PLANT ROOT-SOIL INTERACTIONS 673 



in equilibrium with air— contained only 2.2 ix eq. of strontium. Without 

 discarding any part of the solution or solid, the strontium resin was 

 scooped into a short, vertical Incite tube, the bottom end of which is 

 open and covered with nylon gauze. If final equilibrium had not been 

 achieved (which could not be determined precisely) the error was in 

 favor of the solution phase. 



Roots of alfalfa seedlings were immersed for 18 hours in the equi- 

 librium solution and in the resin slurry. The solution plants acquired 

 2.8 /x eq. of strontium, and in doing so reduced the strontium content 

 of the solution by only 12 per cent. The slurry-plants accumulated 17.8 

 /x eq. of strontium— a five-fold increase. 



In a second type of two-phase experiment, a large volume of clay 

 suspension is centrifuged or passed through an ultrafilter to obtain a 

 large quantity of supernatant liquid or dialysate. Plants are then grown 

 in the original suspension and in the separated phase (Scheuring and 

 Overstreet, 1961). Or, if only a small amount of ultrafiltrate is ob- 

 tained, a large volume of artificial analogue solution is used as the 

 phase partner (Jenny, 1952). As a modification, carbon dioxide is 

 passed through the clay sol, and an uftrafiltrate is obtained which 

 supposedly simulates conditions near the root surface ( Overstreet and 

 Jenny, 1939). 



A third type of two-phase experiment employs the double-column 

 or two-pot technique. Soil is in one vessel, inert sand in the other, and 

 both are shunted, either in parallel or in series ( see Figure 6 ) . A com- 

 mon nutrient solution continually circulates through both vessels, by 

 pump lifting and gravity flow. A detailed illustration is presented in 

 the ensuing sections. 



Ion transfer: a rate problem 



Amberplex-C-1 membranes, manufactured by Rohm and Haas Co., 

 consist of a frame structure of thread molecules carrying negative 

 charges, mainly as sulfonic-acid groups. They may be neutralized by 

 cations, which are then exchangeable. 



In a wet, swollen condition, H-amberplex is 0.80 mm. thick. A slab 

 one square centimeter in cross-section contains 0.127 m. eq. of H + , or 

 2.57 m. eq. per gram of oven-dry membrane. In the wet membrane the 

 mean distance between two exchange sites is 10 Angstrom units. If the 

 exchangeable cations are considered dissolved in the total water con- 

 tent of the membrane, their concentration corresponds to 4.7 normal. 

 According to H. El Hamawi, the H-membranes have a pore space, or 

 "free space," of 9.70 per cent, as determined by HCl uptake (Hope 

 and Stevens, 1952). For calcium membranes the free space is 7.90 per 

 cent. 



