112 SOILS 



nitrogen. Boussingault already had shown, even before 

 Liebig had formulated and promulgated his mineral the- 

 ory, that there were considerable differences between 

 yields of certain groups of plants, and especially that 

 there were noticeable differences in the nitrogen obtained, 

 when cereals and leguminous crops were grown. He had 

 ascertained, also, that when leguminous crops were intro- 

 duced, either before or after cereals in any plan of rota- 

 tion, a greater quantity of nitrogen was secured during 

 the growth of the leguminous crop than during the 

 growth of any cereal crop, and this was the case when 

 additional manure was supplied. 



Liebig was led at this time to revise the theory he had 

 maintained heretofore, by declaring that cereals of all 

 kinds must secure their nitrogen from the soils or from 

 some supplied fertilizer containing this element. 



At this stage of investigation other workers came into 

 the field, notable among whom were Lawes and Gilbert 

 in England. These men carried on experiments quite 

 similar in nature to those of Boussingault, and which 

 seemed to show that the only source of nitrogen supply is 

 from the soil ; in other words, that no cultivated plant is 

 able either to secure free nitrogen of the air, or to estab- 

 lish it in the soil for future use. It is able to secure it only 

 as it does mineral elements: from the compounds of the 

 soil, or from fertilizing compounds supplied with the soil. 



The theory of differences : legumes not like cereals and 

 others. \\~hile the theory of non-fixation of atmospheric 

 nitrogen by any kind of plant generally prevailed (until 

 the true solution was given by Hellriegel and Wilfarth), 

 still there were some among the workers who were not 

 satisfied ; and it is to their agitation and to their unwill- 

 ingness to accept the interpretations of the results that all 

 former theories were proved incorrect and the truth of 



