130 Mineral Nutrition of Plants 



soils contact would be the decisive factor. Also, the lower the salt con- 

 tent of the soil solution the greater will be the contribution of contact 

 exchange. For those micronutrient cations, including iron, which are 

 largely insoluble at higher pH values, contact exchange may well be 

 the dominant mode of acquisition by roots. 



TABLE IV 

 Extraction of Nonexchanseable Potassium from Ramona Soil 



Nonexchangeable 

 Potassium 

 Extractant and Method Extracted, p.p.m. 



Neubauer rye seedlings test, 18 days 188 



Leaching with N NH 4 -acetate, 8.5 liters in 35 days . . o 

 Leaching with C0 2 saturated water (pH 4.0) 



8.2 liters in 35 days ! 5 



10. o liters in 14 days 2 7 



Leaching with N/ 10 NaCl, 1.25 liters in 7 days . ... 23 

 Shaking 5 g. of soil for 7 days with 25 m.e. 



NH 4 -Ion-X, pH = 7.6 . . 68-114 



Shaking 5 g. of soil for 7 days with 25 m.e. 



Na-Ion-X, pH = 7.8 1 10-152 



CONCLUSION 



The contact theory of mineral nutrition of plants in soils is only a 

 theory. If the scientist-philosopher Campbell (6) is right, we shall never 

 know whether or not it portrays the truth, for as he says, theories can 

 never be proven, they can only be disproven. 



However, the contact theory is a productive and stimulating theory. 

 It assigns to the root surface the role of active, individualistic, genetic, 

 and physiologically conditioned participation in the liberation of ad- 

 sorbed nutrient ions. It enables the plant to feed directly upon the solid 

 phase. It insists upon the interplay of soil colloids and plant colloids.* 

 In short, it discloses new vistas on the behavior of plants in soils. 



*As Dr. Norman points out elsewhere in this volume, contact exchange should 

 likewise include the surfaces of the microbial soil population. 



