NOVKMBEE 6, 1908] 



SCIENCE 



627 



other crops than wheat no such mainte- 

 nance of yield is to be seen on the unfertil- 

 ized plot of the rotation field— the barley 

 yield has been reduced to 15.8 bushels 

 against 27.7 on the fertilized plot, the 

 clover yield to 940 pounds against 3,780 

 pounds on the fertilized plot, and the tur- 

 nips to as little as 1,600 pounds against 

 40,000 pounds on the fertilized plot. Here 

 we see that with the barley, clover, and 

 particularly with the turnip crop, a rota- 

 tion is quite unable to do the work of the 

 fertilizer, the yield of turnips is reduced 

 to a minimum on the impoverished soil, 

 even though the crop only comes round once 

 in four years and then grows so poorly 

 that it can do little specific excretion to 

 harm the succeeding crop. Many instances 

 could be given of the incapacity of certain 

 plants to grow in soil the fertility of which 

 had been exhausted by other crops; for 

 eiample, at Rothamsted in 1903 swede tur- 

 nips were sown on Little Hoos field, which 

 was known not to have been cropped with 

 swedes or any kindred crop for more than 

 forty years— and the average yield from 

 thirty-two unmanured plots was only 9.3 

 tons per acre, although an exceptionally 

 good start was made by the plant. In the 

 following sea.son barley was grown and the 

 unmanured plots averaged 24.2 bushels per 

 acre, a relatively much higher yield than 

 the swedes had shown — yet barley had been 

 repeatedly grown on the field in the years 

 immediately before it was brought under 

 experiment. 



As it stands at present Whitney's theory 

 must be regarded as lacking the necessary 

 experimental foundation, no convincing 

 evidence has been produced of the funda- 

 mental fact of the excretion of toxic sub- 

 stances from plants past the autotrophic 

 seedling stage, nor is there direct proof of 

 the initial supposition, that all soils give 

 rise to soil solutions sufficiently rich in the 

 elements of plant food to nourish a full 

 crop did not some other factor come into 



play. If, however, we give the theory a 

 wider form, and, instead of excretions from 

 the plant, understand debris of any kind 

 left behind by the plant and the results of 

 bacterial action upon it, we may thereby 

 obtain a clue to certain phenomena at 

 present imperfectly understood. The value 

 of a rotation of crops is undoubted and in 

 the main is explicable by the opportunity 

 it affords of cleaning the ground, the free- 

 dom from any accumulation of weeds, in- 

 sect or fungoid pests associated with a par- 

 ticular crop, and to the successive tillage of 

 different layers of the soil, but for many 

 crops there remains a certain beneficial 

 effect from a rotation beyond the factors 

 enumerated. 



The Rothamsted experiments have shown 

 that wheat can be grown continuously upon 

 the same land for more than fifty years and 

 that the yield when proper fertilizers are 

 applied remains as large in the later as in 

 the earlier years of the series, any decline 

 that is taking place is hardly outside the 

 limits of seasonal variation and can easily 

 be accounted for by the difficulties of till- 

 age and the increase of one or two trouble- 

 some weeds. Mangolds again in the Roth- 

 amsted experiments show no falling off in 

 yield, though they have now been grown 

 upon the same land for thirty-two years, 

 but with the barley crop, despite the appli- 

 cation of fertilizers, there is a distinct secu- 

 lar decline in the yield. Again, it was 

 found impossible to obtain satisfactory 

 crops of swede turnips upon the same land 

 for more than ten or twelve years in suc- 

 cession, and clover is well known to render 

 the land "sick" for its own renewed growth 

 for a period of from four to eight years on 

 British soil. In this last case the persist- 

 ence of the resting stages of the sclerotima 

 disease in the land may be the determining 

 factor, but there are other crops, e. g., flax, 

 hemp and strawberries, which are consid- 

 ered by the practical cultivator to render 

 the land more or less "sick," so that their 



