Vol. XVII. No. 428. 



THE AGRICULTURAL NEWS. 



301. 



THE EFFECT OF ONE GROWING PLANT 

 ON ANOTHER. 



From time immemorial gardeners have been convinced 

 that certain plants injure others and, in many cases, it is 

 firmly believed that the harmful effect remains in the soil 

 for months, it not for years. This has led to the opinion 

 that certain plants excrete something from their roots which 

 is poisonous to other plants of the same kind, though not 

 necessarily so to those of a different kind. For a long time 

 the plant was considered as completely analogous to the 

 animal, and, thus the scientist agreed with the practical man 

 in admitting the existence of a poisonous excretion in plants. 

 Of recent years, however, much doubt has been thrown on 

 the idea of a poisonous excretion, and serious obstacles have 

 been shown to hinder its acceptance- 'In a good grass field, 

 for example, the plants are as crowded as they can be, yet 

 they show no signs of 'sickness' or poisoning. If the soil be 

 poor the plants may go hungry, but this may be remedied by 

 applying suitable fertilizers; there is nothing in the appear- 

 ance of the plants to suggest that any other factor is con- 

 cerned. 



On the other hand, some years ago Dr. Whitney, Chief 

 of the Bureau of Soils of the United States Department of 

 Agriculture, expressed the opinion that plants do excrete a 

 toxic substance which may, however, be precipitated or 

 iiendered inactive by fertilizers. Therefore the improvement 

 of plants by fertilizers is due, not ■ only to the food they 

 •supply, but also to the above-mentioned action, and perhaps 

 to others as well. Whitney s hypothesis gave rise to much 

 discussion, which led to a great deal of progress being made 

 on the subject. 



Dr. E. J. Eussell, discussing the subject in a recent 

 Issue of Gafdciiers' Chronicle (January 1918), observes that 

 British investigators have usually taken the view that there is 

 no evidence ©f a persistent toxic excretion. The experiments 

 at Rothamsted seem to bear this out. At the present time 

 the famous Broadbalk field is carrying its 7-3th successive 

 crop of wheat, and the plants look as well as any on the 

 farm, and better than a good deal of the wheat in tl'e district. 

 The last crop of mangolds was the 42nd; it was well above the 

 average, and has rarely been exceeded during the whole 

 period. Similarly, barley has been grown for fifty-seven years 

 in succession without showing any signs of suffering- Legumi- 

 nous crops, however, cannot be grown in this way, and, after 

 a short period, fail; they are the only crops which experi- 

 mental evidence has shown cannot be grown year after year 

 on the same land. Observations show, nevertheless, that 

 other plants also fail in the same way; thus foxglove grown 

 splendidly in the soil of a freshly cleared wood (provided the 

 soil is suitable, e-g , the clay patches on the Downes) but for 

 one year only, not longer. It is also said that flax and 

 onions may fail if grown too often in the same soil. These, 

 however, are all simply observations which, even if exact, 

 may have some other explanation. 



The idea that plants excrete poisonous substances has 

 been investigated by Mr. Spencer Pickering. The growth of 

 plants was found to be considerably decreased if they 

 received water which had washed. part of the roots of 

 another growing plant. This eti'ect seems to be general; 

 the washings from the roots of mustard check the growth 

 of mustard; those from grass check the growth of fruit 

 trees, and so on- It was possible to establish the important 

 point that these washings lose their poisonous quality very 

 rapidly, so that they do not necessarily affect the soil after 

 plant growth has ceased. These experiments are, therefore, 

 perfectly consistent with those at Rothamsted described 

 above. 



Another set of P.othamsted experiments is, however, 

 ?!°'"«r •?'^""^ '° reconcile with Mr. Pickering's result. 

 J)r. Wmifred Brenchley grew wheat alone, weeds alone and 

 wheat mixed with wheat; she observed that when poppy 

 (Japaver rhotas), black bent {Alopmcrus agrestis), and 

 (Spergula aruensis) were grown ' with wheat they made 

 less growth than when grown alone; on the other 

 hand, wheat made more growth per individual plant. 

 Ihis, of course, does not mean that wheat should 

 always be grown with weeds; the plants would have 

 done better had no weeds been present, but they suffered 

 less from the presence of the weeds than they would 

 have done from an equal number of wheat plants. In these 

 experiments spurry proved more harmful than the other weeds, 

 because by its straggling habit it badly checked the young 

 wheat, which never recovered properly. Charlock and wheat 

 settle down to some sort of equlibrium as neither masters 

 the other. 



So far as could be seen; the effect was solely one of 

 competition for food, and it made no difference to the indi- 

 vidual wheat whether it competed with another wheat plant 

 or a plant of a completely different order. The whole 

 phenomenon could be explained by the supposition that 

 the number of plants the soil can carry depends on the 

 amount of plant food present in the soil, and the amount of 

 space available for growth; if the food and space are to be 

 divided, each individual will get a smaller share and will, 

 consequently, make less growth than if they were fewer 

 plants present. At first sight these results seem difficult 

 to reconcile with those of Pickering's experiments, which 

 seem to prove that a large number of plants suffer not 

 only from starvation, but also from mutual poisoning, 

 so that growth would be less both individually and collec- 

 tively than when a smaller number is grown. The apparent 

 disagreement may, however, be explained. In another of 

 Mr. Pickering's experiments, plants grown in plots divided 

 into compartments so that each individual root was kept 

 separate from its neighbour made no better growth than did 

 plants in undivided plots where the roots of the plants mixed 

 freely. Thus, the toxin produced by one individual plant 

 does it as much harm as that produced by its neighbour. 

 Further, Mr. Pickering found, in open soil, that the total 

 growth was the same whatever the number of plants (within 

 certain limits of distance apart), or, in other words, that the 

 weights of the plants were inversely propDrtioned to the bulk 

 of soil available. This is in full agreement with Dr- Brench- 

 ley's results, and may be explained perfectly well, without 

 assuming the existence of a toxin, simply by the fact that the 

 full crop-bearing capacity of the soil has been reached. If, 

 with Mr. Pickering, a toxin is assumed to be present, it must 

 be supposed to be at le;ist as harmful to the plant itself as to 

 any other. This assumption involves possibilities which new 

 experiments should investigate. 



Before the war India exported annually 20,0')'), 000 raw 

 and 7,500,000 tanned goatskins. The United Kingdom 

 purchased most of the tanned skins, about half of which were 

 re-exported to the C'ontinent, and about a third of the 

 remainder to America. Of the raw skins 7-"» per cent, were 

 exported to America, 10 per cent, to England, 7 per cent, to 

 France, -5 per cent, to Holland and Belgium, and an insig- 

 nificant quantity to Germany. Since the war America takes 

 8-5 -.S per cent., and Kn|[;lish imports have dropped to 8 per 

 cent. (The Monthly Bulletin oj Agricultural Litelligtnce 

 and Plant Diseases, -June 1918.) 



