May 28, 1917 
Influence of Crop, Season , Water on Soil Bacteria 297 
On the other hand, McBeth and Smith (38) claim that plots con¬ 
tinuously cropped to alfalfa, potatoes, oats, and corn all show a higher 
nitrifying power than do corresponding fallow plots and that the stimu¬ 
lating effect of crop production on the nitrifying power of a soil is most 
marked in alfalfa soil. This is in keeping with the recent findings of 
Vel'bel (60), but is contrary to the findings of many other investigators; 
for Heinze (24) found fallow to increase the pectin, cellulose, and humus 
fermenters and also the ammonifiers, nitrifiers, and Azotobacter. Rus¬ 
sel (48) finds that in late summer fallow land is richer in nitrates than 
cropped, even after allowing for the nitrogen taken up by the crop; 
and Heinze shows that repeated cultivation of fallow soil increases the 
number of organisms in the soil, while Hiltner (25) maintains that no 
nitrification occurs in soils where legumes are growing vigorously and 
fixing large quantities of nitrogen. This latter view, however, is the 
extreme, as is shown by much of the literature on the subject. 
Vel'bel and Winkler (61) found that fallow not only increased the 
assimilable nitrogen but also the available phosphoric acid of the soil 
and that the increased yield of wheat after fallow is due to these factors. 
But Bychikhin and Skalski (5) point out that fall fallow is even more 
wasteful of soil nitrates than is summer fallow, for here the excessive 
rains wash the soluble nitrates from the soil as fast as formed. The 
cultivating of fallow further increases the nitrate content, as was shown 
by Richardson (47). Not only nitrification but all the bacterial activities 
of a soil are increased by fallow, as may be seen from the following table 
from the work of the senior author (20) of this paper. 
Table I .—Average bacterial content and activity of soil with various crops 
Colonies. 
Mgm. of 
nh 3 
, formed. 
Mgm. nitric 
nitrogen 
formed. 
Mgm. of N 
fixed. 
Virgin soil. 
2. 270 
4 840 
3 - 9 11 
3. 980 
43. 88 
52 . 94 
SO- 52 
83.42 
2. 09 
4. OO 
2. 2 K 
6. 22 
14. 28 
6.99 
n.83 
22. 88 
Wheat soil. 
Alfalfa soil. 
Fallow soil, potato fallow, etc. 
The results reported under milligrams of nitrogen fixed indicate that 
in an arid soil the increased nitrogen fixation in a fallow soil more than 
offsets the loss of nitrates, even though rapidly formed, for little, if any, 
would be lost in the drainage waters. These results have recently been 
confirmed by Reed and Williams (46). Moreover, the number of organ¬ 
isms in the soil and the rapidity of the bacterial activity within the same 
is going to vary greatly with the thoroughness and time of cultivation, 
as shown by Deh£rain (11), Neish (42), King and Whitson (29), Chester 
(6), and Quiroga (45), while the number and activity of the organisms 
in the soil may in a degree determine the speed with which the water 
evaporates from a soil (57). 
