12 SOURCES OF THE NITROGEN, OF VEGETATION 



paper, and on his return to England experiments were 

 immediately begun at Rothamsted to check their results. 



A series of small pits were built up of slate slabs out of 

 doors, and these were filled with either soil or washed sand and 

 then sown with various leguminous plants, which were after- 

 wards inoculated or not as desired. The growth was cut away 

 for the determination of dry matter produced and the nitrogen 

 collected ; afterwards the roots were washed out from the soil 

 or sand for the examination of the development of the nodules. 



A more rigorous set of experiments were carried out in 

 glazed stoneware pots in the glasshouse, and some of the 

 results obtained are set out in Table V. (p. 13). 



The table consists of a balance-sheet for the nitrogen only, 

 in which the nitrogen supplied, either in the seed, in the sand 

 or soil used, in the extract employed for inoculation, or in a 

 few cases in the manure, is compared with that recovered in the 

 soil or the plant. The first horizontal line for each plant shows 

 the results obtained when there was no inoculation and the 

 plant grew with simply the store of nitrogen present in the seed 

 and what it could obtain from the soil ; the second and third 

 lines show the results of inoculation, both seed and soil being 

 otherwise similar ; the fourth line shows the result when 

 the seeds were sown in ordinary soil. 



It is needless to elaborate the results thus obtained ; they 

 confirmed, as has repeatedly been done since, the conclusions 

 of Hellriegel and Wilfarth, and showed that the leguminous 

 plants possess the power of " fixing " nitrogen under ordinary 

 conditions of field culture by the agency of the bacteria living 

 in the nodules on their roots. 



The very rigour with which the earlier laboratory experi- 

 ments, like those at Rothamsted on peas and beans in 1857-8, 

 had been carried out, had prevented any fixation of nitrogen 

 by excluding all possibility of inoculation. 



The interpretation of the increased stock of nitrogen 

 obtained with leguminous crops, which, as instanced above, 

 had hitherto been so difficult of explanation, at once became 



