

HORTICULTURAL RESEARCH 497 



substances poisonous to the tree-roots ; or the poisonous effect 

 might be due to an alteration in the bacterial contents of the soil. 



Independently of anything coming from the grass-roots or 

 resulting from their growth, it seemed possible that the grass 

 might abstract something from the soil and alter the proportions 

 of the constituents remaining so as to render the soil virtually 

 toxic. This suggestion, however, has been negatived by some 

 recent experiments in which the grass was grown in such a way 

 that it was impossible for it to draw anything out of the soil in 

 which the trees were growing. These trees were planted in 

 pots and the grass was grown in movable trays resting on the 

 soil in the pots ; the trays were perforated to allow of drainage 

 from them down to the trees but the holes were covered with 

 fine gauze to prevent the grass-roots from passing through and 

 thus there could be no passage of water upwards from the pots 

 to the trays. Yet in spite of this entire separation of the grass 

 from the tree, the grass-effect was still very noticeable and 

 caused a reduction of growth amounting to some 25 per cent. 

 These experiments have since been extended to a study of the 

 effect of grass on other plants besides trees and in every case 

 examined up to the present, a similar action has been observed : 

 in the case of barley the reduction of growth amounted to 15 per 

 cent. ; in that of tomatoes to 46 per cent. ; in that of mustard to 

 58 per cent, and in the case of tobacco to 71 per cent. Some of 

 the results in the last two cases are shown in fig. 3. One other 

 important point in connexion with these experiments should be 

 mentioned, that when the grass is grown in trays as in the 

 preceding experiments and the washings, instead of being allowed 

 to pass immediately to the tree-roots, are left for some time 

 exposed to the air before being used on the tree, their action, 

 instead of being hurtful, is decidedly beneficial; apparently the 

 toxic substance is oxidised and converted into plant-food. 



The proposition which has been made to account for these 

 facts— it cannot at present be termed more than a proposition — 

 is that the growth of grass and probably also of other crops, 

 gives rise, either directly or indirectly, to the formation of some 

 substance in the soil which is toxic towards plant-growth 

 but which, on oxidation, becomes harmless and when oxidised 

 serves to render the soil richer, probably both in organic matter 

 and nitrogen. While the grass is actually growing, there would 

 be a continuous supply of this toxin, which would prevent the 



