HOFFMANN: SOIL PRODUCTIVITY. 93 



This was accomplished by making a suspension of a young 

 culture of the specific organism under consideration in 100 cc. 

 of sterile distilled water. After vigorous shaking, the faintly 

 turbid liquid was filtered aseptically through glass wool, to in- 

 sure a uniform and homogeneous suspension of the organisms. 

 The 10 cc. quantities of the respective sterile soil extracts were 

 then inoculated with these suspensions at the rate of 0.1 cc. per 

 tube. For each extract six tubes were thus inoculated. By 

 making germ content determinations on one tube a day, some 

 idea as to the rate of multiplication of the organism in the 

 respective soil extracts, could be secured. These determina- 

 tions were made daily for a period of five days by which time 

 most of the organisms employed had reached their maximum 

 numbers. Plain gelatin was used as plating medium in some 

 of the experiments, plain agar in others. The dilutions were 

 so adjusted as to give plates upon which the colonies could be 

 readily counted. In all cases an actual count of all the colonies 

 on the plates was made, rather than estimating the total num- 

 ber by counting only a section of each plate. 



In the tables which follow, Tables V to VII, the data secured 

 in a few of these experiments are presented. These have been 

 further arranged graphically in Plates XXV-XXVII which 

 show more clearly the influence exerted by the various crops on 

 the soil extracts. It will be noted that on the whole the results 

 are consistent. This is very strikingly the case with the marsh 

 soil (Plate XXV), where with all organisms employed the rate 

 of multiplication was greatest in the corn-cropped soil, the 

 other soils ranking as follows : clover-cropped, oat-cropped and 

 control. This same order prevails with the three organisms 

 here examined. Apparently the growing of the crops upon the 

 marsh soil changed the composition of the soil solution in such 

 a way as to enhance the growth of the bacteria in the extracts 

 of the same. 



With the loam soil, the growing of crops seems to have modi- 

 fied the soil solution so as to cause a decreased multiplication 

 of the bacteria in the extracts in contrast to the control, which 

 showed the most pronounced growth. With sand there is a 

 very marked retardative influence evident, due to the cropping 

 of the same with corn and oats. This applies to all three 

 organisms. The normal in this case as with the loam, shows 

 the most marked development with the exception of the ara- 

 monifier, where the clover-cropped soil exceeds the normal. In 



