94 KANSAS UNIVERSITY SCIENCE BULLETIN. 



all three cases, the normal and the clover-cropped soils show 

 far more multiplication of the bacteria in their respective ex- 

 tracts than the corn or oat-cropped soils. On the basis of these 

 data it appears as if the soil is the more important factor. The 

 marsh soil when cropped enables a greater multiplicaton of the 

 bacteria than the control or non-cropped soil. The reverse 

 seems to be true with the loam and the sand soils here employed. 

 The three organisms here experimented with are all similarly 

 affected. Azotobacter, B. denitrificans, Sarcina lutea, B. lique- 

 faciens fluorescens have also been used, but owing to contami- 

 nation of the plates, the results have been ignored. It was 

 possible, however, to note that on these plates the same rela- 

 tionship existed as above; invariably the corn-cropped marsh 

 soil showed the most pronounced multiplication. In fact all 

 the marsh soils which had been cropped showed in their respec- 

 tive extracts this stimulating effect on the growth of these or- 

 ganisms. In sand and loam, however, cropping appeared to 

 exert a retarding influence on the growth of bacteria in the 

 extracts from these soils. 



In tables VIII to X further similar data are summarized 

 which show the same general characteristics described above. 

 They give added substantiation to the hypothesis previously 

 stated, that growing crops modify the soil solution so that the 

 latter exerts a more or less pronounced influence, stimulative 

 or retardative, upon the bacterial species of the soil. 



SUMMARY. 



The data which have been secured in the experiments de- 

 scribed above, enable one to make the following conclusions : 



1. The growth of individual species of bacteria in a soil pro- 

 duces changes in the soil solution which manifest themselves 

 by an increased or decreased development of plant seedlings 

 when grown in extracts made from such soils. 



Invariably this influence seems to affect the root develop- 

 ment rather than the leaf development. 



In repetitions of the same experiment consistent results are 

 secured as long as all three factors, soil, bacterial species and 

 crop, are the same. A change of any one factor modifies the 

 results secured. 



2. The growth of individual species of crops in a soil pro- 

 duces changes in the soil solution which manifest themselves 

 by an increased or decreased bacterial multiplication in ex- 

 tracts made from such soils. 



