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 naturally 

 form, by leaf mould or other decaying matter such as 

 manure. The nitrogen contained in rotting manure or m 

 humus is, however, not mainly in the form of nitrates. It 

 IS contained in highly complex organic compounds, 

 while ni manure it occurs largely in urea, a compound of 

 ammonia. These organic compounds require to be acted 

 upon by bacteria, ivhich are foitncL 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 

 w^hich 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 al^^'ays associated with the roots 

 of plants belonging to the pea family. If we pull up a 

 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 ]>eas 

 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 sav, 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 m 



