Microorganisms upon Plant (.rowth 



253 



to indicate that ct-rtain seeds, at least, may eariy a typical microhio- 

 logical population. More recent studies tend to indicate that micro- 

 organisms ma\- produce substances which have a direct beneficial 

 t>flect upon plant growth, due to production of hormones, auxins, or 

 similar substances. 



240 



c 200 

 o 



■-= ™ 160 

 ; t Q. 



i a> 120 



Q. y 



80 



40 







April May 



June 



July 



Aug 



Sept 



Oct Nov 



If) r± 



TT 



I I 



— (/) 

 rl:5 160 



' S; o 120 



CL nj 



in "5 80 



40 h 

 



Fig. 101. Influence of crop upon nitrate content of the soil (from Ly( 



Bizzell). 



When a green manure crop is turned under and seeds are planted 

 immediately, the seedlings may be injured, partly by the action of 

 microorganisms, and partly by the presence in decomposing plant 

 residues of certain substances toxic to seedlings or germinated seeds. 

 As a result of the decomposition of the young plants, numerous 

 fungi develop, some of which are destructive to seedlings. The rapid 

 evolution of CO- and consumption of oxygen also produce conditions 

 imfavorable to oxidation, a phenomenon essential for the germina- 

 tion of the seeds. When seeds are planted two weeks after the plow- 



