120 Mineral Nutrition of Plants 



ever, if living barley roots are immersed in a positively charged iron 

 hydroxide sol, the root surface becomes coated with positive colloids. 

 As this reaction resembles the mutual flocculation process of oppositely 

 charged colloids, the existence of negative charges on the root surface 

 is strongly suggested. 



Good evidence for the existence of hydrogen ion swarms surrounding 

 the root surface is provided by the suspension effect of roots. Living 

 roots, preferably starved of cations, are washed with carbon dioxide- 

 saturated water (pH 4.1). If the calomel electrode of a glass electrode- 

 pH-meter (e.g., Beckman) is gently but firmly pressed against the root 

 mass immersed in water, the instrument records potential differences 

 which correspond to acidities lying between pH 3-4, and often between 





PHYSIOLOGIC ASPECTS OF ROOT-SOIL CONTACT EFFECTS 



The contact theory distinguishes between contact intake and contact 

 depletion of plant nutrients, as illustrated in Figure 5. From clays 

 coated exclusively with potassium ions, excised barley roots sorb large 

 amounts of potassium (Table I). On the other hand, clays devoid of 

 potassium in the ion swarm rob the plant of potassium ions. The de- 

 sorption of root ions bears no simple relationship to pH, and it is not 

 the result of root injury. Needless to say, it cannot be detected if root 

 and clay are separated by a semipermeable membrane (21). There 

 appears to exist for each type of ion on the soil colloid a critical degree 

 of saturation at which the root neither gains nor loses the ion in ques- 

 tion. This null-point would depend on the amount and nature of the 

 clay and on the physiological condition of the plant. As the critical 

 degree of saturation is being approached, the plant suffers severe met- 

 abolic disturbances such as the rosette disease in lettuce (46) which 

 is conditioned by calcium starvation. 



Ratner (]Q, 40) in two papers published in English reported strik- 



*D. E. Williams, and N. T. Coleman, "Cation exchange properties of plant 

 root surfaces," Plant and Soil, 2:243-256 (1950). In this connection the follow- 

 ing paper should be consulted: H. Jenny, T. R. Nielsen, N. T. Coleman, and 

 D. E. Williams, "Concerning the measurement of pH, ion activities and mem- 

 brane potentials in colloidal systems," Science, 112:164-167 (1950). 



