X.] VALUE OF ROTATION OF CROPS 297 



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 applica- 

 tion of fertilisers, there is a distinct secular 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 succession, 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 persistence 

 of the resting stages of the sderotinia disease in the 

 land may be the determining factor, but there are other 

 crops, e.g., flax, hemp, and strawberries, which are con- 

 sidered by the practical cultivator to render the land 

 more or less " sick," so that their growth cannot profit- 

 ably be renewed until an interval of some years has 

 elapsed. 



Again, it is well known that when a plant is sown 

 upon land which has not carried that particular crop for 

 many years beforehand, it starts into growth with a 

 vigour it rarely displays upon land where it forms an 

 item in the regular rotation, even though the new land 

 is so impoverished that the final yield is indifferent 

 In the instance quoted above, where Swedes were 

 sown on the Little Hoos field after a very long 

 interval, although the yield was poor on the unmanured 

 plots yet the seeds germinated and made their early 

 growth in a very remarkable fashion, incomparably 

 better than did the same seed sown upon adjoining 

 land in a high state of fertility, but which had been 

 cropped with Swedes from time to time previously. 

 There is thus some positive evidence that most plants 

 — some to a very slight degree, like wheat and mangolds, 

 others markedly, like clover, turnips, and flax — effect 

 some change in the soil which unfits it for the renewed 



