112 CHEMICAL EQUILIBRIUM AND ANTAGONISM 



change in the position of the soluble salts. Layers of 

 compact soil or heavy clay ordinarily contain more soluble 

 salts than looser ones; and where there is movement of 

 water between different soil layers, there is a change in 

 the concentration of the solution and reactions take place 

 between the salts which have been dissolved from the two 

 types of soil. Small quantities of alkali in the irrigation 

 water may cause profound changes in the chemical com- 

 position of the soil solution. Changes in temperature 

 cause changes in the solubility of salts so that salts may be 

 thrown out of solution or new ones brought into solution. 

 Carbon dioxide and oxygen are frequently brought into 

 the soil by rains, and carbon dioxide is constantly being 

 formed in soils. This disturbs equilibrium of the com- 

 pounds by changing the solubility or causing the oxidation 

 of certain compounds. These and numerous other factors 

 cause the soil solution constantly to vary in concentration 

 and composition. 



Ifi studying alkali, however, these minute and trouble- 

 some changes are not ordinarily of sufficient importance 

 to warrant consideration. The quantity of alkali when 

 it becomes troublesome is generally so large that small 

 changes are practically negligible. Changing a few pounds 

 to the acre of sodium chloride into calcium chloride would 

 make so little difference in the toxicity of the alkali that 

 it could not be noticed. 



With sodium carbonate the condition is somewhat dif- 

 ferent. This salt is relatively unstable when compared 

 with sodium chloride and sodium sulphate. In the pres- 

 ence of solutions of carbon dioxide, as found in the upper 

 soil, sodium carbonate would probably form the unstable 

 sodium bicarbonate to a considerable extent. Sodium 

 carbonate and bicarbonate, on account of their solubility, 



