516 



PRINCIPLES OF SOIL MICROBIOLOGY 



the low nitrogen content of the bacteria, only a small amount of the 

 ammonia is reassimilated. This is the reason why bacteria produce 

 small amounts of C0 2 while considerable quantities of ammonia 

 may accumulate. The amount of ammonia and of nitrate produced 

 and accumulated in the soil will, therefore, depend upon the carbon- 

 nitrogen ratio of the organic matter added. The influence of micro- 

 organisms upon the carbon-nitrogen ratio of the soil itself is discussed 

 elsewhere (p. 702). 



Influence of straw and plant residues upon the growth of cultivated 

 plants. Higher cultivated plants may have to compete with micro- 

 organisms for the available plant food, especially the nitrogen. When 



MGM. 

 OF 



NH 



3 „ -- 



1-- 



1— B. subtilla 

 3— B. megatherium 

 3- — Trichoderma 6p. 



— 1 



rAYS 



Fig. 33. Rate of decomposition of alfalfa meal by pure cultures of micro- 

 organisms and by the mixed soil flora, as indicated by the accumulation of 

 ammonia (from Neller). 



large amounts of green manure or straw are plowed under and a crop 

 is planted soon afterwards, distinct injury to the crop may set in. This 

 injury is largely a phenomenon of nitrogen starvation. Kriiger and 

 Schneidewind 13 submitted definite evidence that the addition of cellu- 

 lose to the soil stimulates the development of various soil organisms 

 which reduce the soil nitrates and prevent the plants from obtaining 

 sufficient nitrogen for their growth. The fact that leguminous plants 



13 Kriiger, W., and Schneidewind, \V. Ursache und Bedeutung der Salpeterzer- 

 setzung im Boden. Landw. Jahrb., 23: 217-252. 1899; Zersetzungen und Um- 

 setzungen von Stickstoffverbindungen im Boden durch niedere Organismen und 

 ihr Einfluss auf das Wachstum der Pflanzen. Landw. Jahrb., 39 : 633-64S. 1901. 



