EQUILIBRIUM IN SOIL SOLUTION 111 



colloidal material appears to be in weak chemical com- 

 bination with certain bases. When the alkali salts are 

 brought in contact with these colloids, there is an apparent 

 exchange of the sodium of the alkali for calcium or mag- 

 nesium. The calcium and magnesium appear in the 

 drainage water in greater quantities where the alkali is 

 present than where it is not, and the sodium is recovered 

 only with great difficulty if at all by leaching. This ac- 

 tion is apparently selective in nature. The weaker acids 

 yield their sodium to the colloids much more easily than 

 do the stronger ones, so that where equal quantities of 

 each of the salts are added to a soil when recovered the 

 quantity of acid assignable to each base will be different. 

 Each soil, and different parts of the same soil, frequently 

 differ considerably so that this interchange may vary both 

 in nature and magnitude in soils not greatly differing from 

 each other. The colloids of organic matter act much the 

 same as those of the soil so that added organic matter may 

 change the nature of an alkali soil. Whether it is due to 

 this exchanging of sodium for calcium in the colloids and 

 the consequent precipitation of calcium carbonate when 

 sodium carbonate is added to soils rich in colloids or in 

 organic matter is not known, but much of the alkalinity 

 of sodium carbonate disappears when added to such soils. 

 In sand where colloids and organic matter are absent, 

 practically all of the carbonates added can be recovered 

 by extraction with water. 



Equilibrium in Soil Solution. - That a complete state 

 of equilibrium is ever established in a soil is hardly probable. 

 The constant removal of water by plants, evaporation 

 from the surface of the soil, addition of water by rains or 

 irrigation, percolation of free water, and all the other causes 

 of movement of water in the soil, cause an incessant 



