A FERTILE SOIL 



DENITRIFYING GERMS 



These germs live on the coarse organic matter of 

 the soil. Like the nitrifying germs they need oxy- 

 gen, and when they cannot get it more readily else- 

 where they take it from the nitric acid and nitrates. 

 This allows the nitrogen of the nitrates to escape 

 as a free gas into the air again, and the work of the 

 nitrogen-fixing and nitrifying germs is undone and 

 the nitrogen is lost. This loss of nitrogen is most 

 apt to occur when the soil is poorly ventilated, be- 

 cause of its being very compact, or when the soil 

 spaces are filled with water. This loss of nitrogen 

 by denitrification can be checked by keeping the 

 soil well ventilated. 



CHEMICAL PROPERTIES OF A FERTILE SOIL 



By the term chemical properties we have refer- 

 ence to the chemical composition of the soil, the 

 chemical changes which take place in the soil, and 

 the conditions which influence these changes. 



The sand, clay and humus of the soil are made 

 up of a great variety of substances. The larger 

 part of these act simply as a mechanical support 

 for the plants and also serve to bring about certain 

 physical conditions. Only a very small portion of 

 these substances serve as the direct food of plants 

 and the chemical conditions of these substances are 

 of great importance. 



In Chapter VIII we learned that plants are com- 



