CHEMICAL CHANGES 239 



Water containing carbon dioxide dissolved in 8 days about one- 

 third of the absorbed potash; acetic acid, (i 13), dissolved about 

 two-fifths; and hot hydrochloric acid (1:3 parts water) dis- 

 solved all the absorbed potash. 



.Replacement of Absorbed Material. If a soil is allowed to 

 absorb one base, and is then subjected to the action of a second 

 solution, a portion of the absorbed base will be replaced by the 

 second base. Other bases in the soil will also enter into solution. 



For example, suppose a soil has absorbed potash from pot- 

 assium chloride. If it be treated with a solution of sodium 

 chloride, some of the soda w r ill be absorbed, and a portion of the 

 absorbed potash, and also some lime and magnesia will enter into 

 solution. Sodium nitrate, ammonium chloride, calcium chloride, 

 calcium sulphate, magnesium chloride, and other salts have a 

 similar action. 



The amount of potash displaced would depend upon the quan- 

 tity present, and the nature and concentration of the salt solu- 

 tion. 



Importance of Absorption. Absorption tends to preserve the 

 potash, phosphoric acid, and ammonia of the soil from being 

 washed out. Soda, lime, and magnesia, for which the soil has 

 less attraction, are more easily washed out. The relative propor- 

 tion of loss, will, however, depend upon the amounts of these 

 bases present in the soil or rendered soluble. As we have seen 

 in the preceding section, soluble salts of lime, magnesia, or soda 

 decrease the absorption of potash or replace absorbed potash. 



Absorption tends also to prevent loss of the soluble plant food 

 placed in the soil in the form of fertilizers ; for nitrates, however, 

 the soil appears to possess little absorptive power. 



The practical importance of absorption lies chiefly in con- 

 nection with the application of soluble plant food. It is im- 

 portant to know whether soluble plant food applied to the soil will 

 be washed out and lost. Nitrates are not absorbed. Potash, 

 phosphoric acid, and ammonia, as we have seen, are fixed by the 

 soil, and as the solution percolates through the soil, coming in 

 contact with fresh masses, the larger part of the soluble material 



