PLANT FOOD AND MANURES 123 



amply provide for the needs of plants. Careful in- 

 vestigations, however, have shown that plants, as a rule, 

 are unable to use this nitrogen (exceptions will be 

 referred to later), but obtain their nitrogen from the 

 soil in the form of complex substances 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. 



All living things, animal and vegetable, contain 

 nitrogen. When they 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 bacteria are spoken 

 of as nitrifying bacteria. In order that they may live 

 and thrive and so carry on their useful work, it is 

 necessary 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 bacteria 

 being suspended, although they themselves are not 

 actually killed, by the cold of winter. In addition, it 

 is essential that there should be some lime in the soil. 

 It will be observed that these conditions are those 

 which have been repeatedly referred to as the objects 

 aimed at in good tillage and cultivation ; that is to say, 

 the presence of moisture, air, and lime, together with 

 vegetable matter which contains the nitrogen to be 

 acted upon. Speaking generally, therefore, we find 

 that those operations and conditions which render the 

 soil best suited for the life and growth of the nitrifying 



