CHAPTER 8 



THE SOIL AS A SOURCE OF 

 MINERAL SALTS 



The multiphase system which the investigator of 

 plant nutrition explores is that of the soil-plant- 

 atmosphere with its innumerable interrelations and 

 interactions. Inherent in the green plant itself are all 

 the complexities common to living organisms, and to 

 these must be added the complexities of the soil 

 medium in which the plant is anchored and finds some 

 of the substances essential for its nourishment. 



D. R. HOAGLAND. 



Lectures on the Inorganic Nutrition of Plants 

 (1944). 



A. Introduction 



An early view of plant nutrition, dating from the time of Aristotle, 

 was that food materials were absorbed in an elaborated form from 

 the soil. This "humus theory", discredited by Van Helmont in the 

 seventeenth century (see p. 1), was not entirely disproved until 

 more than 100 years later, when it was demonstrated by solution 

 culture experiments that neither a supply of organic matter, nor the 

 insoluble mineral particles are essential for plant growth. Since 

 those days, the role of the soil as a source of plant nutrients has been 

 further investigated, and it is known that salts exist in a number of 

 different states in the soil, which can be conveniently classified under 

 three headings : 



(i) water-soluble salts dissolved in the soil solution, 



(ii) sparingly soluble or insoluble substances containing 

 exchangeable ions, and 



(iii) insoluble substances, from which ions are not readily 

 obtained by exchange reactions. 



Salts present in the soil solution are readily available to plants, 



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