54 



MINERAL SALTS ABSORPTION IN PLANTS 



1-0 



E 



0-5 



o 



Q. 



3 



(0 







id 





/ o 

 o 



\/ 



.-0- 



_L 



o 



5 10 



Concn., mg /L 



E 



o. 



Z3 



0-5 



0-8 1-0 



Concn ., mg (PjO^) /L 



1-5 



Fig. 17 (c-d). pH and salt absorption, 

 c. Relationship between nitrate concentrations and the rate of absorption 

 by maize plants at pH 6 ( — ) and pH 7-4 ( — ) (redrawn from Van den 

 Honert and Hooymans, 1955). d. Relationship between phosphate concen- 

 tration and rate of absorption by maize plants at various pH values of 

 the medium. Solid lines represent experimental data; dashed lines calcu- 

 lated on the assumptions that only univalent ions are absorbed, and that 

 pH affects absorption only by altering the ratio of various phosphate ions 

 in solution (redrawn from Van den Honert, 1933). 



than competition with bicarbonate ions was responsible, because 

 varying the concentration of bicarbonate when the pH value was 

 kept constant did not affect nitrate absorption. 



The concentration of hydrogen ions in the medium has an 

 especially important effect on phosphate absorption because over the 

 physiological range of pH values the predominant ionic form shifts 

 from univalent (H_,P04), to bivalent (HPO4) and finally to 

 trivalent (PO^) as the medium becomes more alkaline. Van den 



