THE SOURCES OF PLANT FOOD. 19 



the richness of the soil in nitrogen, the previous conditions 

 as to temperature and moisture, the extent of recent 

 washing by rain, and on whether the soil is or is not 

 under crop. Where a crop is growing the nitrates will 

 be kept nearer the surface, the evaporation of water from 

 a growing crop being far greater than from a bare soil. 

 The nitrates will also be constantly taken up 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. 



Phosphoric acid, potash, and ammonia are very rarefy 

 found in drainage water. If a solution containing phos- 

 phoric 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 

 feebfy 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 

 due, an insoluble basic phosphate of iron being produced. 

 Alumina acts in the same manner. Ferric oxide and 

 alumina have also a retentive power for ammonia and 



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