1096 
Journal of Agricultural Research 
Vol. XXX, No. 12 
of available nitrogen and mineral 
nutrients in the soil is one of the im¬ 
portant factors in crop yields. On the 
other hand, inability of investigators 
to arrive at a satisfactory basis for 
generalization in the use of fertilizers, 
after an enormous amount of experi¬ 
mentation, has brought about a realiza¬ 
tion that there are other factors in soil 
productivity of fundamental impor¬ 
tance. Thus, at the beginning of the 
present century there was a revival in 
modified form of the theory of soil 
toxins wherein these toxins are con¬ 
sidered as of vegetable origin but not 
necessarily consisting of excretions of 
the living plant. 
Among the leaders in the develop¬ 
ment of the newer theory of soil toxins 
were Bedford and Pickering in Eng¬ 
land, and Whitney, Cameron, and Liv¬ 
ingston in this country. While Liebig 
was developing his plant-food theory 
other investigators were pointing out 
that Macaire’s supposed toxic root ex¬ 
cretions might well originate merely 
from exfoliated root material, and, in 
fact, De Candolle himself, as already 
stated, had included consideration of 
the effects of plant debris in outlining 
his theory. Bedford and Pickering ( 1 ) 
made an extended study of the harm¬ 
ful effects of griss on the growth of 
fruit trees, and also made observations 
on many other plants. Whitnejr and 
Cameron ( 20 ). and Livingston, Britton, 
and Reid ( 14 ) found that certain un¬ 
productive soils as well as their aqueous 
extracts contained substances toxic to 
plants, while their toxicity was reduced 
by addition of certain forms of organic 
matter, oxidizing agents, and other 
chemicals, including plant nutrients. 
Schreiner and his associates ( 16 , 17 ) 
isolated from soils several organic com¬ 
pounds which were found to be toxic 
to plants. 
Mention is made also of a recent ex¬ 
tension of the theory of soil toxins to 
include the action of such inorganic 
toxic substances as soluble salts of 
aluminum and iron, more particularly 
as a feature of the soil condition spoken 
of as acidity. There has been consid¬ 
erable controversy between adherents 
of the newer theory of soil toxins and 
those favoring the plant-food theory, 
and this has brought about a great 
deal of valuable experimentation. In 
going over the published data on the 
subject one can scarcely escape the con¬ 
clusion that in many instances inves¬ 
tigators have gone too far in their inter¬ 
pretations of results obtained in reason¬ 
ing from the particular to the general, 
while, on the other hand, some have 
apparently erred in placing unwar¬ 
ranted restrictions in their interpreta¬ 
tions of available data. It appears 
that further work is needed to reach a 
final conclusion as to the exact bearing 
of these two theories on soil produc¬ 
tivity. 
In recent years plant parasitism has 
come to be recognized as an important 
factor in soil productivity. As a rule, 
parasitic microorganisms are more or 
less specific in their ability to attack 
higher plants, so that continuous 
culture of the same or closely related 
crops would favor development of 
parasitic disease and thus reduce crop 
yields. Many cases of this sort have 
been brought to light in recent years. 
Ordinarily the causal organism can be 
readily recognized, but this is not 
always the case, and definite exclusion 
of parasitism as a factor in the un¬ 
favorable influence of one crop on 
another is not always a simple matter. 
Parasitism alone, however, hardly 
furnishes a sufficient basis for general¬ 
ization concerning the effects of crop 
plants on others growing in association 
or in rotation. Finally, while it is 
now known that nonparasitic soil 
microorganisms may greatly affect soil 
productivity, this fact does not seem 
to call for a separate theory of general 
applicability to explain crop effects. 
As to field experimentation dealing 
specifically with the effects of one 
crop on another, Daubeny (6) in 
England was the first to undertake an 
extended study of the subject. Oats, 
barley, flax, tobacco, potatoes, beans, 
clover, and other crops were grown on 
small plots over a period of some ten 
years in continuous cultures and in 
rotation. As an average for all crop¬ 
ping combinations, rotation gave better 
yields than continuous culture for 
most crops, but in many instances this 
was not true of individual crop combi¬ 
nations. Daubeny’s work has been 
cited in support of the plant-food 
theory, but Daubeny himself took a 
very conservative attitude in the 
matter, although he found little evi¬ 
dence of harmful root excretions. 
Bedford and Pickering found that 
clover as well as grass may adversely 
affect the growth of fruit trees, whereas 
weeds do also but to less extent than 
grass. It was concluded that the 
toxic action does not accumulate in 
the soil and lasts only while the surface 
crop is growing. Some varieties of 
apples were affected more than others. 
In supplementary tests with specially 
devised pots, various crops were af¬ 
fected in varying degrees by the teach¬ 
ings from other growing plants. Hed¬ 
rick ( 11 ) observed that on a rich soil 
