32 CROPPING SYSTEMS 



nitrogen are brought down by rainfall, while soil bacteria fix rather 

 large quantities of nitrogen taken from air circulating through soil. 

 These bacteria are the principal natural means by which nitrogen is 

 fixed. Some forms of these bacteria are associated with legumes, 

 while others are capable of living in the soil and fix free nitrogen, 

 if organic matter is present. It is not known which kind have been 

 most important in fixing the present supply of nitrogen in the soil. 

 There is some evidence that in certain soils in rather dry regions, 

 as Colorado or Utah, soil bacteria not associated with legumes may 

 fix large quantities of nitrogen from the air. 



Nitrogen Fixed by Legumes. The nitrogen is not taken from 

 the air by the legume plants as might be implied, but only by the 

 bacteria found in the nodules formed on legume roots. These bac- 

 teria are short-lived and, as they die, their nitrogen becomes avail- 

 able to plants. Legumes also use any available nitrogen in the soil. 



A ton of alfalfa hay may contain 50 pounds of nitrogen, yet four 

 tons a season may be removed, and still the soil be richer in nitrogen 

 than before. This would indicate that an alfalfa crop is capable of 

 fixing 200 to 300 pounds of nitrogen from the air in a season, or 

 forty to sixty dollars worth per acre based on the commercial price 

 of nitrogen. Nitrogen in fertilizers costs about twenty cents a 

 pound. 



In nature, there are great numbers of wild legumes which have 

 been adding their annual deposit of nitrogen to the soil for ages. 

 This is especially true in prairie regions and is one reason for the 

 rich store of nitrogen in these soils. 



Importance of Organic Matter. A large supply of organic 

 matter is important for the following reasons : 



(a) By decaying in the soil, it makes available plant food. 



(&) It is necessary for the life of nitrogen-fixing bacteria. 



(c) The decayed organic matter itself becomes food for plants. 



(d) Vegetable matter in the soil helps to keep the soil moist and 

 in good tilth, so it will not become hard and bake. 



From the above, it would seem that the most important means of 

 keeping a soil productive without the use of fertilizers is to maintain 

 a good supply of vegetable matter, and grow legumes a part of the 

 time to keep up the nitrogen supply (Fig. 14). 



