.lowAL of mom research 
Yol. XXX Washington, D. C., June 15, 1925 No. 12 
EFFECTS OF CROPS ON THE YIELDS OF SUCCEEDING 
CROPS IN THE ROTATION, WITH SPECIAL REFER¬ 
ENCE TO TOBACCO 1 
By W. W. Garner, Senior Physiologist in Charge; W. M. Litnn, Assistant Agronomist; 
and D. E. Brown, Principal Scientific Assistant, Tobacco and Plant Nutrition 
Investigations, Bureau of Plant Industry, United States Department of Agriculture 
INTRODUCTION 
The fact that continuous culture of 
any particular crop on the same soil 
quite commonly results, sooner or 
later, in decreased yields must have 
been observed in the earliest days of 
agriculture. Solution of the problem 
as to the cause or causes involved, 
however, has proved to be decidedly 
difficult. Macaire (15), 2 in 1832, re¬ 
marked that in the broadest sense the 
rotation of crops is as old as agricul¬ 
ture itself, having come into practice 
as a matter of necessity, while he 
further stated that the definite formu¬ 
lation of a theory of crop rotation in 
the early part of the nineteenth cen¬ 
tury marked an important advance in 
the science of agriculture. Two prin¬ 
cipal theories have been offered in ex¬ 
planation of the unfavorable influence 
of one plant on another of the same 
or different species which is frequently 
observed when the two are grown in 
association or when the one occupies 
the soil in advance of the other. The 
oldest of these theories, involving the 
idea that a plant may excrete through 
its roots substances capable of exer¬ 
cising varying degrees of toxicity with 
respect to itself and other plants, was 
developed mainly by De Candolle (3) 
in his Physiologie Vegetale, published 
in 1832. Clements, in a recent mono¬ 
graph (4, pp . 144-152), has given a 
comprehensive review of the fairly ex¬ 
tensive literature on the subject of 
toxic exudates of plants and the rather 
closely related conception of soil toxins 
emanating from plant debris, so that 
for present purposes it will suffice to 
refer very briefly to these theories. 
De Candolle based his theory of 
toxic root excretions largely on the re¬ 
sults obtained by Macaire (15). In 
maintaining well-developed plants in 
pure water for a time Macaire found 
that the water acquired toxic proper¬ 
ties and contained substances which 
were regarded as plant excretions. De 
Candolle considered that plants tend 
to affect the soil unfavorably for other 
plants of the same species, genus, or 
family, and drew a distinction between 
this sort of unproductivity of the soil 
and general soil exhaustion affecting all 
crops and due to lack of plant food. 
He also stated that dead plant material 
may be either harmful or beneficial in 
intermediate stages of decomposition, 
but is generally beneficial when more 
fully decomposed. Macaire suggested 
that the very ancient practice of fal¬ 
lowing or resting the land really in¬ 
volves a rotation of cultivated crops 
and adventitious vegetation, and that 
replacement of weeds by useful crops 
in the rotation is to be regarded as a 
distinct step in the progress of scien¬ 
tific agriculture. In the light of the 
results of cropping tests presented in 
this paper, one needs to be fairly spe¬ 
cific both as to crops to be used and 
conditions under which they are to be 
grown in putting into effect this seem¬ 
ingly logical suggestion of Macaire. 
De Candolle’s theory of toxic root 
excretions soon gave way, at least for 
a time, to the “plant-food theory” 
developed by Liebig, according to 
which soil productivity may be meas¬ 
ured primarily in terms of the avail¬ 
able supply of plant nutrients. Differ¬ 
ences in the effects of crops on the 
yields of succeeding crops would thus 
be explainable on the basis of differ¬ 
ences in the kinds and quantities of 
plant nutrients removed from the soil 
by the different crops. For the greater 
part of a century this theory has largely 
dominated field experimentation in soil 
productivity, though it has as a whole 
undergone considerable modification 
during this time. There can be no 
doubt as to the fact that the supply 
1 Received for publication Sept. 12, 1924; issued July, 1925. Cooperative investigations by the Office 
of Tobacco and Plant Nutrition Investigations, Bureau of Plant Industry, U. S. Department of Agricul¬ 
ture, and the Agricultural Experiment Station, University of Maryland. 
2 Reference is made by number (italic) to “Literature cited,” pp. 1132. 
Journal of Agricultural Research, 
Washington, D. C. 
52244—25f-1 
Vol. XXX, No. 12 
June 15, 1925 
Key No. G-494 
(1095) 
