SOILS AND FERTILIZERS 405 



of nitrogen from the soil in the same manner as most other plants. 

 It has further been proved that these tubercles are caused by a cer- 

 tain kind of bacteria, and that it is really by these bacteria that the 

 nitrogen of the air is absorbed so that it can be used by the clover 

 or by other legumes. There is thus a peculiar interrelation estab- 

 lished between the leguminous plant and the bacteria. The latter 

 obtain their principal nourishment from the legume, while in turn 

 they provide nitrogen from the air which can be used by the host 

 plant. 



Inoculation. Nearly every legume has its own particular 

 strain of nodule bacteria. For example, that of the clover is differ- 

 ent from that of alfalfa, and that of the cowpea distinct from that 

 of the soy bean. In planting a leguminous crop for the first time it 

 will therefore often happen that no tubercles will form on the roots, 

 because the proper kind of bacteria is not present in the soil. These 

 may be supplied by scattering soil from a field where the crop in 

 question has been recently grown, or by using an artificial culture 

 of the proper bacteria. After the field is once thoroughly inocu- 

 lated there is rarely any difficulty afterwards in getting an abundant 

 supply of root tubercles. 



The same strain of bacteria will often inoculate different closely 

 related legumes. Thus alfalfa, bur clover, and sweet clover are tu- 

 bercled by the same strain ; a different strain inoculates most kinds 

 of vetches, as well as the field and garden peas ; still another strain is 

 apparently used by red clover, white clover, and alsike clover in 

 common. 



There is a marked variation among different legumes, both in 

 the extent to which they are naturally tubercled and in the ease 

 with which different ones may be artificially inoculated. Natural 

 inoculation presupposes the existence of the proper germ in the soil, 

 and this is nearly always present over areas where a particular 

 legume has been generally grown. Thus red clover is practically al- 

 ways naturally inoculated in the northeastern quarter of the United 

 States and cowpeas in the southeastern. Alfalfa is naturally tuber- 

 cled as a rule in the West, while in the East artificial inoculation is 

 usually necessary. 



Artificial inoculation is usually accomplished with ease as re- 

 gards cowpeas, red clover, vetches, field peas, and most other leg- 

 umes. Soy beans, however, frequently give negative results. Sweet 

 clover is much more readily inoculated than alfalfa, though both 

 use the same strain of bacteria. In some cases the tubercle germs 

 occur in sufficient abundance on the seeds to provide inoculation. 

 This occurs more particularly on seeds that are tramped out by oxen 

 or which otherwise become covered with dust from the field. 



Sources of Soil Nitrogen. Nitrogen, as purchased in commer- 

 cial fertilizers, such as nitrate of soda, guano, tankage, and dried 

 blood, is the most expensive manurial substance a farmer has to buy. 

 Its cost in these various forms ranges from 15 to 20 cents a pound. 

 To a very large extent, at least, the purchase of the costly nitrogen* 



