POSSIBLE CAUSES OF THE GRASS EFFECT 281 



account for the different behaviour of trees in grassed and tilled 

 ground (XIII, 106). 



Depth, 



A, Fmer Per-ticles. B. Finest Particles, 



10' 



2O" 



80 IOO I2O SO IOO I2O 



Relative proportion of fine Particles. 



FIG. 37. DISTRIBUTION OF FINE PARTICLES IN TILLED 

 AND GRASSED SOIL. 



The Water Supply. It is to an interference in the supply of 

 water to the trees that the deleterious effect of grass has generally 

 been attributed : yet there are several general considerations 

 which tell strongly against such an explanation, perhaps even 

 more strongly than do the results of definite experiments on the 

 subject. The symptoms of injury by grass are not those of 

 injury by drought; there is no wilting of the leaves, as in the 

 case of a sudden drought, nor do the leaves dry up and fall, 

 as iii the case of a gradual, prolonged drought : the grass-effect 

 is evidenced by the light unhealthy colour of the leaves quite 

 early in the season, while the ground is, generally, still moist, 

 and while the vigorous growth of the grass itself proves that 

 the supply of water in the soil is abundant, for the grass would 

 certainly feel the effect of drought before the more deeply 

 rooted tree would: lastly, the grass-effect is manifest in a wet 

 season just as clearly as it is in a dry one. 



A closer examination of these arguments serves only to 

 strengthen them. It is true that the growth of grass may result 

 in the drying up of the soil below : this is especially the case 

 in a light sandy soil, where the average annual evaporation 

 from grassed land may be more than four times as great as 

 that from tilled land, 1 and tillage of the surface, independent 

 of the absence of grass, reduces the evaporation ; 2 but in 



1 Lawes, Gilbert and Warington, Journ. Royal Agric. Soc., 1881, p. 325 

 (see also I, p. 100). 



2 King, New York Experiment Station, Ann. Report, p. 180, and 

 Winconsin, Ann. Report, 8, p. 105 (see also II, 175). 



