CHAPTER XVI 

 LEGUMES AND SOIL FERTILITY 



It has been the experience of practical men from the earliest 

 days of agricultural practice that legumes under appropriate con- 

 ditions render the soil more productive. The farmers of ancient 

 Rome understood that crops following beans, peas, and vetches 

 were usually better than those following wheat or barley. During 

 the middle of the nineteenth century it was considered proved 

 that neither legumes nor non-legumes could use atmospheric 

 nitrogen, yet experiments pointed to the conclusions that legumes 

 possessed peculiar powers. We have seen that during the last 

 quarter of the nineteenth century, it was learned that legumes 

 associated with bacteria have the power of utilizing atmospheric 

 nitrogen, whereas non-legumes must rely on the combined nitro- 

 gen of the soil. Today the best farmers see that the legume 

 enters largely into their systems of rotation. They know that 

 such a practice means bigger and better crops at least for a time. 

 Some farmers grow legumes for a number of years on run-down 

 soil, remove the entire crops, and for a few years get increased 

 yield. They are confident that their soil is becoming richer in 

 plant- food. If this is not true, why this increase? Surely alfalfa 

 and other legumes are not a stimulant to the soil as is tea, coffee, 

 or tobacco to the tired man. Let us examine some results obtained 

 in carefully planned experiments for the answer. 



Essential Elements. — We have seen that plants are com- 

 posed of ten elements, each of which is absolutely essential to 

 growth and seed formation. Only two — carbon and oxygen — 

 are secured from the air by all plants, only one — ^hydrogen — 

 from the water; the other seven are secured by all plants from the 

 soil. One class of plants — the legumes — may, under appropriate 

 conditions, obtain their nitrogen from the air. Six elements— 



