PLANT ROOT-SOEL INTERACTIONS 



685 



oxide (dried, colloidal Fe(OH)a) in the presence and in the absence 

 of contact, and in a COo-saturated environment. 



In contact experiments the Incite tube was filled with COo-satur- 

 ated water. The dry root plug was inserted and, after swelling had 

 ceased, was evacuated to remove trapped air. An "iron-oxide lid" was 

 then placed on top of the wet plug. It consisted of a Incite disc with 

 five holes in it, the bottom side of which had partly fused-in particles 

 of highly radioactive (Fe'^'-*) iron-oxide grains, mixed with solid CaCOa. 

 On top of the Incite disc was laid a cellulose membrane ( \^isking ) , and 

 over it was sprinkled a layer of CaCOs powder. The total amount of 

 carbonate was equal to the exchangeable H in the root plug. The lid 

 was pressed upon the root plug ( to which a few drops of toluene had 

 been added) by a weight of 2.27 kg. In that condition the height of the 

 wet root plug was 1.50 cm. The hollow space inside the pressing tube 

 was filled with 15 ml. of water— designated as supernatant liquid— and 

 CO2 gas was continually bubbled into it. 



In solution experiments, conducted identically except as stated, 

 the iron oxide-Incite disc was suspended in the COo-saturated, super- 

 natant liquid. The root plug was held together, and pressed upon, by 

 a Incite lid with 16 holes. It was also covered with a cellulose mem- 

 brane sprinkled with CaCCs powder. To reach the roots, iron dissolved 

 from the oxide grains in the presence of CaCOs, saturated with CO2 

 ( pH 6.3 ) , had to diffuse through the Visking membrane and the pores 

 of the disc resting on the surface of the root plug. 



Figure 10. Diagram of a lu- 

 cite cell for measuring the de- 

 composition of iron oxide and 

 the diffusion of iron in root 

 piles. At the right is an en- 

 largement of the lid, showing 

 the Visking membrane and 

 the perforated Incite disc. 



W 



H2O. 



Lids 



Root, 

 plug 



^^ 



X 



v 



CO; 



/ 



w 



'^^r%<^' 



CaCO 



Te203 + 

 CaCO, 



