HOFFMANN: SOIL PRODUCTIVITY. 87 



in considerations of soil toxicity problems. Very recently, 

 however, Kellermann has called attention to the possible inter- 

 relationship existing between the soil bacterial flora and the 

 growing crop. 13 It seems plausible to expect such an inter- 

 reaction between the crops and the soil bacteria. We know 

 how very sensitive bacteria are to slight variations in the 

 composition of culture media. Whether in the cosmopolitan 

 life of the soil the variation's occurring in the composition of 

 the soil due to the accumulations of crop residues may affect 

 that life, has not been demonstrated, but is none the less most 

 probable. Lyon and Bizzell 14 have suggested the influence of 

 certain crops on nitrification which would thus influence subse- 

 quent crops. 



Examination of soils in close proximity to the root systems 

 of various plants reveals a marked difference in the quality as 

 well as in the quantity of the bacteria in contrast to that found 

 at some distance from the roots. (See Table I.) There is 

 invariably a much higher germ content in the immediate vi- 

 cinity of the roots. Thus of 32 determinations made on soils 

 adjacent to and remote from the roots of various plants, all 

 but five showed a much higher germ content near the roots 

 than at some distance away. Totalling the 27 counts with an 

 increase near the roots, one finds almost three times as many 

 bacteria near the roots as at some distance therefrom. One 

 must concede accordingly that the root system of plants exert 

 a marked effect upon the bacterial flora of the soil in the im- 

 mediate vicinity of the roots. Whether this influence is due to 

 the respiratory products of the roots or to the products formed 

 as a result of the decomposition of the root cells which are more 

 or less constantly being sloughed off, the author is unable to say. 

 Stoklasa has likewise shown that on the same soil under differ- 

 ent crops the germ content shows marked variations. Thus : 



Sugar beet soil 3-5 million. 



Barley soil 1-2 million. 



Clover soil 7-8 million. 



The work herein reported endeavors to throw some light on 

 this subject of reduced crop yields and soil toxicity. The author 

 maintains that when a particular plant species is continually 

 grown, the plant residues left in the soil occasion a change in 

 the chemical composition of the soil, which in turn influences 



13. Kellermann, Cir. 113, Bureau of Plant Industry, U. S. Dept. Agriculture. 



14. Jour. Indust. and Engin. Chem., 5, 1913, No. 2, p. 136, 



