108 Mineral Nutrition of Plants 



are not free to diffuse in the liquid phase. The ion swarm always re- 

 mains- attached to the colloid particle and goes wherever it goes. 



One might redefine the soil solution as comprising the dissolved 

 electrolytes in the displaced soil extract plus the exchangeable cations 

 and anions of the solid phase. Rather than introducing a new definition 

 of soil solution, the writer prefers Marshall's term "ionic environment 

 of the root" for the combination of adsorbed (exchangeable) ions and 

 the classical soil solution ions. 



Probably the most extreme proponent of a simple soil solution theory 

 of plant nutrition was Cameron (5). As late as 1911, he stated: "There 

 can be no doubt, therefore, that the soil solution is normally of a con- 

 centration amply sufficient to support ordinary crop plants, and is 

 maintained at a sufficient concentration so far as mineral plant nutrients 

 are concerned." To account for the existence of soils giving low yields, 

 Cameron postulated and, seemingly, demonstrated that infertility of 

 soils is caused by the presence of toxic or inhibitory organic substances 

 in the soil solution. 



Cameron labored under the erroneous impression that the liquid 

 phase was nothing but the saturated solution of sparingly soluble soil 

 minerals. He overlooked the contribution of the adsorbed ions to the 

 ionic environment of the plant root. 



Von Liebig, preceding Cameron by half a century, appears more 

 farsighted. After some hesitation he repeated and confirmed Way's 

 famous experiments on base exchange. In 1858 he expounded new 

 ideas on plant nutrition (27) : 



"It is clear that we must abandon this idea (soil solution theory) 

 when it can be demonstrated that rain water, either alone or in con- 

 junction with carbonic acid, does not dissolve enough of the mineral 

 constituents to contribute significantly to plant growth. In this case 

 the uptake of minerals must be the result of an active contributing 

 cause residing in the root, whereby the water surrounding the root is 

 enabled to dissolve certain mineral constituents which, otherwise, it 

 could not do. As further consequence, the quantity of mineral sub- 

 stances consumed must be in proportion to the root surface of the 

 plant and to the sum of those active mineral constituents that are con- 

 tained in such portions of the soil as are in contact with the root sur- 

 face." 



