134 CLOVER CULTURE. 



quite independent of the supply in the soil, the interesting- and 

 important question arises, how does it obtain it ? It was a1 

 first supposed, and quite naturally, that it obtained it by 

 means of its long- tap-root, but, while it does no doubt in thi? 

 way restore a certain portion of the nitrates that have been 

 washed down by rains into the subsoil, the amount has been 

 found to be very inconsiderable compared with the entire 

 amount obtained. It was next supposed that the supply was 

 obtained from the nitrog-en in the form of ammonia in the 

 atmosphere. This ammonia, washed out, as it is, by rains, 

 would be equally available for any other plant, and hence, so 

 far as root action is concerned, in no way aids in the solution 

 of the problem. The same remark applies to the small amount 

 of nitric acid that is known to be formed in the atmosphere by 

 electrical action. 



The question has arisen, whether the clovers do not obtain 

 their supply of nitrog-en from these two last sources through 

 their leaves. To test this matter clovers have been grown in 

 pots with access to air alone that had been washed by being- 

 passed throug-h water, thus removing- the nitric acid or ammo- 

 nia, and the result was precisely the same as in the case of 

 similar pots grown in the open air. Every other means of 

 determining- the source of nitrogen in the clovers having- 

 failed, attention was directed to the tubercles in the roots. 



If the reader will pull up a stalk of any of the clovers, 

 during- the growing- season, he will notice on the roots, some- 

 times on the smaller and sometimes on the larger, certain 

 knots, warts or protuberances. These are the tubercles, a 

 somewhat unfortunate name because associated with disease, 

 but so definitely associated with them that it is useless to 

 attempt to change it now. He will notice that the healthier 

 the plant and the more vigorous its growth, the more of these 

 tubercles will be found, and he will fail to find an^-where a 

 healthy stalk of clover, during the period of vigorous growth, 

 without these tubercles. It was this apparent connection be- 

 tween the number and size of the tubercles and the vig-or of 

 the plant that led Prof. Helrieg-el, of Bernberg-, Germany, to 

 investigate, in 1883 and the years following, the cause of the 

 tubercles on the leg-umes and also the relation they sustain 

 to the plant. As early as 1862 he had been conducting- certain 

 elaborate experiments for the purpose of determining- whether, 

 other things being equal, the magnitude of crops was propor- 

 tionate to the supply of certain essential elements of fertility 

 in the soil. He had no difficulty in determining- that in the 

 case of the cereals, such as wheat, oats and barley, the mag- 

 nitude of the yield was proportionate to the nitrogen in the 



