320 BOARD OF AGRICULTURE. [Pub. Doc. 



ploughed in or allowed to decay upon the soil, the nitrogen 

 which they have taken from the air becomes available to 

 plants belonging to other families which themselves have not 

 the power to draw upon the air for this element. 



It is important to point out, however, that legumes take 

 nitrogen from the air in considerable quantities only when 

 the proper conditions exist. What these conditions are 

 should be made clear. The most important among them are 

 the following: thorough drainage and aeration of the soil, a 

 liberal supply of the mineral elements of plant food, particu- 

 larly lime, potash and phosphoric acid, and a comparatively 

 small amount of available nitrogen compounds in the soil. 

 It is also essential that the bacteria upon which the assimila- 

 tion of atmospheric nitrogen depends should be present in 

 sufficient numbers. 



Good drainage and aeration are essential because these 

 bacteria will not nourish in soils imperfectly drained or 

 aerated. An abundant supply of mineral elements of plant 

 food is essential because without it the plants of the clover 

 family are unable to make a large growth. With an abun- 

 dant supply of these elements they can make a luxuriant 

 development, because the supply of nitrogen in the air is 

 inexhaustible. 



The bacteria upon which the assimilation of atmospheric 

 nitrogen depends are plants, of microscopic dimensions it is 

 true, but just as really plants as the corn and potatoes of 

 our fields. Being plants they must come from seed, or, what 

 amounts to the same thing, from parts which serve the same 

 purpose as seed. We should not expect a crop of corn or 

 potatoes except the seed be planted, so we cannot expect the 

 development of these bacteria without seed. Fortunately 

 these bacteria, as is the case with many weeds, propagate 

 themselves and are self-seeding. When once established 

 in a field the farmer needs not to supply further seed of 

 witch grass or " pusley." These plants will take care of 

 themselves ; so with the bacteria which are connected with 

 the assimilation of atmospheric nitrogen, but there must be 

 a stock of seed to begin with. The beginning was made long 

 ago in the case of all the common plants of the clover family, 

 and the seed of the appropriate bacteria is everywhere abuu- 



