340 FIXATION OF FREE NITROGEN BY BACTERIA. 



205 to 237 kilos, of (combined) nitrogen were obtained per hectare- 

 (180-208 Ibs. per acre). 



For this reason agricultural chemists have termed the Legu- 

 minosoe collectors or accumulators of nitrogen, and have set 

 them up as a class distinct from all other cultivated plants, the 

 latter being grouped under the name of nitrogen consumers. 



This does not, however, imply that the leguminous plants are 

 averse to such manuring; on the contrary, they readily absorb 

 any nitrogenous food present in or added to the soil, this food 

 assisting the plant to tide over the stage of nitrogen-hunger. So 

 soon, however, as this period of stagnation is passed, they no 

 longer require nitrogenous manure, and the latter, when applied, 

 does not influence the size of the crop. Practical agriculturists 

 utilise this observation by planting soils which are poor in nitrogen 

 with such nitrogen accumulators (especially lupins) as a first crop, 

 which is ploughed in when sufficiently developed. This consti- 

 tutes the practice known as green manuring. A field treated in 

 this fashion will then contain a much larger amount of nitrogen 

 (in organic combination) than before, and is rendered capable 

 of properly developing other cultivated plants (cereals, hoed 

 crops, &c.), which, by reason of their high nitrogen requirements, 

 would otherwise have yielded only a miserable crop in such poor 

 soils. 



192. The Discovery of the Leguminous Nodules. 



The source of the free nitrogen accumulated by this class of 

 plants must be sought in the atmosphere alone. 



Formerly the ammonia compounds of carbonic acid, nitrous 

 acid, and nitric acid, always present in the air (i.e. the rainfall), 

 were considered as likely sources of nitrogen. The quantities of 

 the last-named acid brought down in the rain, in temperate and 

 tropical climates respectively, are given in the following table, 

 drawn up (partly from personal experience) by A. MUNTZ and V. 

 MARCANO (I.) : 



I kilo per hectare 0.89 Ib. per acre. 



