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COLLOID CHEMISTRY OF THE SOIL 

 IN RELATION TO PLANT NUTRITION^ 



C Edmund Marshall 



UXIVERSITY OF MISSOURI 



Eleven years ago, at a symposium on the mineral nutrition ol: plants, the 

 author re\'ie\vecl and correlated the diffe'ring concepts of the soil solu- 

 tion and then proceeded to develop the idea of the complete ionic en- 

 N'ironment of plant roots (Marshall, 1951). This includes contributions 

 from dissociating soil colloids as well as from soluble salts. Cationic ac- 

 tivity measurements, made by the use of membrane electrodes, fur- 

 nished the experimental basis for this treatment. Illustrations were given 

 of the differing properties of clay minerals as dissociating colloidal 

 systems, of the relationships of monovalent and divalent cations, and 

 of the consequences of polyfunctional characteristics in the clays. In 

 later publications (Barber and Marshall, 1951, 1952) further details on 

 clay systems were gi\'en, especially of the mutual effects of cations, and 

 the humic colloids of soils were characterized by similar experimental 

 methods ( Marshall and Patniak, 1953). 



Much has occurred since the \\'isconsin symposium which bears 

 on these matters. After a period of considerable confusion in the early 

 1950's, a large measure of clarification has been achieved. This con- 

 tribution seeks to present a balanced view of the position today. 



The definition of chemical environment 



Since the soil is inherently a higiih- complex system with many 

 solid pliases, it will be nccessar\' to simplify the problem by use of a 

 reasonable model. The main properties c^f soils with regard to exchange 



° ContriJmt'ion fnnu the Missouri Aiiricultiiidl Ex])crivivnt Station. A))i)ioi('(l 

 liij the Director. 



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