Hans ] amy 115 



practical validity of the equation in its application to the Neubauer test 

 rests on the rate of conversion of nonexchangeable potassium. 



Unpublished, exhaustive studies by D. E. Williams show that 30 

 days of continuous leaching of Ramona soil with various dilute acids 

 (pH 3-6) will liberate only a fraction of the potassium extracted by 

 the Neubauer seedlings in 18 days. Distilled, carbon dioxide-saturated 

 water, passing through 25 grams of soil, freed of exchangeable potas- 

 sium, contains about 0.10 mg. of nonexchangeable potassium per liter, 

 apparently irrespective of the rate of flow. It is highly questionable (/■?) 

 that plants will grow satisfactorily in a solution containing only 0.10 

 p.p.m. of potassium, even if the solution is continuously renewed. 

 Roots used in the Neubauer test must be endowed with a mechanism 

 of potassium extraction vastly more powerful than carbon dioxide 

 leaching. 



In the author's opinion, the carbon dioxide theory of liberation of 

 adsorbed ions has generally been overrated. To use a metaphor, we 

 have accepted its application blindly, without asking the plant how far 

 it lives up to its tenets. Besides, the theory is unimaginative; soil water 

 is readily enriched in carbon dioxide and little room is left for indi- 

 viduality of plant behavior. We must look for additional theories of 

 plant nutrition in soils. One of these is the contact exchange theory. 



THE CONTACT EXCHANGE THEORY 



The contact exchange theory (/a) describes a mechanism for reac- 

 tions between adsorbents, or solids in general, without the participation 

 of free electrolytes. It was deduced from theoretical considerations 

 concerning the nature of ionic surfaces. Specifically, it rests on the 

 concept of overlapping oscillation spaces of adsorbed ions, or, in another 



Figure 3. Left: Model of ion exchange in salt solution. 

 Right: Model of contact exchange between clay particles. 

 The dashed lines signify overlapping oscillation volumes 

 (75). In these models attention is focused on individual 

 ions. 



