July, 1952 
THE SOUTH AUSTRALIAN NATURALIST 
Page forty-three 
LEGUMES AND SOIL FERTILITY 
By EDGAR W. PRITCHARD. 
When you come to think of it, it is a 
remarkable thing that in a state of nature 
quite a lot of different species of plants 
grow together in great profusion. Why is 
is that, though they are so different, they 
all live on the same soil, in the same situa- 
tion, watered by the same rain and absorb- 
ing the same sunshine? Why is it that the 
strongest species, the one which is best 
suited to that soil and climate, does not 
choke out all the others? 
The answer is that each variety must in 
some way help the other varieties to grow, 
and thus retain its place in the plant com- 
munity. Just how this rule operates is not 
yet well understood, but there are a few 
cases in which it can be clearly seen. 
Now for the healthy growth of plants 
four main elements are needed, namely, 
carbon, phosphorus, sulphur and nitro- 
gen. The carbon is obtained from the car- 
bonic. acid gas of the air, the phosphorus 
chiefly from the phosphate of lime and 
the sulphur from the sulphate of lime of 
the soil. But while there is an abundance 
of carbon in the air, and a good supply of 
phosphorus and sulphur in most soils, 
there is no nitrogen occurring naturally 
in the minerals from which the soils are 
formed. It must be taken from the air, in 
which there is an almost unlimited 
quantity. 
How is this done? By one particular 
group of plants, which we call the legumes, 
the pod-bearers: peas, beans, clover, tree- 
lucerne, and so on. In our native flora they 
occur in abundance: the wild lilac, the 
scarlet runner, the pultenaeas, the davie- 
sias, the swainsonas, and the cassias and 
wattles. There are few, if any, plant com- 
munities, without them. In the wild heaths 
of Europe there are, among many others, 
the broom and the furze, which have been 
introduced into Australia. 
Pull up a bean plant, and you will find 
on the roots clusters of small nodules. 
These are really a kind of plant wart 
caused by colonies of bacteria, which while 
they live on the sap of the plant, extract 
nitrogen from the air and pass it on to the 
plant to be built into its tissues. A lucerne 
crop, for instance, will by this means col- 
lect from the air up to 200 pounds of 
nitrogen to the acre, equal to about 
18,000 gallons of nitrogen gas. For nitro- 
gen is one of the essential ingredients of 
all proteins, and so of the protoplasm of 
all plants and animals.* 
The legumes then use this captured 
nitrogen to form their own tissues. But 
as they die and gradually decay, the nitro- 
gen of the proteins combines with the 
potash of the soil to form nitrate of potash. 
And this substance can be absorbed by 
practically all of the higher plants to form 
their own proteins. This indeed is the only 
way in which they can grow and develop. 
So it will be seen that the non- 
leguminous plants in any plant commun- 
ity are very largely dependent for their 
very existence on the legumes among 
which they grow. Also this is a striking 
instance of co-operation in nature, as con- 
trasted with the all too much emphasised 
competition. 
* NOTE. — A small, but insignificant, amount 
of nitrogen in the form of ammonia, about six 
pounds per acre per year, from the decay of 
animal matter is dissolved out of the air by the 
rain and washed into the soil. 
Secrets of the Swamps— -(continued). 
knees, if he is to find the really tiny things, 
for it is only by very thorough searching 
that it is possible to see the plants which 
are otherwise concealed by the tall sedges. 
Summer is really the best time to roam 
around the swamps as many plants flower 
then, and it is also best to get to the wettest 
places, as these are often inaccessible in 
winter. 
So year after year the botanists visit vari- 
ous marshy areas, and, who knows, some 
new species may come to light, even if it 
is only a new record for the State? 
