4 Grass AND Forest Fires. 
Grass AND Forest Firrs.—The action of these in favouring the spread 
and increase of weeds is twofold. In the first place, the removal of the 
original vegetation provides the conditions and in part acts as a stimulus 
to the germination of the seeds of weeds lying dormant in the soil. In the 
second place, burning off the vegetation involves a great loss of humus, 
which most Victorian soils can badly spare. The loss of humus means not 
only a decrease of water-holding capacity and a diminished aeration, but 
also a fall in the solvent action upon the mineral constituents of the soil. 
In addition to this, the retentive capacity of the soil for soluble mineral 
constituents is diminished by the loss of humus, so that the waste by 
drainage increases. It is for this reason that heavy dressings of soluble 
manures are inadvisable on soils poor in humus. 
The impoverishing action of frequent grass and forest fires on land is 
‘very pronounced, and is very strikingly shown in some parts of Ceylon, 
where it is the custom of the natives to burn off the larger vegetation annu- 
ally, or oftener, in order to secure a temporary grazing ground for stock. 
The result is that certain good forest lands, some of which might be cul- 
tivated after proper clearing, are in the process of conversion into arid 
wastes, or into tracts of scrubby jungle of the most objectionable character. 
Nature cannot be ruthlessly robbed and maltreated with impunity. Re- 
peated fires favour the survival of plants whose demands are easily satisfied, 
and which have either air-borne or long-lived seeds. To this class most 
weeds belong. Its modest demands enable the weed to develop on an im- 
poverished soil, on which long-lived seeds are ready to spring up as soon 
as the way is cleared for them, while air-borne seeds are continually carried 
to the newly-cleared ground from neighbouring unburnt regions. These 
factors operate so constantly, and with such success, that in many parts. 
special plants are recognised as ‘‘ fire-weeds,’’ owing to the way in which 
they spring up after every fire. 
When the amount of ash is great, and especially if it is derived from 
the burning of trees, it contains large quantities of potassium and some 
sodium carbonate,. both of which substances are strongly alkaline when 
present as normal carbonates, and: give an injurious alkalinity to the soil. 
In burning off forests, the ash was often used, especially in the early 
“settling days of the densely wooded States of North America, as a source 
of potassium carbonate (black and pearl ash). These alkaline ashes in the 
soil act upon the hard seed coats of the long-lived seeds in the soil, and 
bv softening them prepare the way for their germination, and this is. 
one of the chief reasons why in Australia, for instance, Wattles so often 
spring up on burnt forest ground on which none grew within the memory 
-of man. In addition, the actual heat of the fire may partially char the 
hard coats, or render them permeable to water, without destroying the seed 
within, which swells and germinates as soon as the soil is moistened. 
I have found the 50 and 60 year-old seeds of various species of Acacia 
to be still germinable on softening or filing the seed coats, and some of the 
plants thus raised are now growing in the Botanic Gardens. 
Once any accumulation of alkali in the form of potassium or sodium 
carbonate has taken place in a soil deprived of its: humus, the soil can 
only be rendered fit for ordinary economic plants by the gradual removal 
of the alkali by the rainfall. When this is slow or imperfect, we get 
sterile’ soil having the character of the bad lands or alkali lands of. North 
America. If the alkalinity is not excessive, and nitrogenous humus is 
present in the soil, nitrification converts the carbonate into nitrate, and so 
