54 RESULTS OF EXPERIMENTS AT ROTHAMSTED, 



other from such fixation. The probability seems to be, that the 

 proportion due to fixation will be the less in the richer soils, and the 

 greater in soils that are poor in combined nitrogen, and which are 

 open and porous. 



Even assuming that, in the case of leguminous crops, there will 

 generally be some gain of nitrogen due to the symbiotic growth 

 supposed, it will nevertheless be well to consider the facts of agricul- 

 tural production, in their bearing on the question of the sources of the 

 nitrogen of crops generally. 



As already said, much of the investigation that has been undertaken 

 in recent years, has been instigated by the assumption that there must 

 exist natural compensation for the losses of combined nitrogen which 

 the soil suffers by the removal of crops, and for the losses which result 

 from the liberation of free nitrogen from its combinations under various 

 circumstances. In some cases, however, the object seems to have been 

 for the most part limited to an attempt to solve the admitted difficulty 

 as to the explanation of the source of the whole of the nitrogen of the 

 Leguminosae. 



As to the losses which the soil sustains by the removal of crops, 

 Berthelot, for example, assumes that 50 to 60 kilog. of nitrogen will 

 be annually removed from a hectare of meadow (= 45 to 54 Ibs. per 

 acre); and that, as only 10 kilog., or less, of this will be restored as 

 combined nitrogen in rain, &c., there will be an annual loss of from 40 

 to 50 kilog. per hectare (=36 to 45 Ibs. per acre) ; so that, if there 

 were not compensation from the free nitrogen of the air, the soil would 

 become gradually exhausted. Further, he considers that the fact of 

 the fixation of free nitrogen, not only explains how fertility is 

 maintained, but how argillaceous soils, which are sterile when first 

 brought into contact with the air, gradually yield better crops, and at 

 length become vegetable moulds. Frank, again, assumes that the 

 average loss of nitrogen by the removal of crops is 51 kilog. per 

 hectare (= 45 Ibs. per acre). 



It is quite true, that a good hay crop may contain as much as 50 to 

 60 kilog. of nitrogen per hectare ; but it may safely be affirmed that, 

 in ordinary practice, even in the case of an unusually fertile meadow, 

 such an amount is not annually removed for a number of years in 

 succession, without the periodical return of manure supplying nitrogen; 

 whilst, taking the average of soils, the annual yield will seldom reach 

 the amount supposed, even with the ordinary periodic return, and 

 without such return gradual exhaustion would be very marked. 

 Indeed, it is well known that there is no more exhausting practice than 

 the annual removal of hay without return of manure; so that, in 

 point of fact, restoration in anything like the degree supposed, 

 certainly does not take place. Next to the removal of hay, the 

 consumption of the grass for the production of milk is the most, but 

 still very much less, nitrogen-exhausting ; whilst, if grass be consumed 

 by store or fattening animals, the loss is very much less still ; indeed, 

 it is then very small. 



