CHOP ROTATION IN BELATION TO SOIL PRODUCTIVITY 
23 
the crops of one rotation represent different seasonal effects than those 
of another, the effects of rotation and the use of manure on the yields 
of each crop in the various rotations, as measured in terms of crop 
increases, are simply averaged. Thus, from the foregoing experi- 
mental data the average yields of each crop in continuous culture 
and in the different rotations are first selected and arranged in table 
form, a table for each crop; then from these yields are calculated the 
increases effected by rotation and fertilization; and each of these 
sets of values, in turn, are averaged. The general averages thus 
obtained for a given crop express truer absolute values of rotation 
and fertilizers, particularly of manure, because such averages include 
different combinations of crops and seasonal effects. 
Two groups of observations are made: (1) A comparison of the 
effects of crop rotation and fertilization when they are practiced 
independently of each other; and (2) a comparison of the effects when 
rotation and fertilization are conjoined. 
ROTATION AND FERTILIZATION PRACTICED INDEPENDENTLY 
In Tables 17, 18, 19, and 20 are summarized, from the foregoing 
tables, those average yields of wheat, corn, oats, and timothy from 
which are determined the effects of both rotation and the use of 
fertilizers on crop yields when practiced independently of each other. 
In the columns to the right of the one containing the average yields 
are given the increases in yield (over the check plots in continuous 
culture) owing to crop rotation, the use of farm manure, and chemical 
fertilizers, respectively. At the bottom of each table are given, in 
terms of bushels of increase, the average absolute values of crop 
rotation and the use of farm manure, and, in case of wheat, an 
additional single comparison of absolute values of rotation and the 
use of chemical fertilizers. 
Table 17. — The effects of rotation and of fertilization on the yields of wheat, showing 
increases over cultivation alone 
[Rotation and fertilization practiced independently of each other] 
Plot No. 
Cultural conditions 
Average 
yield 
per acre 
Average 
increase 
due to 
rotation 
Average 
increase 
due to 
use of 
manure 
Average 
increase 
due to 
use of 
chemical 
fertilizer 
13 
6-year rotation and continuous culture: 
Rotation without use of chemical ferti- 
lizer ._ 
Use of chemical fertilizer without rota- 
tion.. 
Use of manure without rotation... 
Bushels 
20.1 
26.4 
23.4 
12.5 
23.6 
14.8 
4.9 
14.4 
20.1 
9.7 
18.4 
17.9 
9.1 
Bushels 
7.6 
Bushels 
Bushels 
2 
13.9 
6, 10, 21, 24, 30, 36 
10.9 
9 
No manure and no rotation 
39 
4-year rotation and continuous culture: 
Rotation without the use of manure 
Use of manure without rotation... 
18.7 
5, 10, 21, 24, 30, 36 
9.9 
9 
No manure and no rotation 
3-year rotation and continuous culture: 
Rotation without use of manure 
27 
4.7 
5, 10, 21, 24, 30, 36 
Use of manure without rotation 
10.4 
9 
No manure and no rotation 
33 
2- year rotation and continuous culture: 
R otation without use of manure 
9.3 
5, 10, 21, 24, 30, 36 
Use of manure without rotation 
8.8 
9 
Average increase from rotation and use 
of manure 
10.1 
7.6 
10.0 
Increase from rotation and use of 
chemical fertilizer 
13.9 
