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FIRST YEAR SCIENCE 



Besides harboring these visible plants and animals the 

 soil teems with germ life. Some of these germs increase 

 the fertility of the soil and some decrease it. It has been 

 estimated that there are 50,000 germs of various kinds in 

 a gram of fertile soil. It is these that cause the decay 

 of the vegetable and animal matter in the soil. In the 

 course of this decay various acids and gases are formed 



MUD CRACKS. 

 Showing the way clay cracks wheu it dries. 



which help to decompose the rock particles and other 

 compounds which are needed for the food of the plants. 



The most important of these germs to agriculture are 

 the nitrogen-fixing bacteria. Plants must have nitrogen 

 if they are to grow, but they are unable to take it from 

 the air where it exists in greatest abundance. For the 

 use of the plants it must be chemically combined with 

 other substances and these compounds must be soluble in 

 water. Saltpeter is a compound of this kind and is often 



