WINTER MEETING. 313 



France and Italy. Eecently it has received a new impetus from dis- 

 coveries made in plant nutrition, and is being vigorously advocated far 

 ^nd wide by writers on agricultural topics, without proper regard for 

 the conditions under which it must prove an irrational practice. 



The ancients knew that leguminous plants, especially the clovers, 

 left the soil richer after their gi*owth, even when the crop growing above 

 ground was harvested, and they also knew that the soil was enriched 

 in proportion to the size of the crop. Emil John found by a series of 

 analyses more than 40 years ago that this added richness did not con- 

 sist alone in an increase in the humus-forming materials of the soil, but 

 in an actual increase in the nitrogenous matter of the soil. The reason 

 for. all this, however, was not apparent until a few years ago. Investi- 

 gations by Hellriegel, the director of a German experiment station, 

 demonstrated the fact that leguminous plants have the ability to take 

 up or assimilate the free nitrogen of the air and use it in their growth, 

 and that they are enabled to do this by numerous tubercles or nodules 

 which grow on their roots. The tubercles had been noticed before, 

 but their function was unknown. Just how this assimilation of free 

 nitrogen is affected is a problem not fully solved. Further studies have 

 shown that the tubercles contain large numbers of microbes or micro- 

 organisms, which appear to be responsible for the assimilation of 

 nitrogen. It appears to be a result of their life processes. They live 

 in a sort of partnership with the plants, deriving certain things essen- 

 tial to their life and growth from the juices of the plant, and in turn 

 furnishing the plant with nitrogen. This partnership is known in 

 science as symbiosis. Much remains to be found out regarding this 

 mysterious process, and it must be admitted that there is a certain 

 amount of speculation in this theory. The question is an exceedingly 

 diflicult one to get at. But it is sufficient for practical purposes to know 

 that leguminous plants provided with these tubercles possess a nitrogen 

 source not available to other kinds of plants. 



These discoveries throw a new light on green manuring and on the 

 plants best adapted for green manuring. They show that while both 

 leguminous and non-leguminous plants enriched the soil alike in 

 humus-forming materials, in proportion to the size of the crop, they 

 differ in respect to the source of their nitrogenous materials. While 

 non-leguminous plants derive their nitrogen supply almost exclusively 

 from the soil, leguminous plants take theirs from the free nitrogen of 

 the air. Consequently, if spurry, rape, mustard, etc. (non-leguminous 

 jilants ), are grown on the soil and the crop plouwed in, the soil is not 

 materially enriched in nitrogen ; the process is simply returning to the 

 soil all the nitrogen which the crop took from it. Probably a very 



