678 PLANT GROWTH AND PLANT COMMUNITIES 



phase effect. Lagerwerff (1958) carried out an elaborate double-pot 

 experiment in which he added sand to one pot and a mixture of sand 

 and resin grains (0.4 to 0.7 mm. in diameter) to the other. Nutrient 

 solution circulated through both pots. Growth and nutrient uptake 

 were alike in both pots; that is, no phase effect appeared. On the other 

 hand, Lopez-Gonzales and Jenny ( 1958 ) , using a very similar double- 

 column technique with coarse Sr-resin particles, observed a 16.4-fold 

 superiority of the resin column over the sand column. 



These experimental discrepancies appear to be related to the very 

 word "contact." To ascertain the existence of an exchangeable ion 

 effect, it is imperative to provide a relatively large number of contact 

 points between the root and the milieu, and a small quantity of ions in 

 solution. To "disprove" particle influences, one simply chooses a 

 medium with few contact sites but relatively high solution concentra- 

 tions. For roots with low metabolic activity, either mechanism might 

 supply sufficient nutrients to the root surface. 



Quantitative experiment on density of contact sites 



An experiment was designed to measure quantitatively the growth 

 of alfalfa as conditioned by the number of contact sites between root 

 surfaces and iron-oxide particles ( Glauser and Jenny, 1960a ) . 



In a double-column system, as shown in Figure 6, two tubes 17 

 centimeters long and two centimeters in diameter were arranged in 

 series. The lower column was filled with purified quartz sand with 

 grain sizes of 0.5 to 1.0 mm. diameter. The upper column was likewise 

 charged with sand grains, but some of them had a thin coat of iron 

 oxide. The coat had been prepared by dipping the quartzite grains 

 into a sol of Fe(OH)3; drying at 105° G. produced stable brown coats 

 of iron hydroxide in various degrees of dehydration. For brevity's 

 sake, the expression Fe-sand will be used for these particles. The white 

 and brown sand grains could be mixed in any desired proportion. 

 GaGO.3 ( 0.5 per cent ) was added to each column, and, with the aid of 

 a pump, 0.1-strength, complete Hoagland solution was circulated every 

 15 minutes. Its pH was maintained daily between 7.5 and 8.5. At such 

 high alkalinity iron is exceedingly insoluble. 



Six alfalfa seedlings (Medicago sativa), eight days old, were 

 planted in the columns. For analytical purposes each sextet of plants 

 constituted one sample. 



The experiment poses a number of questions which touch upon a 

 broad spectrum of root-soil interactions: Fe-solubility, GO2 influence, 

 excretion of chelating substances, and contact per se. To answer them, 

 pertinent treatments were included. 



As a check, a double column was first charged only with white 



