CROP ROTATION IN RELATION TO SOIL PRODUCTIVITY 17 
The third bar in each of the series of results shown in the chart 
is the most interesting. The total gain above the check plot in 
continuous culture, as indicated in each series, is the actual increase 
effected when to cultivation are added the conjoint effects of rota- 
tion and fertilization. Here no assumptions are involved. 
When to rotation is added the practice of fertilization, the actual 
increase obtained in the yield of wheat is 8.44 bushels. This in- 
crease, which may be indicated by capital F, is shown by the di- 
agonal hachure at the top portion 
of the bar. On the other hand, 
when to fertilization is added rota- 
tion, the increase effected is 8.91 
bushels, which increase may be 
indicated by capital R, and is shown 
by the diagonal hachure below the 
figure, 20.16, indicating the total 
gain. The unhachured portion of 
the bar represents 2.81 bushels, 
being a part of the total gain; but 
there is no way to determine just 
how much of this undivided gain 
should be credited to rotation and 
to the use of fertilizers. This un- 
divided increase measures the in- 
teractive effects of rotation and 
the use of fertilizers when these two 
practices are conjoined. These 
effects may be interpreted in three 
ways: (1) When, under the condi- 
tions of these fertility experiments 
on wheat, rotation of crops and the 
use of fertilization are conjoined, 
the effectiveness of rotation, as 
determined when rotation is prac- 
ticed in the absence of fertilizers, is 
reduced and the efficiency of the 
fertilizer -remains the same as when 
it acts apart from rotation; or (2) 
the effect of crop rotation remains 
the same and the effectiveness of 
the use of fertilizers is diminished; 
or (3) it may mean that the efficiency of* both rotation and the use 
of fertilizers is diminished in the same or in different degrees. 
The unallocated value as shown by the unhachured portion also 
shows the difference (2.81 bushels) between the sum of the increases 
effected by rotation and the use of fertilizers when acting indepen- 
dently of each other and the actual increase obtained as a result of 
their combined effects, but just how much should be subtracted from 
the separate gains effected by rotation and the use of fertilizers can 
not be determined. 
In case of barley, R has a negative value of —6.62 bushels, due 
probably to the fertilizer treatments not being exactly comparable. 
Whether or not rotation would have effected any increase at all if 
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Fig. 2. — Chart summarizing the comparable 
yields obtained with wheat and barley at 
Rothamsted, showing the proportion of each 
yield (in bushels) that is credited to culti- 
vation alone, the increase effected when crop 
rotation or the use of fertilizer is added to 
cultivation, and the total increase over cul- 
tivation when rotation and the use of fer- 
tilizer are conjoined. A6-0, plot 6-0 on the 
Agdell field; B6, plot 6 on the Broadbalk 
field; H4-A, plot 4-A on the Hoos field; 
A2-C, plot 2-C on the Agdell field 
