118 NATURE TEACHING. 



vide for the needs of plants. Careful investi- 

 gations, however, have shown that plants, as 

 a rule, are unable to use this nitrogen (except- 

 ions will be referred to later), but obtain their 

 nitrogen from the soil in the form of complex 

 bodies known as nitrates. We are familiar 

 with nitrates in the form of saltpetre which is 

 nitrate of potash, and nitrate of soda largely 

 used as a manure. 



2. All living things, animal and vegetable, 

 contain nitrogen. When these decay in the 

 soil, their nitrogen is converted into nitrates, 

 this change being brought about by the 

 agency of microbes or bacteria which live in 

 the soil in countless numbers In consequence 

 of the results they bring about these bacte- 

 ria are spoken of as nitrifying bacteria. In 

 order that they may live and thrive and so 

 carry on their beneficient work, it is necess- 

 ary that the soil should present certain 

 conditions. Moisture and air are needed, 

 for in their absence the nitrifying bacteria 

 cannot live. A certain amount of warmth 

 is also necessary, the activity of the bac- 

 teria being suspended, although they them- 

 selves are not actually killed, by the cold of 

 winter in temperate climates. In addition, it 

 is essential that there; should be some lime in 



