May 28,1917 
Influence of Crop, Season, Water on Soil Bacteria 
33 3 
UNIRRIGATED SERIES 
All of the crops which have appeared in the previous series, together 
with fallow, appear in the unirrigated series. The summarized results 
for the three years are given in Table XXXII. 
Table XXXII .—Number of colonies of bacteria , milligrams of ammonia , and milli¬ 
grams of nitric nitrogen from soil with various crops receiving no irrigation water 
NUMBER OR COLONIES DEVELOPING IN 4 DAYS ON SYNTHETIC AGAR 
Plot No. 
Crop. 
Sampled May 1. 
Sampled Aug. 1. 
Sampled Nov. 
28. 
Average. 
34 . 
Alfalfa. 
8,133,000 
6, 333 jOOO 
5,967,000 
3, 867, OOO 
5, 667, OOO 
6, 711,000 
46. 
Oats. 
7 > S 33 .000 
6,100,000 
7,167,000 
6, 000, 000 
4, 676, 000 
6, 500,000 
4,911,000 
.• ••*••••• 
$0 . 
Com. 
38. 
Potatoes.... 
6, 500,000 
3,633,000 
5,833,000 
6, 500,000 
2,833,000 
42 . 
Fallow .... 
3>100,000 
1, 767, 000 
MILLIGRAMS OF AMMONIA PRODUCED IN IOO GM. OF SOIL IN 4 DAYS 
34 -.. 
46.. . 
5 °*** 
38 --. 
42.. . 
Alfalfa. 
53-5 
52-7 
43 *o 
Oats. 
48.8 
50. 2 
44 - 7 
Com. 
55*2 
57*2 
46. 6 
Potatoes.... 
56.4 
65.0 
50.6 
Fallow.. ,. 
61. 2 
$ 5 - 7 
52. 6 
MILLIGRAMS OF NITRIC NITROGEN PRODUCED IN IOO GM. OF SOIL IN 21 DAYS 
Alfalfa. 
i *5 
6.2 
3.1 
3 * & 
Oats. 
4.0 
5*4 
6.8 
5*4 
Com. 
1.9 
2. 1 
i* 5 
1.8 
Potatoes.... 
1.1 
■ 13* 6 
3*8 
6. 2 
Fallow.... 
1.6 
7 * 7 
4 * 2 
4*5 
The organisms are highest in the alfalfa and lowest in the fallow soil 
throughout the year. With the exception of spring and summer for the 
series which received the maximum quantity of water, the alfalfa soil has 
been much higher in bacteria than any of the other soils. And in most 
of the series the decrease in number from spring to fall is much more 
pronounced in the fallow than in any of the cropped soils, thus indicating 
that something develops in the fallow soil during the summer which 
limits the number of organisms. However, it is not the water applied, 
for we find the decrease just as great in the unirrigated as in the irrigated 
soil. Furthermore, the results show conclusively that the crops have 
stimulated growth of organisms which will develop on synthetic agar. 
Considerable of this stimulation is produced by the plant residues which 
are left on the cropped soil but are missing from the fallow soil. 
The ammonia produced by this series is highest in the fallow and con¬ 
siderably lower in the oats and alfalfa plots. The quantity produced in 
