■September, 1952 THE SOUTH AUSTRALIAN NATURALIST 
Page seven 
Ecology and Co-operation in Nature 
by Edgar W. Pritchard 
The science which deals with the 
relations of plant to the soil and 
climate, and also to one another, 
is called ecology. The first two- 
have been studied in agriculture 
for generations where the suita- 
bility of each plant to the environ- 
ment, in which it grows, is the basis 
of farm and orchard practice in 
every country. 
But it is only in the last few 
years, that the relations of plants 
to each other have received any 
direct attcntiop. In scrub, where 
the varieties of plants are so num- 
erous and diverse, this is a very 
complicated and time-consuming 
occupation. These plant associa- 
tions, as they are called, have been 
mapped out and described for this 
State in Prof. J. G. Wood’s book, 
“The Vegetation of South Aus- 
tralia: ’ But little appears to have 
been done on the affect of each 
plant on the life history, wellbeing 
and nutrition of its associates, 
probably for the reason given 
above. 
In cultivated land, however, the 
association is usually much simpler. 
1 hus it may be useful to describe 
one such example, consisting of 
only two plant species to show how, 
while competing with each other 
lor the plant food in the soil and 
room in the air, they may yet co- 
operate for mutual benefit. 
Subterranean clover ( Trifolium 
subterraneurn ) is one of the best 
fodder plants for converting poor 
scrub country in fertile agricultural 
land. But when planted by itself it 
will, in a few years, begin to deteri- 
orate, the land becoming what is 
called “clover sick.” If, however, at 
this stage seed rye grass [Loliurn 
sp.) is sown in it, the clover will 
recover, and a good well-balanced 
pasture result. Notwithstanding 
this the rye grass would not grow, 
if planted in the first place with 
the clover! 
What is the explanation? First 
that scrub land contains very little 
nitrogen, which is an essential 
plant food, and therefore cannot 
provide for the heavy growth of 
rye grass. Second that the clover 
can obtain its nitrogen from the 
air by means of bacteria in small 
nodules on the roots; and so with 
its growth aided by annual applica- 
tions of superphosphate it can 
thrive on even this poor soil. 
It is well known that clovers in 
a grass lawn can be killed without 
killing the grass by a heavy appli- 
cation of a nitrogenous fertilizer, 
such as sulphate of ammonia. So 
what happens with the subter- 
ranean clover is that the roots, 
stems and leaves, all containing 
nitrogen, die each year and are 
gradually incorporated with the 
soil, until it is so rich in this 
element as to limit the growth of 
the clover. 
Thirdly no grass of any kind can 
obtain its nitrogen direct from the 
air; thus it must depend for its 
supply on the soil. When the rye 
grass is planted it uses the accumu- 
lated nitrogen, and while flourish- 
ing itself enables the clover once 
more to thrive. 
Thus w T e have a mutually bene- 
ficial association of two plants, 
which provides a permanent, pro- 
ductive and economic pasture, 
well-balanced from the dietetic 
continued on page 14 
