May a, 9211 
Cotton Rootrot in the San Antonio Rotations 
123 
bottom to the depth of 4 to 6 inches additional. In the five rotations 
that are not subsoiled the mean of the mean annual rootrot injury is 
14.2 per cent, ranging from 1.1 to 31.9 per cent. In the five subsoiled 
rotations the mean of the means is 7.4, and the range is from 1.2 to 26.3 
per cent. Direct comparisons between the members of each pair shows 
that in only three of the five cases does subsoiling show a reduction of 
injury, and in only two cases is this difference sufficiently great to be 
significant. 
4. Effect of barnyard manure. —Direct comparison as to the effect 
of barnyard manure is possible in two pairs of the two-year rotations, in 
one pair of the annual or continuously cropped plots, and in one pair 
of the three-year rotations discussed below, making four in all. In those 
four cases the mean of the mean annual injury on the unmanured plots is 
13.7 per cent, ranging from 1.1 to 42.2 per cent, while for the manured 
plots the mean is 9.4 per cent and the range from 1.1 to 19.3 per cent. 
Direct comparisons between the members of each pair shows that in two 
cases they have the same annual means, while in one case the manured 
rotation has the higher mean and in the other case the lower mean. From 
this it must be concluded that the use of farmyard manure has not ma¬ 
terially reduced the injury from rootrot. 
5. Effect of green manure crops. —The effect of a green manure crop, 
either cowpeas or field peas, may be compared in four pairs of rotations of 
which two are in the two-year group, one in the continuous-cropping 
group, and one in the three-year group. The mean of the annual means of 
the four rotations which do not have green manure crops is 14 per cent 
of rootrot injury, ranging from 1.1 to 42.2 per cent, while the mean for the 
four in which green manure is used is 10.8 per cent, with the range from 
1.9 to 18.5 per cent. These differences are clearly not significant, and the 
conclusion must be that a green manure crop is not an effective remedy 
for cotton rootrot. 
In addition to the foregoing simple and direct comparisons between 
several groups of rotation pairs, it is possible to compare the effect of com¬ 
binations of different treatments against the control rotations. For exam¬ 
ple, rotation B-6a is a simple corn-cotton rotation, the land being plowed 
immediately after each crop and kept fallow and in good tilth until the 
next crop is planted. Rotation B-6b is the same, except that the land 
is subsoiled after the com crop. In rotation B-6c the land is manured 
after the com crop. In rotation B-6d the land is manured after the corn 
crop and planted to field peas, which are plowed under preceding the corn 
crop. In rotation B-6e the land is manured, planted to field peas, and 
plowed and subsoiled during the period between the com and cotton crops. 
These five rotations show progressive combinations of treatments that 
are often recommended for controlling cotton rootrot. Reference to 
Table III shows that the mean annual percentage of rootrot injury was 
much the same in all these rotations, though slightly higher in the 
