS AND FERTILIZERS 



107. Composition of Typical Soils. A few examples 

 are given, in tabular form, of the chemical composition 

 of soils from different regions in the United States. On 

 account of variations in the same locality, the figures 

 represent the composition of only limited soil areas. 

 There have been made in the United States a large 

 number of -soil analyses which as yet have not been 

 compiled or studied in a systematic way. 



108. Alkaline Soils. When a soil contains enough 

 alkaline salts, as sodium sulphate, sodium or potassium 

 carbonate or chloride, to be destructive to vegetation, 

 it is called an ' alkali ' soil. These soils are found in 

 semi-arid regions, and wherever conditions have been 

 such that the alkaline compounds have not been drained 

 from the soil. Occasionally calcium chloride is the 

 destructive material. Sodium sulphate is a milder form. 

 Alkaline carbonates are destructive to vegetation when 

 present to the extent of more than I part per 1000 parts 

 of soil. When evaporation takes place, the alkaline 

 compounds are deposited as a coating on the surface 

 of the soil. Of these sodium carbonate is one of the 

 most injurious ; it exerts a solvent action upon the 

 humus, forming a black solution which evaporates and 

 leaves the so-called 'black alkali.' Many soils sup- 

 posed to be strongly alkaline, because a white coating 

 is formed on the surface, simply contain so much lime 

 carbonate that a deposit is formed. Excellent soils have 

 been passed over as ' alkali ' soils when in reality they 

 are limestone soils. 



