Ch. V, 6] 



STRUCTURE OF SOILS 



245 



of the most important, perhaps the most important, factor 

 underlying soil fertility. Moreover, the supply needs con- 

 stant renewal to compensate for loss by drainage and removal 

 from the land with the crops. Now it happens that some 

 kinds of soil Bacteria have the power to change certain nitrog- 

 enous substances, nota- 

 bly ammonia, common 

 in soils but not usable by 

 the higher plants, into 

 other nitrogenous sub- 

 stances, notably nitrates, 

 readily usable by those 

 plants ; and such nitri- 

 fication of soils, while it 

 only transforms, and does 

 not add nitrogen com- 

 pounds, is yet an impor- 

 tant element in soil fer- 

 tility. Further, there are 

 other kinds of soil Bac- 

 teria which possess the 

 power to take free nitro- 

 gen from the air and 

 incorporate it into com- 

 pounds in their own 

 bodies ; and such nitrogen 

 fixation, on decay of their 

 bodies, adds nitrogen to 

 the soil, and is the chief 

 source of supply in soils of that indispensable substance. 

 Both kinds of Bacteria live in the humus, or at least are de- 

 pendent thereon for most of their food, in which fact lies the 

 principal reason for the association of humus with good soils. 

 The nitrogen compounds formed by these Bacteria become 

 ultimately dissolved in the soil solution, whence they are 

 absorbed by the roots of higher plants. In a few families, 



Fig. 173. — Typical root nodules (or tu- 

 bercles), on roots of Lupine ; X \. (DrawD 

 from a photograph.) 



