FERTILIZERS 107 



of nitrogen to plants. Practically all the nitrogen used by 

 plants comes from the humus in the soil. Nor can the plant 

 use all the combinations of nitrogen derived from humus. 

 In the soil this element may exist as ammonia, nitrites, and 

 nitrates, but plants can use only the nitrates. 



Bacteria and nitrification. The changing of nitrogenous 

 substances in the humus to forms that are available to plants 

 is accomplished by bacteria, of which there are some fifty 

 million in every cubic centimeter of rich soil. These bacteria 

 are the smallest of living things. They are most numerous 

 near the surface of the soil, but are found in lessened 

 numbers as far down as the lowest layers of the subsoil. 

 Like other plants, they need warmth, moisture, and oxygen 

 for growth, but, unlike them, must have organic food. They 

 are very intolerant of acids and will not live in sour soils. 

 In bogs and other water-soaked soils the absence of air pre- 

 vents the growth of bacteria, the soil becomes sour by the 

 accumulation of acids from the dead vegetation, and the 

 organic material, instead of being turned to nitrates, forms 

 peat. The addition of lime to such soils corrects the acidity, 

 but draining is necessary to promote the activities of the 

 bacteria. The fertility of the soil is then effected exactly as 

 it would be by the addition of more nitrogen. 



Three sets of bacteria are concerned in the work of nitrifi- 

 cation. The first group simply turns the nitrogenous parts 

 of the humus to ammonia, a process which is often called 

 ammonification. In this process the animal and vegetable 

 matter in the soil serves as food for the bacteria which may 

 be said to digest it, excreting ferments, or enzymes, for the 

 purpose. Considerable carbon dioxide is also liberated in this 

 process, and this serves to further weather the soil particles. 

 The ammonia produced by the bacteria combines with soil 

 water to form ammonium hydroxide (NH 4 OH), and at this 

 point a new group of bacteria, known as Nitrosococcus, turns 



