12 
the soil as far as the nutrition of plants is concerned. We 
can realise therefore the usefulness of nitrate of soda as 
a fertilizer. In nature the nitrogen for plant food are 
largely obtained from the humus, or decaying vegetable 
matter, which is present in all soils. In cultivation, 
where plants are removed when they die down or where 
they are taken up to be used as vegetable, it becomes 
necessary to replace the humus, which would naturaily 
form, by leaf mould or other decaying matter such as 
manure. The nitrogen contained in rotting manure or in 
humus is, however, not maihly in the form of nitrates. It 
is contained in highly complex organic compounds, 
while in manure it occurs iy in urea, a compound of 
ammonia. These organic compounds require to be acted 
upon. by bacteria, which are found in the soil and in dung 
before they are available as plant food. On the other 
hand we find.in the soil micro-organisms of another kind 
which have the power of combining the free nitrogen of 
the air with the oxygen and ultimately build up the 
nitrates so important to plants. These nitrifying bacteria 
are constantly at work, and when a field lies fallow the 
soil will be found at the end of the fallowing to be richer 
in nitrogen compounds than it was at the beginning. In 
addition to these organisms referred to above, there’ are 
special forms which are always associated with the roots 
of plants belonging to the pea family. If we pull upa 
pea or bean we find that the roots bear curious swellings 
called root tubercles, which are found when examined to 
contain numerous minute bacteria. The roots must not 
be considered diseased, though they mav look like it. 
They are in a normal condition, and the bacteria inhabit- 
ing these nodules do not injure the plants but enrich 
them with nitrogenous material which they obtain from 
the air found in the interstices of the soil. It is therefore 
particularly important to keep the ground around peas 
and beans open by hoeing, so that the roots may not only 
get the necessary oxygen for breathing purposes, but also 
the nitrogen they require for their nutrition. It is bv the 
activity of these nitrifying bacteria that leguminous 
plants, as those belonging to the pea family are called, can 
grow in very poor soil, that is to say, in soil in which 
there are very few nitrogen compounds, and yet produce 
seeds which are very rich in nitrogen, and therefore very 
important as food for mankind. Of further interest in 
