1 24 TILLA GEMO VEMENTS OF SOIL WA TER [chap. 



The following table shows the percentages of water 

 in the fine earth of an orchard on heavy soil, part of 

 which was under grass and part kept tilled ; it will be 

 seen that in the winter the grass land carries as much 

 or even more water than the bare soil, but towards the 

 end of the summer the drying effect of the grass becomes 

 very pronounced, even down to the third foot. 



Few crops so effectually dry the surface soil as grass 

 does, because of the intimate way in which its roots 

 traverse the soil ; hence a fruit tree cannot compete 

 with grass for water as long as the two sets of roots are 

 confined to the same layer. The experiments at the 

 Woburn Fruit Farm of planting fruit trees and sowing 

 the seed of coarse meadow grasses at the same time, 

 show this competition at its highest degree, but even 

 when trees are planted in old pasture care should be 

 taken to keep a ring round the tree free from grass 

 and well cultivated or mulched for at least two years. 



Other causes are doubtless at work ; it has been 

 argued that the grass roots excrete substances toxic to 

 the roots of the fruit trees, among which we maj' 

 enumerate carbon dioxide as a very probable factor in 

 checking development. Again, the grass roots may- 

 induce an injurious disturbance of the bacterial flora, 



