148 SCIENCE PROGRESS 



tend to disappear. Soil, we know, is the seat of many far- 

 reaching changes. Thus we have never got a perfectly clear 

 issue : the experiment has always been complicated by other 

 changes taking place simultaneously. Our conclusion, there- 

 fore, is that there is no unexceptionable evidence of any toxic 

 excretion by plant roots. 



Many statements may be found in agricultural literature 

 to the effect that certain crops poison others, or even young 

 plants of the same kind, perhaps the most picturesque being 

 an account many years ago by Sir Hans Sloan of a wonderful 

 Eastern plant, the Scythian Lamb, that devastated the ground 

 for some distance around it. 1 It is a commonplace among 

 practical men that ground becomes "sick" if a crop is grown 

 too frequently. Careful examination, however, always shows 

 complicating factors at work. Many years ago Daubeny [9] 

 investigated certain cases at Oxford from the standpoint of 

 de Candolle's excretion hypothesis but failed to find any evidence 

 of the existence of toxins, and his work still remains among the 

 best we have on the subject. Even the strikingly harmful effects 

 of grass land on fruit trees demonstrated by Pickering's well- 

 known experiments have not been shown to be the result of any 

 toxic excretion. 



On the Broadbalk field at Rothamsted wheat has been 

 grown continuously since 1843, Dut there is no sign that the 

 unmanured plot is suffering from a toxin ; the young plant, 

 which ought to show symptoms of poisoning if a toxin were 

 present, is always healthy and develops normally. But there 

 is the greatest possible difficulty in keeping down weeds 

 because the opportunities of "cleaning" land under continuous 

 wheat culture are not great, and no practical man could 

 possibly bear the expense such a system entails. Again, 

 continuous turnip growing leads to difficulties because the 

 particular cultivations involved soon cause the formation of 

 a hard layer of soil — technically a " ploughsole " — some 5 inches 

 below the surface. Continuous mangold growing on Barn- 

 field has brought about a diminution of the soil organic matter 

 so that a tilth can only be got with difficulty and the soil cakes 



1 See Evelyn, Terra. " This vegetable is called the Tartarian lamb from its 

 resemblance in shape to that animal. It has something like four feet, and its body 

 is covered with a kind of down. Travellers report that it will suffer no vegetable 

 to grow within a certain distance of its seat." 



