June is, 1925 Effect of Crops on Yields o f Succeeding Crops in Rotation 1127 
Comparing the results collectively 
with those in the rotations of hoed 
crops and small grains without soiling 
crops (Tables XIII and VIII), it is 
apparent that, qualitatively, the rela¬ 
tive effects of tobacco, potatoes, and 
corn remain the same. On the other 
hand, there are some very striking 
quantitative differences in the two 
series. The general effect of the 
legumes has been to intensify the differ¬ 
ences in effects of the three hoed crops 
on the yields of succeeding crops of 
small grains. It is an interesting fact 
that in both series of tests the compara¬ 
tive average effects of tobacco, pota¬ 
toes, and corn are practically the same 
for the straw as for the grain of the 
wheat, oats, and rye. In other words, 
these crop effects apply to the growth 
of the plant as a whole rather than that 
of particular parts and in this way 
differ somewhat from the effects pro¬ 
duced by the fertilizer treatments 
(Table VIII). The large increases in 
yields of small grains resulting from the 
residual effects of the legumes empha¬ 
size the fact that the latter have fur¬ 
nished an ample potential supply of 
nitrogen for the hoed crops, even in the 
case of soy beans. 
The yields of wheat on the control 
plots indicate that on Field IV the soil 
of the corn section is somewhat more 
productive than that of the tobacco and 
potato sections, while on Field V the 
reverse appears to be true. Making 
due allowance for these soil irregulari¬ 
ties, it is apparent that corn, in com¬ 
parison* with tobacco and potatoes, de¬ 
presses the yields of all the small grains 
(pi. 6). The effect of tobacco on oats 
resembles that of potatoes rather than 
that of corn, as in the previous tests, 
while the yields of wheat and rye after 
tobacco are about midway between the 
yields after potatoes and after corn. 
An outstanding feature is the large 
increase in the yields of oats after 
tobacco and potatoes resulting from 
the residual effects of the legumes. In 
some years these yields have exceeded 
80 bushels per acre. Wheat and rye 
are less responsive to the residual effects 
of the legumes. The simplest assump¬ 
tion concerning the cause of the marked 
residual action of the legumes on the 
small grains after tobacco and potatoes 
is that tlje latter crops were unable to 
make use of the nitrogen supplied by 
the legumes, thus leaving in the soil 
considerable supplies for the small 
grains. Corn, on the other hand, prob¬ 
ably has appropriated most of the nitro¬ 
gen, leaving but little for the wheat, 
oats, and rye. There is an interesting 
question, however, as to why oats after 
52244—25t-3 
tobacco and potatoes are benefited to a 
far greater extent than are wheat and 
rye through the residual action of the 
legumes. It seems that some factor 
other than the nitrogen supply has lim¬ 
ited the growth of the wheat and rye. 
The comparative effects of tobacco, 
potatoes, and corn on the yields of the 
small grains as influenced by the soiling 
crops are shown graphically in Figures 
15, 16, and 17. 
As to the individual soiling crops, it 
appears that all the legumes have been 
about equally effective as a whole, 
although in some cases oats and rye 
have shown larger gains in yield from 
the' winter legumes than from the 
summer legumes. It is of considerable 
interest to note that the depressing 
action of grass on growth of crops 
immediately following, so plainly 
shown by tobacco and potatoes and to 
a lesser extent by corn, does not extend 
to the succeeding crops of small grains, 
the yields on the grass plots checking 
satisfactorily with those on the fallow 
plots. This indicates that the un¬ 
favorable effects of the grass are only 
temporary. 
RELATIVE EFFECTS OF TOPS AND 
ROOTS OF CERTAIN PLANTS ON 
GROWTH OF TOBACCO 
The results of the preceding field 
tests show that, as compared with such 
crop plants as corn, tobacco is quite 
sensitive to the effects of preceding 
crops. These crop effects are not due 
entirely to the action of the preceding 
crops on the supply of available plant 
nutrients in the soil. In some cases 
unfavorable effects are noted from pre¬ 
ceding crops, even when their above¬ 
ground portions are removed from the 
field in harvesting. It seemed of in¬ 
terest, therefore, to compare the effects 
of the tops and roots of some plants on 
the growth of tobacco, and some pre¬ 
liminary pot and field tests have been 
made along these lines. 
Glazed 6 and 2 gallon earthen jars 
were used for the pot tests. Soil from 
the experimental field which had not 
been previously cropped was used, and • 
no fertilizer was added. With the 
larger jars 8,330 grams of soil, and for 
the smaller jars 2,500 grams were used. 
The moisture content was maintained 
at approximately optimum. In the 
larger pots 200 grams of the plant 
material were incorporated in the soil, 
and 66 grams in the smaller pots. 
The tobacco plants were set out on 
December 26, and harvested March 6. 
Detailed observations were made on 
green weights of tops and roots of the 
