May 28,1917 
Influence of Crop , Season , Water on Soil Bacteria 
329 
face again by the water, but we could not expect this to be as great for 
the bacteria as it is for the soluble nitrates, and it has been seen that, 
even in the case of these where the greater quantities of water were used, 
the nitrates never concentrate in the surface foot during the summer, 
the results showing a decrease in most plots, due to water. It is there¬ 
fore quite reasonable to expect that bacteria which developed upon 
synthetic agar would be carried down in about the same proportions as 
were the other organisms; hence, the increased bacterial activities noted 
must be due to an increased physiological efficiency of the organisms. 
Moreover, had samples been taken to a depth of 3 feet for the bacterio¬ 
logical analysis, we would have obtained just as pronounced an effect 
upon number of colonies of bacteria, ammonifying, and nitrifying powers 
of the soil as we have in the case of the nitrates, which is the summation 
effect noted in the 6 feet. 
During the first season determinations were made of the nitrogen - 
fixing powers of the soil, but during the succeeding years we were so 
crowded with other work that it became impossible to continue this 
phase of the work. While the results for one season are not sufficient to 
warrant their consideration in detail, the average results are of interest, 
as they show the best fixation where 15 inches of water were applied to 
the soil. The averages for the various treatments were as follows: 
37.5 inches of water.1.4 mgm. of nitrogen fixed in 100 gm. of soil. 
25 inches of water.2.1 mgm. of nitrogen fixed in 100 gm. of soil. 
15 inches of water.8.5 mgm. of nitrogen fixed in 100 gm. of soil. 
None.3.5 mgm. of nitrogen fixed in 100 gm. of soil. 
INFLUENCE OF CROP WITH THE DIFFERENT QUANTITIES OF WATER ON 
BACTERIAL ACTIVITIES 
The experiment was so planned that, besides giving information upon 
the influence of water upon the bacterial activities of the soil, it should 
also give definite information upon the influence of crop on these same 
properties. This being the case, the results have been rearranged so 
that the crop is the variable and the quantity of water a constant. We 
can therefore compare the results from the alfalfa with the various 
quantities of water with those obtained where other crops receiving like 
amounts of water were grown, and each of these in turn can be compared 
with the fallow. 
PLOTS RECEIVING 37.5 INCHES OF IRRIGATION WATER 
In this series the alfalfa, oats, com, potato, and fallow soil each 
received 37.5 inches of irrigation water. The average summarized 
results for the three years are given in Table XXIX. 
The number of organisms in the soil is greatest in the spring and least 
in the fall. During the spring the fallow has many more organisms than 
any of the cropped soils. During the summer the cropped and uncropped 
plots contain about the same number of organisms, while in the fall all 
