27 



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, particularly 

 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 assimilation of atmospheric nitrogen depends 

 should be present in sufficient numbers. 



Good drainage and aeration are essential because these bacteria will 

 not flourish in soils imperfectly drained or aerated. An abundant sup- 

 ply 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 abimdant supply of these elements they can make a luxuriant devel- 

 opment, because the supply of nitrogen in the air is inexhaustible. 



The bacteria upon which the assimilation of atmospheric nitrogen de- 

 pends 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 de- 

 velopment of these bacteria without seed. Fortunately these bacteria, 

 as is the case with many weeds, propagate themselves and are self-seed- 

 ing. 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 assimi- 

 lation of atmospheric nitrogen, but there must be a stock of seed to be- 

 gin 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 abundant. To secure the development of a suffi- 

 cient number of them it is just as unnecessary to supply more seed as it is 

 to scatter seed of" pusley " in the garden to secure a crop of that weed. 

 It is only when a legume is new in a given locality that it becomes nec- 

 essary to consider the question of supplying seed of its appropriate bac- 

 teria ; for while some of the bacteria seed may be present, adhering to 

 the seed of the new crop in the form of dust, the quantity will be insuf- 

 ficient for the best results the first few years. 



To supjjly this deficiency the farmers of to-day may easily purchase ni- 

 tragin, often spoken of as a germ fertilizer, or he may secure soil from 

 the locality where the new crop is known to flourish. Nitragin is not 

 expensive, and if used according to directions accompanying it, it has 

 often been found beneficial. The quantity of earth needed is not large, 

 and if scattered and mixed with the soil, as fertilizer would be, it will 

 usually produce the desired eftect. 



In one other important particular green-manuring has a relation to the 

 supply of nitrogen in the soil. This element is rapidly converted into sol- 

 iible forms during the summer, and the soil has not the ability to retain 

 these soluble compounds. If the rainfall is heavy and water leaches 

 through the soil it will take these soluble nitrogen compoimds with it. 



