Scientific Agriculture. 



238 



[March, 1912. 



nitrates 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, therefore, 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 grouud for 

 some distance around it. * It is a 

 commonplace among practical men that 

 ground becomes " sick " if a crop is 

 grown too frequently. Careful examin- 

 ation, however, always shows complic- 

 ating factors at work. Many years ago 

 Daubeny [9] investigated certain cases 

 at Oxford from the standpoint of de 

 Oandolle's excretion hypothesis, but 

 failed to find any evidence of the exist- 

 ence 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, t 



On the Broadbalk field at Rothamsted 

 wheat has been grown continuously 

 since 1843, but 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 keep- 

 ing down weeds because the opportun- 

 ities of "clearing " 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 culti- 

 vations involved soon cause the form- 

 ation of a hard layer of soil— technically 

 a " ploughsole "— some 5 inches below 

 the surface. Continuous mangold grow- 

 ing on Barnfield has brought a diminu- 

 tion of the soil organic matter, so that a 

 tilth can only be got with difficulty and 

 the soil cakes very badly after rain. If 

 a storm happens to come any time 

 after the young plant is through, and 

 before it has grown very much, there is 

 great danger that it may be beaten 

 down into the sticky soil and not pick 

 up again ; an excellent illustration is 

 afforded during the current season. 

 These cultivation difficulties are very 

 serious from the practical point of view 

 and effectually prevent continuous 

 cropping with any one plant except 

 where a liberal amount of organic 

 matter is present in the soil, when till- 

 age is a much simpler matter. A rotation 

 is therefore always followed in practice 

 —the benefit to the crop is seen in the 

 following table of wheat yields at 

 Rothamsted : — 

 and Total Grain lb. per Acre. 





Rotation. 

 (Agdell Field.) 

 Unmanured since 

 1848. 



Alternate. 

 (Hoos Field.) 

 Unmanured since 

 1851. 



Continuous. 



(Broadbal 

 Unmanu 

 18c 



k Field.) 

 red since 

 S9. 



Plot. 6. 

 artificial 

 including 

 per acre pi 



Complete 

 manure 

 43 lb. N 

 3r annum. 





Total 

 produce. 



Total 

 grain. 



Total 

 produce. 



Total 

 grain, 



Total 

 produce. 



Total 

 grain. 



Total 

 produce, 



Total 

 grain. 



1907 ... 

 1903 ... 

 1899 ... 

 1895 ... 

 1891 ... 

 1887 ... 

 1883 ... 

 1879 ... 

 1875 ... 

 1871 ... 



lb, per 

 acre. 

 3,459 

 3,219 

 4,785 

 3,(166 

 4,868 

 4,689 

 5,140 

 2,162 

 4,412 

 3,004 



lb. per 

 acre. 

 1,066 

 1,255 

 1,704 

 1,436 

 1,927 

 2,184 

 2,146 

 669 

 1,579 

 929 



lb. per 

 acre. 

 3,094 

 2.111 

 2,620 

 2,129 

 3,645 

 2,365 

 2,461 

 1,187 

 2,718 

 1,892 



lb. per 

 acre. 

 849 

 915 

 1,004 

 978 

 1,404 

 1,153 

 1,160 

 379 

 993 

 605 



lb. per 

 acre. 

 1,715 

 1.078 

 1,825 

 1,384 

 2.142 

 1,801 

 1,878 

 1,093 

 1,575 

 1,715 



lb. per 

 acre. 

 618 

 485 

 769 

 664 

 828 

 906 

 872 

 33C 

 567 

 615 



lb. per 

 acre, 

 5,073 

 3,175 

 3,182 

 2,916 

 4,466 

 3,329 

 4,386 

 2,283 

 3.073 

 3,386 



lb. per 

 acre. 

 1,554 

 1,094 

 1,274 

 1,590 

 1,568 

 1,477 

 1,781 

 691 

 1,065 

 1,089 



Aver- 

 age. 



3,880 



1,490 



2,422 



944 



1,621 



665 



3,527 



1,298 



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 Buffer no vegetable to grow within a certain distance of its seat. 

 •j-The latest evidence is however in favour of such an effect.— Ed. T.A, 



