190 MOVEMENTS OF SALTS IN THE SOIL 



calcium carbonate may occur throughout a considerable depth 

 of soil, though most vigorously in its upper layer. The pro- 

 duction of nitrate is practically confined to the surface soil. 



Besides the calcium and magnesium salts derived from 

 the calcareous matter in the soil, soluble salts of potassium 

 and sodium may also arise if the soil contains imperfectly 

 weathered silicates. 



Considerable quantities of salts are sometimes applied to 

 the surface of the soil in the form of manure. Nitrate of 

 sodium, sulphate of ammonium, potassium salts, chloride 

 of sodium, soluble phosphates, and sulphate of calcium are 

 applied in this way. Some of the substances thus applied 

 are chemically retained by the soil, and need not therefore 

 be considered when we speak of the movements of salts 

 within the soiL This retention by the soil depends however 

 upon the quantity of the material applied, and if this quantity 

 exceeds the retentive power of the surface soil the movement 

 of even these salts becomes quite possible. We need not 

 at this point discuss the relative retentive power of soils 

 for the various bases and acids ; we need only note that 

 soil has apparently no retentive power for the acid radical 

 of nitrates, chlorides and carbonates, and very little for the 

 acid radical of sulphates ; and that when these acid radicals 

 occur in soil solutions they are usually combined with calcium 

 or sodium. On the other hand, phosphoric acid, potassium, 

 and ammonium are only rarely present to any considerable 

 extent in soil solutions, these substances being generally 

 precipitated on the surface of the soil particles, and retained 

 there in a difficultly soluble condition. 



In a wet soil the solution of salts moistening the solid 

 particles is usually very weak. As a soil dries the solution 



