CHAPTKR X 



THEORIES OF FERTILISER ACTION 



Liebig's Ash Theory — Part played by the Soil in the Nutrition of 

 the Crop— Villa's Theory of Dominants — Liebig's Law of the 

 Minimum — Law of diminishing Returns — Limiting Factors 

 in Plant Growth — Is the Composition of the Soil Water 

 unaffected by Fertilisers? — Attack of the Plant's Roots upon 

 Insoluble Fertilisers — The Part played by Carbon Dioxide in 

 the Soil — Excretion of Toxic Substances from Plant Roots — 

 Rotations as a Substitute for Fertilisers — Unexplained Factors 

 in the Nutrition Problem. 



It is to TJcbig that we owe the first general theory of 

 the nutrition of the plant and the function of fertilisers : 

 although Liebig himself did not add anything to the 

 knowledge of the process of carbon assimilation which 

 had been acquired by Priestley, Senebier, and others, 

 nor to the study of the nitrogen and ash constituents 

 which had been begun by de Saussure, he yet drew up 

 from these facts a coherent theory of the course of 

 nutrition, and put it before the world with such vivid- 

 ness that it forthwith took its place in the general body 

 of accepted scientific opinion. Liebig argued that since 

 the ash constituents alone are drawn from the soil, it 

 is only necessary that there shall be no deficiency in 

 such inorganic materials as are left behind when the 

 plant is burnt, in order to ensure the complete nutrition 



of the plant. According to Liebig, the function of the 

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