February, 1912.] 



137 



Scientific Agriculture, 



certain experiments on the growth of 

 plants in confined spaces supplied with 

 soil and fertilisers of known composition, 

 so as to be able to draw up a balance 

 sheet of the nitrogen in crop and soil as 

 against the original nitrogen in seed 

 and soil. Experiments of this kind are 

 subject to considerable error and no 

 conclusive results had been obtained by 

 several previous investigators, but the 

 Rothamsted experiments, which showed 

 that the nitrogen in the plant had been 

 wholly derived from the soil, were gener- 

 ally taken to demonstrate the fact that 

 the living plant cannot bring into com- 

 bination the free nitrogen of the atmos- 

 phere. As part of the same question, 

 systematic measurements were made of 

 the ammonia brought down in the rain- 

 water, a large guage measuring one- 

 thousandth part of an acre in area being 

 constructed in order to obtain sufficient 

 . rain-water for analysis. During forty 

 years systematic analyses have thus 

 been made month by month, which show 

 that, on the average, the rainfall at 

 Rothamsted brings down about 4 lb. of 

 combined nitrogen per acre per annum, 

 whereas the ordinary crop will remove 

 from 40 to 150 lb. per acre, thus disprov- 

 ing Liebig's opiuion that the rainfall 

 provides a supply of ammonia sufficient 

 to maintain the crop without any ex- 

 ternal assistance from nitrogenous 

 manures. 



The leguminous crops provided certain 

 facts inexplicable on this theory of the 

 source of the plant's nitrogen. Several 

 of the Rothamsted records showed that 

 the growth of clover, in addition to 

 yielding a crop containing more than 

 the normal amount of nitrogen, also 

 left the soil richer in nitrogen than it 

 was at the outset ; the clue to these 

 facts was finally supplied only in 1887, 

 when Hellriegel and Wilfarth discovered 

 the existence of certain bacteria living 

 " symbiotically " upon the roots of 

 clover and possessing the power of fixing 

 gaseous atmospheric nitrogen. Novel as 

 was this conception, it served to explain 

 the anomalous results which had been 

 recorded at Rothamsted and the later 

 work of Lawes and Gilbert consisted 

 largely in confirming and giving practi- 

 cal shape to this discovery of Hellriegel 

 and Wilfarth, Still later, after Gilbert's 

 death in fact, other bacteria were 

 discovered living free in the soil which 

 also possess the power of fixing nitrogen 

 and some of the evidence accumulated 

 at Rothamsted served to demonstrate 

 that it is to these later bacteria we must 

 attribute the storing up of nitrogen 

 which has been going on for long ages in 

 some of the rich virgin soils, such as the 

 18 



black lands of Manitoba and Russia. 

 Even the temporary putting down of 

 land to grass and any other farming 

 process which will enrich the soil with 

 carbonaceous matter leads to the fixation 

 of nitrogen, by providing material from 

 which the appropriate bacteria can 

 derive energy. Thus, in a very wide 

 sense, Liebig's opinion is vindicated : 

 the growth of plants, though not of the 

 higher plants, is the only method iby 

 which any considerable amount of 

 nitrogen is brought into combination in 

 nature and it is possible to devise a 

 system of farming which will continue 

 to produce good crops without any ex- 

 traneous sources of nitrogen, provided 

 the land be supplied with lime, phos- 

 phoric acid and potash. 



With the first twenty years of the 

 field trials, a period which was suffi- 

 ciently loug to eliminate the experi- 

 mentiil error and to smooth out the 

 inequalities due to varying seasons, it 

 might have been considered that the 

 original Rothamsted experiments had 

 done their work ; it is indeed true that, 

 as regards the immediate effect of the 

 application of the manures to the crops, 

 final conclusions had been reached of 

 which no revision has been necessary. 

 But here comes in one of the chief con- 

 tributions of Gilbert to the Rothamsted 

 Station, Possessed of a conservative 

 temperament and persuaded that much 

 still remained to be learnt, he held out 

 for the continuance of the experiments 

 without change ; as a result, the treat- 

 ment of the plots has been repeated 

 without variation from at least 1852 

 until the present day. Thanks to this 

 continuity, the Station now possesses 

 unrivalled material for the study of a 

 number of questions which were hardly 

 appreciated or even suspected at the 

 time the experiments were designed ; in 

 fact, there is hardly any portion of the 

 theory of nutrition of the plant which 

 they do not serve to elucidate. It is 

 even to-day impossible to guess in what 

 directions they may not next prove to 

 be of value ; as new points of view and 

 lines of research successively open before 

 us, they will doubtless provide material 

 for investigation, either from the past 

 records or the crops actually growing. 

 A few illustrations of the further 

 developments that have come to light 

 may be given here. 



When the Rothamsted experiments 

 were begun, nothing was known as to 

 how such processes as fermentation and 

 decay took place. Liebig and others had 

 attempted certain explanations but that 

 the actions were carried on by living 

 organisms was entirely vinsuspected. 



