494 MISCELLANEOUS FARM SUBJECTS 



white clover, Canada peas, soy beans, peanuts, vetch, velvet beans, 

 Japan clover, and bur clover. A few more are cultivated to a less 

 extent, as sweet clover, beggarweed, grass peas, fenugreek, and horse 

 beans. Many others have been tested in an experimental way, but 

 as yet are not grown as crops. 



From an agricultural point of view legumes may be classified 

 into three groups: (1) Summer annuals, including cowpeas, soy 

 beans, peanuts, beans, velvet beans, and in the North common vetch 

 and Canada peas; (2) Winter annuals, comprising crimson clover, 

 bur clover, hairy vetch, and in the South common vetch and Can- 

 ada peas; (3) Biennials or perennials, embracing red clover, white 

 clover, alsike clover, alfalfa, and sweet clover. 



Each of these crops can be grown advantageously only in a 

 more or less definitely limited region. For the particular purpose 

 in view, it rarely happens that a choice of two or more equally 

 valuable legumes is offered. Usually one is so much superior to any 

 other available that substitution is practically out of the question. 

 In a few cases, however, the use of one legume in place of another 

 is practicable. Thus cowpeas and soy beans are agriculturally 

 much alike and are adapted to nearly the same regions. In a like 

 manner crimson clover, bur clover, and the vetches over a large area 

 may be used one in place of another. In some sections the culture 

 of red clover is no longer profitable, principally owing to diseases. 

 Alsike clover has been used to some extent as a substitute, but the 

 yield is ordinarily much less. There is also an increasing use of 

 alfalfa in place of red clover, but with alfalfa the best practice is to 

 keep the fields in this crop three years or longer. 



How Legumes Get Nitrogen From the Air. It was known 

 even in ancient times that much larger crops of various kinds could 

 be produced on land that had been in clover or lupines the previous 

 season. 



In modern agriculture the value of legumes is quite generally 

 appreciated, as seen in the common practice of growing clover or 

 some other legume in rotation at frequent intervals. The reason 

 why legumes have a beneficial effect was discovered by Hellriegel 

 and Wilfarth in 1886, though many accurate experiments had long 

 before proved the fact. 



An examination of the roots of leguminous plants will reveal 

 on many of them nodules or tubercles; sometimes very few, some- 

 times very many. These vary in size and shape according to the 

 kind of plant. Thus, on red clover they are more or less round, and 

 quite small ; on the cowpea they are also round and nearly smooth, 

 but much larger; on the velvet bean they may even reach the size 

 of a pigeon egg; on the vetches they are irregular, both in shape 

 and size. The differences in the tubercles are such that in many 

 cases it is possible to determine the plant to which the root belongs. 



Hellriegel and others have proved beyond any question that 

 when leguminous plants have these tubercles on the roots they can 

 make use of the free nitrogen of the air ; when they do not have these 

 tubercles they are powerless to do this, but must obtain their supply 



