COLLOID CHEMISTRY OF THE SOIL IN RELATION TO PLANT NUTRITION 727 



Amberlite only) the potassium activity was 1.1 x 10~* molar, and the 

 lowest calcium activity was 4.7 x 10'^ molar. Again there was no in- 

 jury, but evidence of deficiency was shown in a steep rise of calcium 

 uptake with increasing activity. In these experiments, as discussed 

 below, the importance of the bonding-energy relationships of cations 

 was demonstrated ( Marshall and Upchurch, 1953 ) . 



In the third series (Higdon, 1958) a number of low cationic ac- 

 tivities were included, and definite evidence of deficiencies, both direct 

 and indirect, was obtained. The sprouting procedure used, combined 

 with the low calcium content of the seed, gave seedlings which were 

 beginning to suflFer from calcium deficiency. The study was thus con- 

 cerned with recovery from or enhancement of this deficiency. 



Where calcium was the only cation of the substrates, the calcium 

 chloride solution at 5.3 x 10'^ molar did not arrest losses of calcium, 

 whereas the calcium bicarbonate at the same concentration just did so. 

 In calcium-potassium systems, plants grown on the chlorides at 6.8 x 

 10"^ molar calcium lost this element when the K/\/Ca ratio was 0.078, 

 but they just maintained constancy at K/vCa = 0.021. Thus the steep 

 part of the curve for uptake of calcium was around 7 x 10"^. 



In the presence of large amounts of calcium ( 4 x 10'^ ) , the chloride 

 system grown at a potassium activity of 3.5 x 10"^ showed much greater 

 uptake than that at 1.0 x 10"^. In the presence of small amounts of 

 calcium (6 x 10"^), the chloride system grown at aK = 6.4 x 10"^ gave 

 slightly greater uptake than that at aK = 1.7 x 10"^ Thus the steep part 

 of the curve for uptake of potassium in relation to activity lies in the 

 range 1 to 5 x 10"* molar. 



Where uptake lay on the steep parts of the respective curves, the 

 absolute activity of the cation was found to be of greater importance 

 than the K/\/Ca ratio, although this also had its effects. 



Weak electrolyte effects 



There is a considerable body of evidence to sho\\' that plant roots 

 take up larger quantities of nutrient cations from the salts of weak acids 

 than from corresponding salts of strong acids. In highly dilute systems, 

 bicarbonates have been shown to be superior to chlorides ( Higdon and 

 Marshall, 1959). In our most recent comparisons (Table II), the abso- 

 lute activities of the cations are similar, whilst their ratios to one an- 

 other vary only slightly. The factor that influences uptake is the rela- 

 tionship of the hydrogen ion to the anion in solution. Formation of a 

 weakly dissociated acid favors uptake of nutrient cations. 



This has been formulated in terms of a cation exchange reaction 

 between the plant root and nutrient salt, b\- whicli the root takes up the 

 metallic cation and loses Indrogen ions. Ob\iousl\- any bonding be- 



