494 SCIENCE PROGRESS 



in which trees were grown in pots, the condition of moisture 



being so regulated that it was the same whether or no grass 



was present. 



The Food Supply 



Similar experiments in pots supplied the most conclusive 

 evidence that the grass-effect is not explicable as a consequence 

 of the lack of the recognised food material of plants any more 

 than it is by lack of water. In some of these experiments the 

 grass-roots were effectually prevented from coming into contact 

 with the tree-roots by placing a layer of fine gauze about four 

 inches below the surface and adding all the water and food from 

 below, so that the tree obtained all that it wanted before any 

 reached the grass. In spite of this and in spite of the supply of 

 food being liberal, the tree suffered nearly as much from the 

 grass as when grown in the ordinary way without gauze, the 

 food being supplied from above. 



General considerations are equally conclusive that the grass 

 effect is not due to lack of nourishment in the soil : thus the 

 grassed plots receive the same annual dressings of manure as do 

 the other plots and the grass, when cut, is not removed but 

 allowed to rot into the soil again, so that in the case of our original 

 grassed plots nothing will have been removed from the soil 

 during the last eighteen years other than the food material con- 

 tained in the one grass crop at present on the ground together 

 with the small amount of material removed by the feeble growth 

 of the trees ; whereas from the neighbouring tilled plot the 

 material removed has been that contained in the annual crop of 

 fruit and in the wood formed by the vigorously growing trees. 

 The grassed plot must evidently be richer in food than the tilled 

 plot : not only do analyses of the soil show that this is so but 

 when samples of soil are taken from these two plots and trees 

 are grown in them under similar conditions, it has been found 

 that those in the soil from the grassed plot flourished more than 

 twice as well as those in the soil from the tilled plot. 



That the behaviour of the trees under grass is due to some 

 form of starvation cannot be doubted — the colour of the leaf is 

 itself proof of nitrogen starvation ; but it is starvation in a land 

 of plenty — due to the tree not being able to utilise the food which 

 is there, not to any deficiency in the supply of that food. 



It has been suggested several times that if the grass were fed 

 off by sheep, as is the practice in the Kentish orchards, instead 





