670 PLANT GROWTH AND PLANT COMMUNITIES 



colloidal particle. It governs exchange, and determines electrokinetic 

 behavior. The second is the Donnan membrane equilibrium, which 

 stresses the electrolyte nature of colloidal systems. It has been produc- 

 tive in describing the bio-ionic potentials of ion-exchange membranes 

 (HelflFerich, 1956; Robertson, 1956). 



Flow and counter-flow of ions between root and soil 



In one of the earliest quantitative experiments ( Jenny and Cowan, 

 1933) on two-way rhigrations of ions in root-soil systems, soybean 

 seedlings were grown for 35 days in a pure Ca-clay suspension which 

 had an initial pH of 6.3. The plants removed from the clay 1.02 milli- 

 equivalent ( m. eq. ) of calcium, \\'hereas the clay gained simultaneously 

 0.95 m. eq. of exchangeable hydrogen— a nearly equivalent exchange. 



In standard Hoagland solution, barley plants accumulate large 

 quantities of nutrients. They develop "high-salt roots." Transferred to 

 distilled water, these roots will leak appreciable amounts of potassium. 

 But plants raised in dilute Hoagland solution develop "low-salt roots." 

 Tenaciously they hold on to their nutrients, and the release of potas- 

 sium to distilled water is nil or negligible. Four grams (dry-weight 

 basis) of excised, low-salt barley roots were leached continuously for 

 ten hours with 380 liters of distilled water (Jenny and Overstreet, 

 1939). The potassium content was but slightly lowered: 2.1 per cent, 

 from 43.1 to 42.2 m. eq., which is within experimental error and root 

 variability. 



Clay suspensions, on the other hand, drastically deplete low-salt 

 roots by a two-way flow of ions. As the results have been reported 

 previously (Jenny and Overstreet, 1939; Jenny cf ah, 1939), they will 

 be merely sketched here. 



1. Excised, low-salt roots were immersed in dilute K-H-clay sus- 

 pension. The roots absorbed potassium ions and increased their potas- 

 sium content by 28.5 per cent; at the same time they lost calcium ions 

 to the clay, to the extent of 22.2 per cent of their supply. 



2. When placed in Ca-H-bentonite sols, corresponding roots took 

 up calcium ions (6.4 per cent of their initial content) but experienced 

 a loss of potassium of 19.4 per cent. 



3. Roots ( 4.51 grams, oven-dry basis ) of intact barley plants con- 

 taining radioactive potassium were placed in K-Ca-bentonite suspen- 

 sions (0.35 per cent). After five hours the roots had given ofiF 0.114 

 m. eq. of their potassium to the suspension, as detected by radioactivity 

 of the clay. At the same time the roots accumulated 1.155 m. eq. of po- 

 tassium from the clay suspension. Potassium ions must have moved 

 simultaneously in both directions— from root to clay and from clay to 



