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ing instances of depletion of ions from barley plants at different stages 

 of growth. Loss of cations to clays containing 30 per cent of calcium 

 and 70 per cent of sodium was not restricted to roots; it extended to 

 stalks, leaves, and even to ears. 



While loss of nutrients to clays is an observation of long standing 

 (/, 18,23), it nac ' never been systematically explained. Now, in the light 

 of the colloid interaction theory it reveals itself as the legitimate partner 

 of contact sorption. 



TABLE I 

 Gains and Losses of Potassium by Barley Roots in Various Clay Systems 



D = degree of potassium saturation 



In presence of clay strongly pronounced counter migration of cations 

 can be demonstrated. From potassium-hydrogen clays (pH 4.4) having 

 not too low degrees of potassium saturation, barley roots accumulate 

 potassium and increase their potassium content by 28.5 per cent. At the 

 same time the roots lose calcium ions, to the extent of 22.2 per cent. 

 From calcium-hydrogen clays (pH 5.25), roots take up calcium ions 

 (6.4 per cent), but experience a loss of potassium ions amounting to 

 19.4 per cent. With radioactive indicators it is possible to prove that a 

 given ion species simultaneously moves into the root and out of the 

 root. We must think, therefore, in terms of net accumulation and net 

 depletion. 



