200 THE LIVING ORGANISMS OF THE SOIL [chap. 



receive their nitrogen almost entirely by the absorption 

 of the soluble organic substances." This view was 

 displaced by the so-called "mineral theory" of Liebig, 

 who, in laying down the broad principle that the plant 

 only derived certain necessary mineral constituents, its 

 " ash," from the soil, but the whole of its carbon 

 compounds from the atmosphere, was led to regard 

 the nitrogen as well as the other combustible matters 

 of the plant as due to the atmosphere, largely because 

 of the exaggerated estimate which then prevailed as to 

 the amount of ammonia from the air that was brought 

 down in the rain. Boussingault had already shown, by 

 weighing and analysing the crops on his own farm for 

 six separate courses of rotation, that from one-third to 

 one-half more nitrogen was removed in the produce 

 than was supplied in the manure. The gain of nitrogen 

 was little or nothing when cereal crops only were 

 grown, but became large when leguminous crops were 

 introduced into the rotation. Liebig, however, con- 

 sidered that cereals, as well as the other plants, were 

 able to draw their ammonia from the atmosphere, 

 and that, provided sufficient mineral plant food were 

 forthcoming, there was no need of ammonia compounds 

 in the manure. 



This view of Liebig's, though modified later, when 

 he admitted that cereals must obtain their nitrogen 

 from a manurial source in the soil, led to considerable 

 investigation of the source of the nitrogen in the plant. 

 Boussingault himself carried out a long series of 

 laboratory experiments, in which weighed seeds con- 

 taining a known proportion of nitrogen were grown 

 in artificial soils containing no nitrogen, but supplied 

 with the ash constituents of the plant. Care was 

 taken to remove all ammonia from the air in which 

 the plants were grown, and from the water and 



