O ver street an d Dean 1 03 



soils is a field for a great deal more experimentation. In the case of 

 phosphate it begins to appear that its availability may be rather closely 

 related to its concentration in the soil solution; that is, in a filtrate of 

 the soil. This is additional indirect evidence that the absorption rate of 

 the phosphate ion may be determined by its activity rather than con- 

 centration in the soil medium. 



Importance of the question of reversibility 

 of the ion absorption reaction 



A knowledge of the reversibility of the absorption by roots of a 

 given nutrient ion is of paramount importance in the estimation of its 

 availability. This is the case because the effective rate at which a reversi- 

 ble reaction proceeds will be influenced by the concentrations or activi- 

 ties of the products as well as of the reactants. For example, if it were 

 definitely established that the following absorption reaction were 

 reversible 



R' ■ OH + H 2 P(V > R' ■ H 2 P0 4 + OH - , 



then obviously the extent to which H 2 P0 4 _ would be absorbed would 

 depend on the amounts of R'H 2 PO, in the plant and of OH - in the 

 culture medium as well as on R'OH and H 2 P0 4 ~. Thus, eventually 

 it may be established that the availability of certain ions are functions 

 of ratios of concentrations or activities. 



REFERENCES 



1. Allison, F. E., and Sterling, L. D., Soil Sci., 67:239 (1949). 



2. Anderson, M. S., Keyes, M. G., and Cromer, G. W., U. S. Dept. Agr. 

 Tech. Bull. 813 (1942). 



3. Barbier, G., and Chabannes, J., Compt. rend., 218:519 (1944). 



4. Berger, K. C, and Truog, E., /. Am. Soc. Agron., 32:297 (1940). 



5. Berger, K. C, Adv. in Agronomy, 1:321 (1949). 



6. Borland, J. W., and Reitemeier, R. F., Soil Sci. (In Press) (1950). 



7. Bower, C. A., Soil Sci., 59:277 (1945). 



8. Brooks, S. C, Trans. Faraday Soc, 33:1002 (1937). 



9. Broyer, T. C, and Overstreet, R., Am. /. Botany, 27:425 (1940). 



10. Dean, L. A., /. Agr. Sci., 28:234 (1938). 



11. , and Rubins, E. J., Soil Sci., 59:437 (1945). 



12. , Soil Sci., 63:377 (1947). 



