574 
. 
PROCEEDINGS OF SECTION G. 
Without such matters in them, the changes 
which air, moisture, and variations of tempera- 
ture could make would be of comparatively 
little importance; and soils which are unpro- 
vided with them are, for the continued. growth 
of crops, to all intents and purposes, barren. 
These substances are practically never in a state 
of stability. They are always making more or less 
rapid progress towards decomposition, and their 
presence in the soil is the direct or indirect cause 
of changes taking place in it incessantly. If 
these substances are moist and warm, and air, 
with the bacterial organisms it contains, has 
free access to them, their decay is rapid; but 
slower as the access of air becomes, after a certain 
point, less. If the temperature be too low (and 
that is probably never the case in the interior 
of Australia), or if moisture or air be entirely 
absent, decomposition ceases. Whether the 
destructive changes which these substances - 
undergo be rapid or slow, substances are formed 
from them which react on the mineral con- 
stituents of the soil, and bring about decompo- 
sitions and fresh combinations amongst them, 
with the result that mineral potential plaut-foods 
are incessantly being changed into forms which 
plants can take in and assimilate. 
These dead vegetable substances also bring 
into the soil nitrogen in inert combinations for 
the nitrifying bacteria, which are in it, to change 
into assimilable plant-food. Not the least 
valuable of the products of the decay within the 
soil of vegetable substances is humic acid. This, 
which is a relatively stable substance, plays a 
part of no little importance in the soil. It 
releases some of the most valuable of the plant- 
foods from insoluble combinations, and uniting 
with them, forms double humates. These sub- 
stances are relatively insoluble, and are there- 
fore not liable to be washed out of the soil; 
while the state of chemical union in which they 
exist is such that they do not offer very great 
resistance to decomposition. Plants are able to 
break up these double humates, and appropriate 
the plant-food elements they contain; and in 
doing so set free their humic acid, which, by 
