C. Edmund Marshall 77 



colloidal suspensions was vitiated to some extent by the fact that the 

 bentonite systems released some magnesium to the plants, whereas 

 none was supplied in the true solutions. The plants grown on the 

 calcium-potassium bentonite systems were uniformly larger than those 

 grown on the true solutions but were of the same percentage composi- 

 tion as regards potassium and calcium. The most surprising feature of 

 these experiments was that the systems giving Ca = 0.7 X io -4 and 

 a K = 1.8 X io -4 apparently provided ample supplies of both calcium 

 and potassium, since larger amounts and varied ratios of calcium and 

 potassium showed no increase in uptake. Future experiments will 

 therefore have to be designed to give comparisons at even lower activi- 

 ties. The experimental methods for measurement will therefore be 

 pushed to the utmost, since our membranes begin to deviate from the 

 ideal behavior according to the Nernst equation below io -4 molar in 

 the case of potassium and below io — 5 molar in the case of calcium. We 

 intend to repeat this type of experiment with better control of the 

 colloidal medium since it is of such critical importance. 



REFERENCES 



1. Barber, S. A., Ph.D. Thesis, Univ. of Missouri (1949). 



2. Burd, J. S., Soil Sci., 64:223 (1947). 



3. Chatterjee, B., and Marshall, C. E., /. Phys. Chem., 54:671 (1950). 



4. Jarusov, S. S., Soil Sci., 43:285 (1937). 



5. Jenny, H., and Ayers, A. D., Soil Sci. 48:443 (1939). 



6. McLean, E. O., Ph.D. Thesis, Univ. of Missouri (1948). 



7. , and Marshall, C. E., Soil Sci. Soc. Am. Proc, 13:179 (1948). 



8. Marshall, C. E., Soil Sci. Soc. Am. Proc, 7:182 (1942). 



9. , and Ayers, A. D., Soil Sci. Soc. Am. Proc, 11:171 (1946). 



10. , /. Am. Chem. Soc, 70:1297 (1948). 



11. Marshall, C. E., and Bergman, W. E., /. Am. Chem. Soc, 63:1911 

 (1941). 



12. , /. Phys. Chem., 46:52 (1942). 



13. , /. Phys. Chem., 46:327 (1942). 



14. Marshall, C. E., and Eime, E. O.. /. Am. Chem. Soc, 70:1302 (1948). 



15. Marshall, C. E., and Krinbill, C. A., /. Phys. Chem., 46:1077 (1942). 



16. Marshall, C. E., and McLean, E. O., Soil Sci. Soc. Am. Proc, 12:172 



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