1132 
Journal of Agricultural Research 
Vol. XXX, No. 12 
The comparative effects of the three 
crops, tobacco, potatoes, and corn, on 
succeeding crops have been studied by 
growing each of these three crops in 
(1) continuous culture and in alterna¬ 
tion with the other two; (2) rotation 
with each of the small grain crops, 
wheat, oats, and rye; and (3) rotation 
with each of the three small grain 
crops, including hairy vetch, crimson 
clover, cowpeas, soy beans, and a 
mixture of grasses as soiling crops in 
the rotations. Except where the soil¬ 
ing crops were used, four different 
fertilizer treatments have been used in 
each system of cropping. Although 
the potato crop removes from the soil 
only small quantities of plant nutrients, 
while corn removes much larger quan¬ 
tities, tobacco for the most part has 
resembled potatoes rather than corn in 
its effects on succeeding crops. In con¬ 
tinuous culture, tobacco has shown a 
tendency to decline in yield, but more 
recently the growth after corn has been 
poorer than in continuous culture. 
Potatoes have given best yields 
after tobacco and poorest yields after 
corn. The corn crop has not been 
greatly affected by preceding crops. 
In comparison with tobacco and po¬ 
tatoes, the corn crop has decidedly 
reduced the yields of succeeding crops 
of wheat and oats, and, to a lesser ex¬ 
tent, rye. In earlier years, the growth 
of small grains after potatoes was con¬ 
siderably better than after tobacco, 
but this difference has not been fully 
maintained. Addition of legumes to 
the cropping system has intensified 
rather than overcome the differences in 
effects of tobacco, potatoes, and corn 
on the small grains. The effects of vetch 
crimson clover, and cowpeas on to¬ 
bacco have been the same as noted in 
the previous paragraph. Soy beans 
have given especially poor results with 
tobacco and potatoes. Grass as a soiling 
crop has markedly reduced the yield of 
tobacco and potatoes, and to a lesser ex¬ 
tent, the yield of corn. Neither tobac¬ 
co nor potatoes have been able to effec¬ 
tively utilize the nitrogen of the legumes. 
Corn and the small grains, on the other 
hand, have been greatly benefited by 
the legumes, readily taking advantage 
of the nitrogen furnished by them. 
The results of these cropping tests 
furnish a group of crop effects which 
are not wholly explainable either on 
the basis of the plant-food theory or 
of parasitic disease. These crop effects 
are of a transitory character, in so far 
as they are toxic or injurious, and are 
markedly influenced by the character 
of the soil and the weather conditions. 
They are also influenced to some extent 
by fertilizers and lime. Preliminary 
field and pot tests with tobacco indi¬ 
cate that the injurious effects of pre¬ 
ceding crop plants come mostly from 
the roots rather than the tops of these 
plants. If these results are confirmed, 
they would seem to suggest new prob¬ 
lems in the matter of most effectively 
supplying the soil with organic matter. 
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