126 THE ROTHAMSTED EXPERIMENTS. 



Large ac- Without assuming that the figures represent accurately 



l/nltrogen ^ ne amounts of nitrogen accumulated per acre, it cannot be 



—where doubted that the surface-soils had become considerably richer. 



jr i tlY° me If > for the sake of illustration, we assume that 300 lb. of 



nitrogen were removed per acre in the crop, and that 150 lb. 



were accumulated in the surface-soil, we have 450 lb. of 



nitrogen to account for, as gathered by the crops within a 



period of little more than two years. 



It is clear that we have in the experimental results them- 

 selves no conclusive evidence as to the source of so large an 

 amount of nitrogen. As the surface-soil became determinably 

 richer, it is obvious that it must have been derived either 

 from above or below it — from the atmosphere or from the 

 subsoil ; and, if from the subsoil, the question arises, whether 

 it was taken up as nitric acid, as ammonia, or as organic 

 nitrogen ? Eesults relating to these points will be referred 

 to presently ; but it must be admitted that there is nothing 

 in the experimental results themselves to show that so large 

 an amount of nitrogen could have been available as nitric 

 acid. There remains the question whether the free nitrogen 

 of the atmosphere has in any way been brought into combi- 

 nation, either within the soil or within the plants ? Evidence 

 on these points will be adduced further on. 



Various Leguminous Plants grown after Bed Clover. 



We have now to adduce another and even much more 

 striking instance of successful growth, and of great accumu- 

 lation of nitrogen, by plants of the leguminous Order, on soil 

 where another plant of the same order had failed, and where 

 the surface-soil had become very poor in nitrogen. 



The experiments were made on the plots where it had been 

 attempted to grow red clover year after year on ordinary 

 arable land ; where, in fact, clover had been sown twelve 

 times in 30 years, and where, in eight out of the last ten trials, 

 the plant had died off in the winter and spring succeeding the 

 sowing of the seed — in four cases without any crop at all, and 

 in the other four yielding very small cuttings. 



In 1878, the land was devoted to experiments with various 

 leguminous plants, differently manured, having regard, how- 

 ever, to the previous manurial history of the plots. 

 Object of The object was to ascertain whether, among a selection of 

 the expert- p] an ts all belonging to the leguminous Order, but of different 

 habits of growth, and especially of different character and 

 range of roots, some could be grown successfully for a longer 

 time, and would yield more produce, containing more nitrogen, 

 as well as other constituents, than others ; all being supplied 



