98 



PLANT PHYSIOLOGY 



while in France and many other sections of Europe the sewage 

 from large cities is placed on the truck farms of the vicinity, 

 the molecules of nitrogen are taken up by the growing foodstuffs, 



Animal protein 



fuTial 



Plant protein 



— "^ 



nodule\bacteria 



Free Nitrogen 

 of the air 



electric spark 



tcogen fixing 

 bacteria in soil 



denitrifying ^ 

 bacteria 



mmonifying 

 Xr 



ectric s 



pack 



^bacteria/ 

 Cd^cay/) 



Ammonia or 

 ammonium salts 



■^-_ - 



Free Nitrogen 

 of the soil 



nitrite/bacteria 



nitrate^bacteria 



I Nitrites" 

 Fig. 6. — The nitrogen cycle. Arrows indicate the direction of the change. 



and within three months they are continuing their interesting 

 and variable careers as animal foods. 



Figure 6, which shows the various phases of the nitrogen cycle, 

 may serve as a pictorial summary of its manifold features. 



QUESTIONS 



1. Under what conditions may the addition of humus be injurious to 

 plants? 



2. What are the elements generally contained in commercial fertilizers? 

 Why? 



3. An average crop of wheat removes about 50 pounds of nitrogen per 

 acre from the soil. If nitrogen is 80% of the atmosphere by weight, how 

 many years would the nitrogen in the air last if it were available? 



4. At what point in the nitrogen cycle is loss of available nitrogen most 

 likely to occur? Why? 



5. Why does barnyard manure lose much of its fertilizing value when left 

 exposed to the air? 



6. Does old manure have the same value as fresh manure? Explain. 



