26 THE CHEMISTRY OP THE FAEM. 



by the roots and employed as plant food. The loss of 

 nitrates by drainage is thus far less when the land is under 

 crop than in the case of a bare fallow. 



7. Absorptive-Power of Soil. The surface of a moist 

 fertile soil is capable of absorbing ammonia from the 

 atmosphere. This absorption may proceed continuously 

 if the ammonia absorbed is continuously converted into a 

 nitrate. The amount that may be so absorbed may 

 apparently become considerable when the soil is in a 

 suitable condition as to porosity, and as to its capacity 

 for effecting nitrification. Whether free nitrogen is ever 

 brought into combination by any of the constituents of 

 soil is at present a disputed point. 



If a solution containing phosphoric acid, potash, or 

 ammonia is poured on a sufficiently large quantity of 

 fertile soil, the water which filters through will be 

 found destitute of these substances. This retentive 

 power of soil for phosphoric acid, potash, &c., is of the 

 utmost importance in agriculture. The action is a com- 

 plex one. All salts are doubtless retained to some extent 

 by soil through mere mechanical adhesion ; salts, thus 

 feebly retained, as nitrates and chlorides, can be easily 

 removed by washing with water. Other substances are, 

 on the contrary, retained by chemical affinity ; these are 

 not removed by washing, or but to a small extent. The 

 ingredients of the soil which exercise a chemical retentive 

 power, are the hydrates of ferric oxide and alumina, the 

 hydrous silicates of aluminium, and humus. 



Ferric oxide is a common ingredient of soils ; to it the 

 red colour of many soils is owing. To the presence of 

 ferric oxide the retention of phosphoric acid is chiefly 



