THE QUESTION OF BOOT EXCRETIONS 261 



corollary to his theory of mineral requirements ; since plants take their 

 essential constituents in such established ratios they must in time 

 destroy the necessary ratio of these elements in the soil, but when 

 another plant drawing its ash constituents in a different ratio is sub- 

 stituted, it obtains a sufficient supply of nutrients and the soil is thus 

 relieved of exhaustion. 



Under the domination of Liebig's theory of mineral requirements, 

 the theory of De Candolle was practically abandoned. Subsequent to 

 his time the mineral matter of both soil and plant claimed paramount 

 attention, and the biological factors connected with soil problems were 

 almost entirely neglected. Unscientific as it now appears, it must be 

 admitted that for several decades Liebig's dictum had more weight 

 than any amount of experimental evidence. It would even appear 

 that his word has been regarded so infallible in certain quarters that 

 further scientific research has been regarded as unnecessary. 



The inadequacy of the theory of mineral requirements alone to 

 explain the productivity of soils has been aptly set forth by Coleman 

 in an essay " On the Causes of Fertility or Barrenness of Soils," pre- 

 sented to the Eoyal Agricultural Society of England. This essay, 

 although written fifty years ago, expresses the status of the problems 

 of the mineral requirements theory as well now as at the time it was 

 written. He says: 



The causes which operate in producing the fertility or barrenness of soils 

 have hitherto to a great extent been shrouded in mystery, not from any want 

 of study, but owing to the difficulties which meet the inquirer at every step, 

 and the fact that most important results frequently depend upon causes which 

 have eluded the search of the experimenter. The science of chemistry it was 

 hoped would afford the key wherewith to unlock the mysteries of nature, but 

 though its discoveries have conferred much practical benefit on the agricul- 

 turist, it has up to a very recent period effected comparatively little toward 

 settling the causes of fertility or sterility. The theories of scientific men led 

 us to expect that fertility depended upon the presence of certain mineral sub- 

 stances which were found invariably present in the ashes of plants, and the 

 analysis of a soil it was believed would confirm the practical experience of the 

 farmer; these hopes have been falsified except in the few cases of almost simple 

 soils, such as pure clays and sands. In all other instances the analysis pre- 

 sented the existence in varying proportions of those substances supposed to 

 induce fertility equally in the barren as the fertile soil. The proportion of the 

 various ingredients was next proposed as a sign of quality, but researches into 

 the amount of inorganic matter abstracted by each crop have demonstrated that 

 soils of a mixed character contain abundant supplies of mineral food for 

 numerous crops. 



From the time of Liebig and the establishment of his theory of 

 the mineral requirements of plants, there appears to have been no 

 serious discussion of the subject of root excretion until recent years. 

 The reports of the Woburn Experimental Fruit Farm of the Eoyal 



