120 
Journal of Agricultural Research 
Vol. XXI, No. 3 
The plowing in these rotations is ordinarily done to the depth of about 
8 inches; where subsoiling is indicated, this is done in each furrow after 
the plow to the depth of 12 to 14 inches. After plowing, all plots are 
kept in good tilth and free from weeds by harrowing and disking until 
the subsequent crop is planted. Where manure is indicated, it is applied 
at the rate of 16 tons per acre and plowed under. 
It will be observed that cotton is the only crop used in these rotations 
that is subject to rootrot injury and that the diversity of tillage methods 
fairly well covers the range of farm practice in this region. 
In the first of the categories listed above, cotton recurs on the same 
plot each year. In the second case, cotton recurs on the same plot each 
alternate year, so that to give an annual expression as to the rootrot 
injury each rotation may be regarded as occupying a field of X acre in 
size, one-half of which is in cotton each year. Likewise, for the third 
category, each of the four rotations may be thought of as occupying a 
plot of X acre in size, of which one-third is in cotton each year, per¬ 
mitting an annual expression of the rootrot injury for each rotation. In 
the fourth case the rotation would be regarded as covering 1 acre of 
ground, of which one-quarter is in cotton each year. 
This method of comparison is faulty, of course, in that it assumes a 
degree of uniformity as between adjacent plots with respect to rootrot 
infection that probably does not exist. But, on the other hand, in view 
of the fact that the different rotations were located in the fields without 
previous knowledge as to the location of rootrot areas there was no 
chance for bias in their distribution. It would be fair to assume that if 
any system of rotation or any method of tillage were really effective in 
controlling the disease this effect would be shown by a lower annual 
percentage of plants dying from the disease within the area subjected 
to that system or method. 
COMPARISON OF THE ROTATIONS 
In order to compare the effect of the disease as observed in the different 
rotation plots, it is necessary to present in some detail the annual records 
as to the percentage of plants killed by the disease. 
In the first of the categories listed above there are four plots on which 
cotton has been grown continuously during the period of record. The 
extent of rootrot injury in each of these plots is shown in Table II. 
Table II .—Percentage of plants killed by rootrot in the continuously cropped cotton 
plots at the San Antonio Experiment Farm, ipi2 to igiQ 
Plot No. 
1912 
1913 
1914 
1915 
1916 
1917 
I9l8 
1919 
Plot 
mean. 
A-4-19 . 
(a) 
2.7 
4.2 
11. 8 
42. 0 
10.6 
25.6 
3 2 * 3 
18. 4 
A-6-3. 
0. 7 
.8 
•5 
•7 
7-4 
I 5 - r 
9. 2 
19-5 
6. 7 
B- 5-3 . 
•9 
3-8 
17. 6 
49 - 7 
96. 2 
43-7 
30-3 
95*5 
42. 2 
B-5-4 . 
. 2 
.8 
2-S 
10. s 
60. 5 
36.8 
42. 0 
74-3 
16. 0 
Annual mean. . . 
.6 
2. 0 
6. 2 
18. 2 
5 i -5 
26. s 
26.8 
55-4 
° Not planted to cotton in 191a. 
