j6 Mineral Nutrition of Plants 



total exchangeable quantities. This has been done in Figure 6 in which 

 the activity ratio K/Ca is plotted vertically and the concentration ratio 

 horizontally. It covers the range from 70-100 per cent calcium and 

 30-0 per cent potassium. The curves for the different clay minerals are 

 spread over a wide range. The value of such a comparison can readily 

 be seen by considering the case of a soil dominated by kaolinitic clay 

 such as the Cecil in comparison with a beidellitic soil such as the 

 Putnam. Let us suppose that both are 10 per cent saturated with potas- 

 sium and 90 per cent with calcium. The ionic concentration ratio K/Ca 

 is thus 0.2. Then in the kaolinite soil the activity ratio K/Ca is 0.8 

 while in the beidellitic soil it is 4.0. This means an enormous difference 

 in the chemical environment of the plant root in the two cases. 



Thus a considerable variety of factors play a part in establishing the 

 mean chemical environment of the plant root. The nature of the col- 

 loids, their concentration, and the particular proportions of exchange 

 ions present are all powerful in their effects. Much further investigation 

 in this field is needed, but the experimental methods are well under- 

 stood. They have proved reliable precisely in those ranges of activities 

 in which plant roots live and grow. 



CATIONIC ACTIVITIES AND PLANT GROWTH 



From the composition of the nutrient solution used by plant physi- 

 ologists it is possible to calculate approximate cationic activities. It has 

 been shown that plants thrive over a considerable range of such activi- 

 ties, the range being widest where the cations concerned (usually K, 

 Mg, and Ca) are in proper balance. So far as the author is aware, no 

 studies have ever been published in which colloidal systems and true 

 solutions have been compared at equal cationic activities as regards 

 their effects upon plant growth. Such experiments are now entirely feas- 

 ible. E. O. McLean (6) has made a preliminary study of this kind using 

 montmorillonite clay with calcium and potassium together, and com- 

 paring this medium with dilute true solutions of the same activities. 

 Soybeans were grown in both sets of systems at a prescribed series of 

 calcium and potassium activities. In order to maintain constant condi- 

 tions, frequent changes of the nutrient systems were made. The over- 

 all comparison of the effects of the solutions as compared with the 



