626 



SCIENCE 



[N. S. Vol. XXVIII. No. 723 



the same plant only recurs after an in- 

 terval long enough to permit of the de- 

 struction of its particular self-formed 

 toxin, its yield will be maintained without 

 the intervention of fertilizers. The func- 

 tion of fertilizers is to precipitate or to put 

 out of action these toxins, and various 

 bodies such as lime, green manure and 

 ferric hydrate are also effective in this 

 direction; the same result of destruction 

 of the toxins excreted by the plant may 

 even be brought about by minute quanti- 

 ties of certain bodies like pjTogallol. Ac- 

 cording to this theory the function of fer- 

 tilizers is to remove toxins rather than to 

 feed the plant; they are only required 

 when the same crop is gTown continuously 

 and the need for them may be obviated 

 by a judicious rotation which permits of 

 the destruction of the toxins by natural 

 causes. Careful consideration will show 

 that this theory can be made to fit a good 

 many of the phenomena of plant nutrition ; 

 it would also explain the difficulties experi- 

 enced in growing certain crops continu- 

 ously on the same ground ; it is, in fact, an 

 elaborated revival of one of the earliest 

 explanations of the value of rotations, 

 originally suggested by de CandoUe. 

 Furthermore, Whitney's colleagues have 

 succeeded in extracting certain substances 

 from the soil — oxystearic acid, pyridin 

 derivatives, tyrosin, etc., which when in- 

 troduced into water cultures are toxic to 

 seedling plants. The compounds isolated 

 are, however, all of them products of the 

 oxidation and decay of proteins, fats and 

 other compounds contained in plant resi- 

 dues; there is no evidence to show that 

 they are specific excretions from particular 

 plants or that they are more abundant in 

 soil impoverished by the growth of a par- 

 ticular crop than in soil which would be 

 usually termed rich. Again, it has not 

 been demonstrated that such substances, 

 although harmful to young plants in water 

 culture, are toxic under soil conditions; 



it is well known how exceedingly sensitive 

 are plants in water culture, where growth, 

 for example, is inhibited by traces of cop- 

 per not to be detected by ordinary methods 

 of analysis. A body like ammonia, itself 

 a product of protein decay and present in 

 the soil, is exceedingly toxic to water cul- 

 tures, yet when applied to the soil it in- 

 creases the growth of the plant. Turning 

 to the fertilizer side of the theory, evi- 

 dence is yet lacking to show that fertilizers 

 in such dilute solutions as they form in 

 the soil water can exert any precipitating 

 or destructive action on such toxic sub- 

 stances as have been extracted from the 

 soil ; particularly it is the specific action of 

 fertilizers which is difficult to explain. 

 Why should substances so dissimilar as 

 nitrate of soda and sulphate of ammonia 

 exert the same sort of action on the same 

 toxin? Why should phosphates cause all 

 classes of plants to develop in one direc- 

 tion, or why should it be appropriate to the 

 toxins of all plants on one particular type 

 of soil, whereas potash answers on another 

 soil type? Lastly, there is a lack of evi- 

 dence for the fundamental thesis that the 

 rotation will take the place of fertilizers 

 and that the yield only falls off when a 

 particular crop is grown continuously on 

 the same land. 



On the rotation field at Rothamsted the 

 yield of wheat on the unfertilized plot has 

 been remarkably maintained; for the last 

 five courses (tenth to fourteenth of the 

 whole series) it has averaged 26.2 bushels 

 per acre, but it is below the yield of the 

 fertilized plots on the Broadbalk field, 

 which averaged 35.7, 32 and 39.7 bushels 

 for the same years, and also below the 

 fertilized plot on the same rotation field, 

 which averaged for the same period 37.1 

 bushels per acre, although the fertilizer is 

 only applied once in four years to the 

 swedes, which are followed by barley and 

 either clover or a bare fallow before the 

 turn of the wheat comes round. But with 



