1106 
Journal of Agricultural Research 
Vol. XXX, No. 12 
tobacco. Cowpeas in the two-year 
rotation give an increase of something 
more than 200 pounds over continuous 
tobacco culture without cover crop 
but there is no increase in value of the 
product. Vetch as a cover crop gives 
about the same gain in yield as the 
cowpeas in the two-year rotation and 
also gives some increase in value of the 
tobacco. Of the several legumes, crim¬ 
son clover continues to give the lowest 
yields, while as a cover crop in con¬ 
tinuous tobacco culture it has been less 
effective in maintaining yields than in 
the two-year rotation which includes 
wheat. Comparing the period 1916 
to 1923, inclusive, with the period 
on all plots. These relationships are 
more clearly shown in Figures 1, 2, 
and 3. In some years vetch as a cover 
crop has given very large increases in 
yield while in other years the yield has 
been actually depressed. Evidently 
the effect of the vetch is dependent 
largely on weather conditions. By 
comparing the yields of tobacco from 
year to year with the rainfall during 
the summer months (Table II), it will 
be found that the large increases in 
yield of tobacco usually occur in rela¬ 
tively dry seasons and the poor yields 
usually in wet seasons. 
This relationship between the effect 
of the cover crop and the rainfall, which 
Fig. 1.—Annual yields of tobacco, 1912-1923, inclusive: (1) in continuous culture, with hairy vetch 
as a cover crop; (2) in a 3-year rotation of tobacco, wheat, and red clover; (3) in continuous cul¬ 
ture, with no cover crop. Note the extremely wide fluctuation in yield from year to year and 
the downward trend in yield after vetch, in contrast with the relatively stabilized yield after 
clover and with no cover crop and the upward trend in yield after the red clover 
1912 to 1923, inclusive, it will be 
observed that the three-year rotation 
which includes red clover is the only 
cropping system showing a gain in 
yield of tobacco, all others showing no 
change or a decline. 
An outstanding feature of the results 
with vetch as a cover crop is the ex¬ 
tremely wide range in yield of tobacco 
from year to year. Similar, though 
somewhat smaller, fluctuations in .yield 
are obtained from other legumes in the 
rotation and from rye as a cover crop. 
By way of contrast, the yields in the 
red-clover rotation and on the fallow 
plot are relatively constant, except in 
1922, when poor yields were obtained 
seems to hold also for other cover 
crops, is important. It has often been 
suggested that turning under large 
quantities of green manures may cut 
off the normal upward capillary move¬ 
ment of moisture in the soil. This 
factor could hardly be of significance 
in the present case, since the unfavor¬ 
able action of the vetch occurs in 
seasons of abundant rainfall. , Again, 
cowpeas in the rotation have much 
the same effect as vetch, and since the 
cowpeas are grown in the precedng 
year they would certainly decay suffi¬ 
ciently before the tobacco crop is trans¬ 
planted to obviate interference with 
movement of soil moisture. It is plain, 
