98 



ELEMENTS OF AGRICULTURE 



plants are in consequence forced to draw their sup- 

 plies of nitrogen from the soil, like wheat, oats, or 

 any crop having no power to use the nitrogen of the 

 air. Without the proper kind of bacteria legumes are 

 unable to make any use of the nitrogen of the air. 

 To supply deficient soils, these bacteria are now pre- 

 pared for sale, and they may also be supplied by adding 

 to the deficient soils quantities of another soil known 

 to contain them. It is probable that in a few years the 

 various nitrifying bacteria will be sold in the market 

 much as fertilizers are sold to-day. 



97. Forms of Nitrogen in Soils. — We have learned 

 the sources from which the soil is supplied with nitro- 

 gen; now, how are these compounds held in the soil? 

 A part of the nitrogen is held in the undecayed organic 

 matter, some is combined in the humus, and the air in 

 the soil contains free nitrogen. In all of these forms 

 the nitrogen is unavailable to plants for food, and 

 must first be converted into nitrates by the bacteria 

 before it may be used. The soil contains some nitrates, 

 as does the soil water, but these compounds are in 

 small quantities because they are so easily washed from 

 the soil. 



Questions 



1. Why is nitrogen particularly important as a plant 

 food? 2. From what three sources is the soil's supply of 

 nitrogen derived? 3. What happens to the protein com- 

 pounds when the plant decays in the soil? 4. What causes 

 the decay of plants in the soil? 5. What nitrogen com- 

 pounds are formed when plants decay in the soil? 6. What 

 is the process of the formation of nitrates called? 7. When 

 is nitrification most active? 8. When plants decay is there 



