Effect of Soil Conditions 57 
diseases of foliage are present, such as those produced by 
insects or fungi, the diagnosis is comparatively easy and 
the remedy readily suggested; but in the absence of such local 
disturbances, when the evident abnormality of the foliage 
is due to more remote causes, the difficulty is greater. .\s 
long as fungus growth or insect depredations on bark, woud 
and root, can be discovered as likely to have produced the 
disturbance in the condition of the foliage, even a layman 
may find the cause; but to diagnose the so-called physiolog- 
ical diseases which are due either to permanent changes, or 
to some unusual temporary conditions in the environment, 
the services of an expert may be required. 
Changes in the conditions to which the tree was originally 
adapted may take place in the soil or in the light supply 
(when neighbors are removed or are crowding), or they 
may simply be in the weather conditions of the particular 
season, or, In many cases, the combined conditions of soil 
and season etfect the sickness. 
Indeed, whenever the conditions of nutrition are inter- 
fered with, the foliage will soon give evidence of it. And 
in this connection, the soil conditions, especially the mechan- 
ical conditions which influence water and air supply, are 
of the greatest importance, as these are the ones which can 
be more or less controlled. 
Effect of Soil Conditions. It must not be overlooked that 
a soil changes in its stucture and thereby in its capacity for 
water conduction and aération by the compacting of rain- 
drops, by frost, and in many other ways, so that the same 
soil which was originally satisfactory to tree growth, may in 
time become less so. Not only soils exposed directly to 
rains, but also those under sod, or even under sidewalks, 
become constantly more and more compacted, and hence a 
eradual change in their permeability to water and air takes 
