HOFFMANN: SOIL PRODUCTIVITY. 95 



The extracts made from a marsh soil cropped by corn, oats 

 or clover in all cases stimulated bacterial multiplication. This 

 stimulation was consistently greatest in the case of the corn- 

 cropped soil. In contrast to the marsh, the extracts from the 

 cropped loam and sand soils invariably retarded the multi- 

 plication of the bacteria grown in the same. 



3. There is a definite relation between the growth of crops 

 in soil and the growth of bacteria therein. It is possible that 

 one crop in a given soil may stimulate certain bacterial species 

 which in turn will produce changes in the composition of the 

 soil solutions. These changes may or may not influence bene- 

 ficially subsequent crops. 



Owing to the variability of the three factors involved, soil, 

 crop and bacterial flora, it is impossible to establish any hard 

 and fast laws for all cases. 



It seems evident from the work that if these relationships 

 could be established, one would have a definite basis on which 

 to determine crop rotation systems. If corn favors the de- 

 velopment of organisms whose growth stimulates oats, then 

 oats can follow corn in a rotation ; whereas, if corn retards the 

 development of species favorable to oats or favors others detri- 

 mental to oats, then corn should not precede oats in a crop 

 rotation. 



Future work along the lines suggested by this paper should 

 throw much light on this all-important subject. 



