June 1907.] 



373 



Scientific Agriculture, 



from the air. As it has been proved, however, that the organism itself in certain 

 forms can take up nitrogen and store it up in itself as nitrogenous matter even when 

 it is isolated from the plant nodule, there seems little doubt that the organism in the 

 nodule also absorbs nitrogen in this way. 



The present view of the case briefly stated is that, firstly, the bacterium in 

 its minute form enters the root of the plant, and secondly, in the root this minute 

 form changes to the rod-like form, multiplies, and fixes nitrogen, and then, thirdly, 

 in the nodule it changes to the branched form Avhich is finally destroyed by an 

 enzyme secreted in the plant, and the nitrogenous matter is dissolved and absorbed 

 by the plant, whilst the nodule gradually diminishes in size. 



Although green manuring is occasionally practised with other than legu- 

 minous plants, the use of such plants can only increase the organic matter in the 

 soil, whereas leguminous plants not only do this but also increase the nitrogen 

 content of the soil by the direct absorption of atmospheric nitrogen, and consequently 

 it seems that the use of non-leguminous plants is much less advantageous. 

 Leguminous green manures are, moreover, of great value, as they may often take 

 the place of other and more expensive nitrogenous manure such as sodium nitrate, 

 ammonium sulphate, guano, etc. 



By the use of leguminous crops such as alfalfa (Medicago sativa), clovers 

 (Trifolium sp.) or cowpeas (Vigna Catiang), poor or exhausted soils maybe readily 

 improved. Such plants will generally grow upon these soils, if supplied with the 

 requisite amount of phosphoric acid and potash, which constituents are of small cost 

 compared with that of the nitrogen in nitrogenous manures. 



As an example of this it may be stated that the United States Department 

 of Agriculture in 1888 commenced some experiments in the Jack Pine Plains of 

 Michigan where the soil is light, sandy and almost barren. Green manures were 

 principally used together with cheap fertilisers, and in three years an improvement 

 was effected, both in the physical character of the soil and in the yield of the crops 

 grown on it. 



Prom the experiments of Hellriegel and Wilfarth and others it is evident 

 that if leguminous plants used as green manures are to fulfil their purpose of the 

 fixation of nitrogen, it is absolutely essential that the specific organism should be 

 present in the soil. 



It appears that while many soils contain the necessary bacteria, some do 

 not, or only contain it in a form which has lost its activity and cannot produce the 

 desired effect. The first remedy suggested for this deficiency was to inoculate the 

 sterile soil with some soil known to contain the organism. This method involves the 

 disadvantage of the cost of transport and labour as well as the danger of simulta- 

 neously introducing insect or fungoid pests and objectionable weeds. In order to 

 obviate these difficulties many attempts were made to prepare cultures of the 

 organism on a large scale. 



Of these preparations the "nitragin" of Nobbe was probably tne most 

 important, which was a culture of the organism in nutrient agar solution and was 

 said to give good results in Germany, but did not meet with much success in the 

 United States of America. 



A complete scientific investigation of the nature of the organism and its 

 action was, therefore, undertaken by the Laboratory of Plant Physiology of the 

 United States Department of Agriculture, the results of which are published 



