CHAPTER XXVII 



POSSIBLE CAUSES OF THE GRASS EFFECT 

 (Reports, III, 2 ; XIII, i) 



IN the preceding chapter an account has been given of the 

 main features presented by the action of grass on trees ; the 

 present chapter contains a review of the search made at Woburn 

 for an explanation of this action. The more obvious sugges- 

 tions are, naturally, those based on an interference with the 

 water- or food-supply required by the tree ; but, before discussing 

 these, it will be well to clear the ground by some account of the 

 examination of other possible causes. 



The aeration of the soil is no doubt different under grass from 

 what it is in tilled ground, but this appears to have little or no 

 value in determining the action of the grass. Trees planted in 

 meadow land, with the soil round the roots aerated to different 

 extents by making and keeping open holes in the ground, 

 showed no differences corresponding with the extent of the 

 aeration. Thus 



No Slight Much Turf 



holes. aeration. aeration. removed. 



Tree-Vigour . 55 45 57 98 



These experiments (III, 39) were made at Harpenden, whilst 

 similar ones were conducted at Ridgmont, and with like results 

 (III, 29). At this latter place there were also other experiments 

 in which the air-supply was reduced below that which must 

 obtain in grassed soil, by driving down into the ground round 

 the trees, two feet from their stems, iron hoops, reaching to a 

 depth of eighteen inches below the surface, and covering the 

 earth within the hoops with a layer of two inches of cement, 

 so that no air (or water) could reach the trees, except that which 

 made its way through the stiff clay subsoil below them. Yet 

 such drastic treatment failed to reproduce any of the evil effects 

 of grass, and, indeed, until the soil within the drums became 

 exhausted, these trees did even better than those in tilled ground, 

 probably owing to the enclosed soil being less subjected to 



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