Fertilizing 79 
soil acts precisely like a mulch, the capillaries having been 
destroyed. Hence no water can rise from below into the 
surface layer, and thus loss of water by evaporation is pre- 
vented. In applying water to the surface, therefore, the 
compacting of the soil by the water must be avoided, and 
this is best done by applying the water through ditches or 
holes near the tree, by sub-surface watering. 
Where, due to soil and topographic conditions, continued 
excess of moisture is experienced, and aération thus im- 
peded, the drainage must be improved. Usually it is better 
simply to lower the water table than to attempt thorough 
draining. An open or covered ditch properly located at 
some distance will do the work. Such ditches may be filled 
up with rubble and then be again covered and sodded. 
It is, however, advisable to be very cautious in attempt- 
ing the regulation of the water-supply by drainage, for not 
only do species vary in their water requirements, but the 
individual trees, having once adapted themselves to a wet 
situation, may resent any sudden change of condition. 
Time must, therefore, be allowed the tree to adapt itself, 
by changing the water level gradually, while at the same 
time the process of adaptation may be assisted by judicious 
pruning. 
Fertilizing. Troubles due to deficiency in mineral salts 
in the soil are of rare occurrence, since most soils contain 
sufficient quantities of the needful minerals of which trees 
require only small amounts, their bodies being mainly built 
up from the carbon of the air. The only mineral likely 
to be deficient is nitrogen, the lack of which would produce 
a yellow or even whitish appearance of foliage. In nature 
this mineral is provided by the decay of fallen leaves, in 
which the nitrogen-gathering bacteria work. So the practice 
of raking off the leaves in parks is one way of starving trees. 
