DAMAGE TO TREES BY GASES 169 



of the best means of accomplishing this is to dig a trench 

 6 or 8 feet from the tree and as deep as the lowest 

 part of the gas main from which the gas has escaped. If 

 the gas has penetrated the soil on all sides, this trench 

 should completely encircle the tree. Otherwise it will be 

 necessary for it to be only on that side from which the 

 flow of gas has come. This trench must be open suffi- 

 ciently long to give the gas abundant time to escape. Ordi- 

 narily 30 days should be allowed for this. 



When the gas has been given time to escape, the trench 

 may be refilled with the soil if it has been thoroughly 

 refreshed. It is safer, however, to provide entirely new 

 earth, fortified by the addition of well-rotted compost in 

 quantity equal to one-fifth of the bulk of the soil. 



If these steps do not restore health, the tree should be 

 removed. It is wise, however, to wait until an entire 

 summer has passed before removal. In case removal 

 proves necessary care should be taken to make sure that 

 the gas is entirely eliminated from the soil, or fresh soil 

 should be provided before planting a new tree. 



Atmospheric influences are also a source of serious 

 harm to shade trees. Gases and vapors in the air often 

 seriously damage and sometimes cause the death of trees. 

 This menace is especially prevalent in manufacturing 

 communities, in which stacks and chimneys discharge 

 their gas-laden fumes and smoke. 



A very common offender is the smoke from soft coal, 

 because of the large percentage of sulphur. The sulphuric 

 acid resulting from combustion attacks vegetation. 

 Smoke also causes trouble from soot, which is deposited 

 on the leaves, obstructing light and clogging the breathing 

 pores to an extent which sometimes results in asphyxia- 

 tion. Heavy discharge of smoke also deposits soot parti- 

 cles on the ground, impregnating the soil with smoke acids 



