394 



LEGUMES 



LEGUMES 



soils poor in combined nitrogen are enriched in this 

 substance when crops of legumes are grown on 

 them, even though the crop of vines and seed is 

 removed, because of the large amount of fixed ni- 

 trogen in the bacteroids still within the tubercles 

 in the soil; while with the cereals and grasses the 

 nitrogen content of the soil is decreased. This 

 explains why it is that leguminous crops are more 

 important for green-manuring than the cereals and 

 grasses when there is need of an increase of com- 

 bined nitrogen. 



Races of nodule bacteria. 



While the nodule bacteria are widely distributed 

 in the soil, the fact that there are several different 

 races which dwell in the roots of certain genera 

 of hosts, which cannot attack the roots of others, 

 explains why it is that the bacterial races to which 

 certain genera of legumes are susceptible, are not 

 present in all soils, especially in soils where these 

 hosts do not grow, while other races are present 

 in those soils. This is shown in the case of the pea 

 and lupine organism, which will not attack the 

 roots of cytisus, robinia, trifolium, serradella and 

 others, as shown above. It is also shown by ex- 

 periences with the soybean from Japan. When 

 the seed of this bean was planted in America and 

 Europe, no nodules were developed on the roots. 

 It was only when soil from Japan, in which the 

 soybean had grown, was imported and mixed with 

 soil in which the soybean was planted, that the 

 nodules were developed. This organism of the soy- 

 bean nodules was thus considered by Kirchner to 

 be a different species and was named Rhizobae- 

 terium Japonieum. 



Besides the distinct races which cannot infect 

 certain genera of hosts, there are probably sub- 

 races or initial races which can infect a wide range 

 of genera, but, by being confined to a limited num- 

 ber or to single genera for several years, infect 

 certain genera much more readily than others. 



Soil inoculation. 



This leads to an important method in practice, 

 i. e., the inoculation of soils with the specific 

 organism to which the legume which it is 

 desired to grow on the particular plot of ground 

 is susceptible. This method has been developed by 

 the United States Department of Agriculture, 

 especially through the work of Moore and Keller- 

 man, and by some of the experiment stations. It 

 consists in obtaining pure cultures of the needed 

 different races on a medium poor in nitrogen com- 

 pounds so as to create a state of nitrogen hunger 

 in the organism, which makes it more likely to 

 attack the roots of the legumes than organisms 

 which have a nitrogen surfeit of food. Pure cul- 

 tures were distributed, after being dried on cotton, 

 or other suitable material, to the planters, who 

 place them in a quantity of liquid nutrient media 

 for a day or so in order to multiply the germs. 

 This liquid is then scattered on the soil, or, better, 

 the seed is sprinkled with the infusion before 

 being planted. Under certain conditions this prac- 

 tice, or some modification of it, promises good 



returns, especially in soils poor in nitrogen, where 

 the crop in question has not grown for several 

 years or where for any reason the specific organism 

 for the specific crop is absent, or present in small 

 numbers. When the specific organism is present in 

 quantity or in soils already rich in nitrogenous 

 plant-food, the increase in the crop is slight or nil 

 as a result of inoculation of the soil. 



A method has not yet been perfected for sup- 

 plying and applying cultures of the germ which is 

 reliable under all circumstances, due to deteriora- 

 tion or contamination of the organisms in cultures, 

 either because of fault, careless or unscrupulous ' 

 methods on the part of manufacturers, or to imper- 

 fect methods of multiplying the organism at the 

 farm and of inoculation of the seed and soil. With 

 some crops it is now a practice to transport the 

 organisms with the soil in which the specific crops 

 have been grown, for inoculation of soils. In this 

 method, however, there is danger of the transpor- 

 tation of the germs of fungus and bacterial diseases, 

 which may be present in the soil. [Soil inoculation 

 is fully discussed by Lipman, Vol. I, pages 447- 

 450.] 



Relation between nodule bacteria and their host. 



The relation which exists between the nodule 

 bacteria and their host is an interesting one. The 

 bacteria can live in the soil for several years with- 

 out the presence of the legume host, — how long is 

 not known. Nor is it known what permanent bene- 

 fit the organism derives from its association with 

 its host. There is at least a temporary gain by the 

 rapid increase in the number of bacteria which are 

 formed within the nodule, but the larger number 

 of these become surcharged with the nitrogen 

 which they fix, pass into abnormal and involution, 

 forms and die. It may be, however, that the living 

 ones which escape again into the soil form an in- 

 crease over what the increase would be in the soil, 

 and also that the association with the legumes may 

 give them new vigor. The host benefits by the 

 association from the increased nitrogeneous sub- 

 stance placed at its disposal. This is abundantly 

 shown by experiment where there is an increase in 

 size and product when the organism is present over 

 that under the same conditions when the organism 

 is absent. The few cases which have been observed 

 under experimental conditions where the bacteroids 

 assume a firm condition so that they cannot be dis- 

 solved by the host, cannot be taken as proof against 

 the general and almost universal benefit derived by 

 the host from the association with the bacteria, ex- 

 cept when the soil is already very rich in nitroge- 

 neous plant-food. Even under these conditions, 

 although the number of nodules is smaller than in 

 nitrogen-poor soil, there may be an increase of 

 nitrogen in the plant, though no increase in the 

 crop. It cannot be denied, therefore, that there is 

 a mutual benefit derived from this association of 

 the bacterium and the legume in the nodules. The 

 bacterium lives within the nodular root, and thus 

 the nodules are endotrophic mycorhiza. 



This relationship of the bacterium and the legume 

 is a good example of what is ordinarily called sym- 



