Febbuart 3, 1911] 



SCIENCE 



173 



would depend in part on the degree of the 

 accumulation of the toxic substances in the 

 soil. If the toxic substances were removed 

 or destroyed sufficiently rapidly, no dele- 

 terious result would occur; but, otherwise, 

 there would be one. We should also con- 

 clude that the secretions from the wheat 

 would not necessarily be toxic to other 

 crops. These a priori predictions seem to 

 be confirmed by the facts. 

 Cameron^* points out that 



It has been found that as a general rule the 

 continued growth of one crop in any soil results 

 in a low crop production. Pot cultures have 

 given even more pronounced results in the same 

 direction. The explanation long accepted is that 

 the soil has, as a result of the continued cropping, 

 become deficient in one or more of the " avail- 

 able" mineral nutrients. Pot experiments, where 

 the garnered crop was returned to the soil and 

 still diminished yield was obtained, throw doubt 

 on this explanation. Still further doubt results 

 from water cultures which, by growing a crop in 

 them, become " poor " for subsequent crops, al- 

 though there is maintained in them an ample 

 supply of mineral plant nutrients, and they are 

 easily renovated by good adsorbers. These facts 

 find a more satisfactory explanation as being due 

 to the production in the nutrient medium of 

 deleterious organic substances originating in the 

 growing plant itself. This idea seems to have 

 been advanced first by De Candolle, in 1832, to 

 account for the beneficial results obtained by 

 employing a rotation of crops. It appears to 

 have been held by Liebig at one time, although 

 he subsequently abandoned it in favor of the view 

 that the benefits of a crop rotation are due to the 

 several crops requiring different proportions of 

 mineral nutrients, and that the disturbance of 

 the balance in the soil produced by one crop is 

 not unfavorable to the growth of some olher crop. 

 Although lacking direct experimental confirma- 

 tion, this latter view of Liebig's has long pre- 

 vailed among agricultural investigators, partly 

 by reason of his authority, partly by reason of 

 the dominance of the plant food theory of fertil- 

 izers, and partly by reason of the fact that the 

 ideas of De Candolle, as originally advanced, in- 

 clude certain errors soon detected. The trend of 

 recent investigations has been distinctly in favor 



"Jour. Phys. Chem., 14, 425, 1910. 



of a modified form of the view of De Candolle. 

 It has been recognized that other factors enter 

 into crop rotations, such as the elimination of 

 associated weeds, various kinds of animal, insect 

 and plant parasites, preparation of the soil by a 

 deep-rooted crop for a shallow-rooted following 

 crop, etc. It has come to be recognized that there 

 are natural associations of plants and natural 

 rotations of vegetation certainly determined by 

 other than plant-food factors. Thus, in the east- 

 ern United States, wheat is followed by ragweed 

 naturally, while, across the fence cocklebur and 

 wild sunflowers come in after the corn, the differ- 

 ence in vegetation being as sharply marked after 

 the removal of the crops as when ihey still occu- 

 pied the land. Analyses of the ragweed, for in- 

 stance, altnough it is a shallower rooted crop 

 than wheat, show that it takes from the soil as 

 much of the mineral nutrients as does the pre- 

 ceding wheat crop. The investigation of Lawes and 

 Gilbert on fairy rings showed that the continual 

 widening of the rings can not be satisfactorily 

 explained by the comparison of the mineral con- 

 stituents in the soil within and without the rings. 

 Work at Woburn on the effect of grass on apple 

 trees finds no other plausible explanation than 

 that the growing grass produces in the soil 

 organic substances detrimental to young apple 

 trees. A number of similar cases have been 

 recorded. 



CLIMATE 



When seeds are planted in a new local- 

 ity, a great many conditions are varied 

 simultaneously and it is difficult, usually 

 impossible, to tell to what extent each fac- 

 tor causes the changes that actually take 

 place or why any given change is benefi- 

 cial. If we compare two varieties of the 

 same plant, it seems reasonable to suppose, 

 unless proof to the contrary is given, that 

 the local variety is fairly well adapted to 

 the local conditions, in which ease one 

 would expect the foreign variety to vary 

 towards the local one. Darwin''^ cites a 

 case of this sort. 



The effects of climate of Europe on the Amer- 

 ican varieties [of maize] is highly remarkable. 

 Metzger obtained seed from various parts of 



"* " Animals and Plants under Domestication," 

 2d ed., 1, 340, 1890. 



