(24. 
W. Git, | 
THE AUSTRALIAN GARD ENER. 
Plantation of Sugar Gums, 18 years old, Bundaleer. 
“Jtine, 1910. 
[Pxo'r0. 
Other explanations which suggest d 
themselves laye been investigated, and 
“found equally unacceptable , these were 
differences in soil temperature, differences 
‘in aeration or proportion or carbon 
dioxide, and difference in the physica} 
‘condition of tho soil. ‘The only other 
6xplanation which appears to be possible 
‘is that the growth of the grass results in 
‘the formation of some substance which.is 
‘poisonous to the tree. This may be an 
‘active poison—a toxin—or the poisonous 
‘action may result from the alteration in 
‘the proportion of various substances 
“present in the soil. An active poison 
“may be produced in various ways, such 
88 by the decomposition of the débris of 
she grass, actual excretion from tho grass 
‘roots, or as a product of the bacteria. 
“present'in the soil, As t the origin of 
‘the toxin no definite evidence has yet 
: been obtained, but it has been found that 
produced by grass on trees. Thus, on 
heating soil, substances are produced 
_Which are toxic towards the germination 
of seeds, and these have been found tobe 
toxic towards plant growth also. That 
established plants which grow better in 
heated than in unheated soil, is due to 
the fact that herting causes a considerable 
increase in the soluble nitrogen present 
in the goil, and also in the composition 
of the bacterial flora of the soil. More- 
ever, the toxin formed asthe result of 
heating the soil soon becomes oxidised 
aud destroyed, allowing the favourable 
If, how- 
ever, the toxin is present in sufficient 
quantity, it is not all destroyed before 
the plant grows, and its delecterious effect 
becomes apparent. It is noticeable that 
this effect varies greatly in “different 
cases, and is very mach less in the case 
of grasses that in than of the other plants 
conditions to assert themselves. 
toxins may be formed” in soils by» heats. which’ have been examined, Earth for 
-and other means, producing effects which 
are pnelbpgnan in Many respects with thoso 
| 
‘grassed ground behaves in the game way 
‘as earth which have beon slightly heated 
and which contains only a limited amount 
of toxic matter, for trees planted i in it (the 
grass being removed) do better than in 
soil taken from tilled ground, such -toxie 
matter as there was present in it having 
evidently become destroyed before the 
tree started into growth ;. whether its 
presence originally in soil can be estab- 
lished by its effect on germinating seeds, 
still remains to be seen. 
Ifthe formation of the toxic ahbebinde 
is the explanation of the grass effect, we 
might naturally expect great variations 
in this effect in different ‘soils; 3 and this 
is certainly the case. At Ridgmont the 
effect is, perhaps, greater,than i in any other 
instance which has come - under. the 
writer’s observation, but cases_of very 
nearly the same inteusity have been found 
in various parts of the Kingdom, whilst 
only one instance hae been noticed where 
the grass had apparently had no -offect, 
This variation in intensity with the nature 
of the soil i is, probably, the chief reason 
why ‘the action i is not more widely recog- 
