in. sou. 
(a) Unsuitable soil. Alkali. 
(6) Sand encroachment. 
(a) Unsuitable Soil. Alkali. The next great barrier to tree-growth is unsuitable soil. Often 
there is insufficient soil ; often the soil contains alkali or salt, and very often there is hard-pan near the 
surface which prevents the penetration of roots. It is very seldom that tree-growth is prevented by a 
lack of nourishing food material in the soil. Although trees are often in a half-starved condition, it is 
an extremely poor soil which will not support tree-growth of some kind. The formation of grass or savanna 
land is often due to hard-pan, which is hardened or indurated soil. Hard-pan not only prevents the 
penetration of roots, but, because of imperfect drainage, the ground is sour and stagnant in nature, and 
therefore not conducive to tree-growth. 
This is how John Gifford* discusses the subject in dealing with barriers to forest 
extension, and although we theoretically realise the truth of what he says, the subject 
is practically very little considered in New South Wales. 
Taking examples from Sydney that are familiar to so many, let us note the hard 
pan at the Centennial Park, which renders tree -planting such a costly process. It is at a 
varying depth below the surface, very hard, and thoroughly impervious. The water 
which accumulates on the top has a slightly acid reaction. Tree-planting in such country 
cannot be carried out except by removal of this rock (sometimes several feet below the 
surface) and replacement of it by better soil. 
Take the Lower Botanic Garden, which, for the greater portion of its area has 
been reclaimed from the salt water. The original salt has never been quite leached out 
lower than a foot or two, and there is always a fresh supply of salt-water by soakage 
underneath. So that a balance has to be arrived at with trees which are tolerant to 
more or less diluted salt-water. In time circumstances arise to induce concentration of 
the salt in a particular area, and the roots of a particular tree push forward and meet 
this saline mass, and stunting of growth, and even death may result. Some trees are 
more tolerant than others. Indeed, the environment precisely suits a number of our 
trees which are denizens of brackish swamps. 
The following experience is instructive in the present context : 
A young man was driving me a number of years ago near Edensor Park, Liverpool, 
and I saw some She-oaks in the distance which looked to me like the Salt-water Swamp 
Oak (Casuarina glauca). (At that time the species was not known so far from the sea 
or a salt-water creek). I asked the driver what name he gave to those oaks, and he 
said, " Oaks." A little later I was able to collect specimens, and then knew that the 
trees were truly C. glauca. I told the driver that I had never before seen this oak except 
on the banks of brackish creeks or lagoons, and he said nothing. But he was evidently 
pondering the matter, for in a few minutes he said that all round about those oaks the 
water was so salt in a dry time that the cattle could not drink it. Here was the solution. 
Although far away from the sea that infinite source of brackishness there was a stock 
of salts in the soil hereabout. (What is the original source of these deposits is for the 
geologists to say.) These salts were sufficient to enable C. glauca to live and flourish. 
* " Practical Forestry," p. 84. 
E 
