June is, 1925 Effect of Crops on Yields of Succeeding Crops in Rotation 1115 
per acre, indicating an approximately 
equal productiveness as a whole for the 
plots involved. The trend of the 
yields under the three cropping sys¬ 
tems for the period covered is shown in 
Figure 5, which makes clear the pro¬ 
gressive increase in yields after tobacco 
as compared with results after corn. 
The corn crop, which is in no wise 
related to potatoes, seems to produce 
an unfavorable influence on the pro¬ 
duction of tubers which is not materially 
modified by any of the fertilizer treat¬ 
ments used. 
Good yields of corn have been main¬ 
tained during the period of the tests, 
the yields of the last year being but 
little below those of the first. The 
average yields of tobacco on the control 
plots for corn in continuous culture, in 
culture are due at least in part to 
original lower productivity of the soil. 
Thus far the indications are that corn is 
less affected by preceding crops than to¬ 
bacco and potatoes, although other 
crops are greatly affected by corn. 
In the rotations with tobacco and 
potatoes, the fertilizer treatments have 
had little effect on the corn crop, 
whereas in continuous corn culture 
the yields have been materially re¬ 
duced by omission of each of the three 
elements from the fertilizer. Where 
the complete fertilizer mixture is used 
the differences in yields as between 
continuous culture and rotation are 
small and hardly significant. 
As regards the bearing of these re¬ 
sults on the plant-food theory and the 
theory of soil toxins, it might be 
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Fig. 5. —Comparative effects of tobacco, potatoes, and corn, as preceding 
crops, on the yield of potatoes. The “crop eflects” evidently are influ¬ 
enced by seasonal conditions, but the yield of potatoes has been somewhat 
better after tobacco than after potatoes, and much better after tobacco 
than after corn. It appears that these differences are increasing. The 
yields here shown are the averages of all fertilizer treatments 
rotation with tobacco and in rotation 
with potatoes, are, respectively, 823, 
905, and 868 pounds of leaf, indicating 
that the productiveness of the plots 
occupied continuously by corn is some¬ 
what less than the average of the plots 
occupied alternately by tobacco and 
corn. The data in Table IY indicate 
that this difference is due entirely to 
the superiority of the cropping unit 
represented by plot 13 of Section A 
over that represented by plot 13 of 
section C, the yields of the latter and of 
plot 3 of Section C being the same. 
It should be stated, also, that the 
differences in corn yields on the two 
cropping units have remained ap¬ 
proximately constant from year to 
year. It seems probable that the 
reduced yields of corn in continuous 
expected, perhaps, that in comparison 
with the tobacco-cropping tests already 
discussed and the rotation tests, which 
follow, the present series, which in¬ 
cludes only hoed crops, would sooner 
or later involve more particularly the 
phenomenon of general soil exhaustion 
due to heavy annual withdrawals of 
plant nutrients. It is generally con¬ 
sidered that soil toxins of vegetable 
origin are, for the most part, transitory 
products, so that intervals of several 
months each year when the soil is not 
occupied by the crops, such as occur 
in this series, would tend to reduce 
injurious effects on succeeding crops 
from this cause. Moreover, broadly 
speaking, the toxic effects of any par¬ 
ticular crop tend to become evident in 
the early years of cropping, and these 
