Hoagland — 12 — Plant Nutrition 



ter are of signal importance in that the entrance of 

 nitrate ions into the soil solution must of necessity be 

 accompanied by the entrance of cations from the solid 

 phase of the soil. Some of the soils also developed 

 relatively high amounts of sulphate ions and in soils 

 of the type studied bicarbonate ions likewise are sig- 

 nificant anion components of the soil solution. The 

 biological generation of anions, and of hydrogen ions, 

 thus plays a dominant role in determining the con- 

 centration and composition of the soil solution, to- 

 gether with the reactions of soil colloids, now to receive 

 consideration. 



The Role of Soil Colloids : — In the early stages of 

 the investigation of soil solutions there was lacking 

 an essential key to an understanding of these phe- 

 nomena; that is, adequate knowledge of the nature 

 and behavior of clay colloids. Of greatest interest is 

 the phenomenon of base exchange in colloids of this 

 type. In the earlier period to which I have referred, 

 ideas of base exchange were rather vague. Some- 

 times it was thought that base exchange was par- 

 ticularly a property of special minerals known as 

 "zeolites." Then the systematic and valuable researches 

 on soil colloids of the Russian chemist, Gedroi2, for 

 a long time hidden in the Russian language, were 

 translated into English and became familiar to Ameri- 

 can investigators, and the work of the Dutch inves- 

 tigator, HissiNK, also became known. The principle 

 of stoichiometric replacement of adsorbed bases, pri- 

 marily calcium, magnesium, potassium and sodium, 

 and that of the large dependence of the physical prop- 

 erties of the colloid on the nature of adsorbed bases, 

 were rapidly recognized to have profound consequences 

 in soil science and plant nutrition. Interest in the 

 subject of base exchange has increased, rather than 

 diminished, in the years that have followed. 



The significance of base exchange in clay colloids 

 and of organic colloids also, is not difficult to appre- 



