124 



SOIL CONDITIONS AND PLANT GROWTH 



no relationship to the amount of crop growth. Other field experiments 

 have given similar results. Yet in laboratory and pot experiments 

 bacterial counts have often proved most valuable ; numerous instances 

 are given in the Rothamsted papers on soil sterilisation (240, 241, etc.). 

 This discrepancy between field and laboratory experience is cleared 



Bacterial Numbers. NH 3 and Nitrate. 



Limit 



200 



TOO 



200 



100 



UNTREATED SOIL 



1 9 40 70 



130 



^'UNTREATED SOIL 



19 40 70 



130 

 Days 



Case i. Small amounts of NH 3 and nitrate initially present. A relationship is 

 indicated between bacterial numbers and the rate of production of NH 3 and nitrate. 

 Bacterial Numbers. NH, and Nitrate. 



250 



150 



50 



TOLUENED SOIL 



r 

 i 

 i 



I 

 i 

 i 

 i 



4 

 f 

 I 

 I 



I 



BOO 



300 



UNTREATED SOIL 



100 



Limit 



14 



14 



110 

 a^s 



No relationship 



Case 2. Large amounts of ammonia and nitrate initially present, 

 like that in Case i is indicated. 

 FIG. 6. Relation between bacterial numbers and amounts of nitrate and ammonia formed. 



up by a closer examination of the nature of the relationship between 

 bacterial activity and plant growth. The connection as already pointed 

 out lies in the fact that bacteria decompose the organic matter of the 

 soil and make new plant food out of old plant residues. If the factor 



