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 
