116 PLANT FOOD 



piles. This gas contains about eighty per cent nitrogen. 

 The soil absorbs ammonia to a marked degree and -changes 

 it to available compounds of nitrogen. Even ammonia 

 itself is slightly available for the uses of plants, but not 



NODULES ON BEAN ROOTS. 

 Where the nitrogen-gathering bacteria grow. 



to the extent of other nitrogen compounds, which are 

 called "nitrates." 



b. Ammonia is also set free for plant growth when the 

 decay of vegetable matter forms humus. In a general 

 way it may be said that the amount of nitrogen in the 

 soil is in proportion to the humus it contains. 



