POSSIBLE CAUSES OF THE GRASS EFFECT 287 



influence it was always raised in the trays two or three weeks 

 before the tree started into activity, and not placed in the pot 

 till it was in full growth. 



The Food Supply. The question of the food-supply is almost 

 identical with that of the water-supply, though not quite so, 

 for, though plants absorb their nourishment mainly by absorbing 

 the soil-solution as a whole, they do exercise a certain amount 

 of selective absorption, abstracting some of the constituents 

 from the soil-solution, and leaving others. It was possible, there- 

 fore, that the grass might act on trees by depriving the trees of 

 some substance necessary for their welfare. The experiments 

 described above prove, however, that it is not what the grass 

 abstracts from the soil, but what it adds to it, that does the 

 damage. But before such proof had been obtained various 

 investigations on the food-supply had been undertaken. 



In all the field experiments there was, to start with, a strong 

 probability against there being any interference with the food- 

 supply due to the grass, for the crop of grass was never removed 

 from the ground, and such food-material as was contained in the 

 crop actually growing at the time would represent the sum total 

 of the impoverishment of the soil; but this impoverishment 

 would be far less than that brought about by the growth and 

 cropping of the trees themselves, and it has been found that 

 such growth and cropping can be continued in the unmanured 

 Ridgmont soil for twenty-two years without effecting any 

 reduction in the vigour of the trees (p. 90). 



Additional evidence was also supplied by an examination 

 of trees in the numerous manurial experiments at the farm, 

 for the manurial plots abutted on paths which had been grassed, 

 and the action of this . grass began to tell on the end trees 

 of the plots after a time : but the particular trees affected 

 bore no relation to the manurial treatment of the plots: the 

 percentages of trees suffering were 33, 56 and 40 in the case of 

 plots receiving an excess of manure, a moderate dressing, and a 

 deficiency, respectively (XIII, 71). 



The supply of extra nourishment to trees under grass would 

 appear to be equally ineffectual in counteracting the effect of 

 the grass. Trees grown with a weekly dressing of manure water, 

 conveyed to the roots below the grass in the manner described 

 on p. 284, produced the same effect as water containing no 

 nutrient; and, though trees planted with a liberal supply of 

 dung around their roots suffered from grass less than those 

 without such a dressing, they still showed only half the vigour 



