284 FIELD CROP PRODUCTION 



soil to tide it over until it is able to draw upon the supply- 

 in the air. Soils for alfalfa should have enough available 

 plant food to permit the plants to reach the stage where 

 they can make use of the nitrogen in the air. Poor soils 

 are usually in a poor physical condition, and do not pro- 

 vide a suitable place for the growth of bacteria. The 

 physical condition of such soils may be greatly improved 

 by applying barnyard manure and plowing under green 

 manure crops a few years previous to the seeding of alfalfa. 

 Organic matter not only improves the physical condition 

 of the soil, but it also prevents heaving and winter-killing. 

 The bacteria that produce the nodules on the roots of 

 the alfalfa plant should be present in the soil, for it is due 

 to these bacteria that nitrogen is gathered from the air. 

 When they are not present, if the plant makes a success- 

 ful growth, it must secure its nitrogen from the soil. 

 In order to grow alfalfa without the aid of these bacteria, 

 it is necessary to have a very fertile soil and one that is 

 well supplied with nitrogen. However, it is not usually 

 desirable to do this. Sometimes the bacteria that grow 

 upon the roots of alfalfa are not present in the soil and it 

 then becomes necessary to supply them artificially. As 

 a rule it is best to inoculate a new field unless one is certain 

 that the bacteria are present. 



The methods employed in inoculating the soil for alfalfa 

 are the same as have been discussed in Chapter XIII. 

 The bacteria that grow upon the roots of sweet clover and 

 bur clover will also grow upon the roots of alfalfa. Soils 

 that have successfully grown these crops may be used for 

 inoculating alfalfa fields. 



288. Use of Alfalfa. — The reason why alfalfa is so 

 highly esteemed is not difficult to see. The farmer, as a 

 business man, considers both the cost of production and 



