CHAPTER 



w Contact Phenomena Be- 

 tween Adsorbents and 

 Their Significance in 

 Plant Nutrition 



HANS JENNY 



S, 



peculations on the nature of mineral uptake by roots 

 in soils are encountered very early in botanical literature. Two states of 

 existence of nutrients in the soil were recognized: first, nutrients in 

 the solid portion which were considered unavailable to plants; second, 

 nutrients dissolved in the liquid phase which could be ready assimi- 

 lated by plant roots. These dissolved nutrients were known to diffuse 

 freely in the soil moisture, and they were known also to move with the 

 flow of water in the soil. Water containing dissolved nutrients consti- 

 tuted the soil solution, a fruitful concept which is still in use today. In its 

 essence, the soil solution corresponds to the nutrient solution of the 

 plant physiologist. 



SOIL SOLUTION CONCEPT 



Investigators who study the soil solution usually define it on an 

 operational basis. The soil solution is that part of the liquid phase 

 which can be separated from the bulk of the soil by some sort of 

 displacement method (5), extraction or centrifugation. 



To the colloid chemist it is clear that the extraction or displacement 

 technique does not measure the adsorbed (exchangeable) cations, in- 

 cluding hydrogen, which remain attached to the colloidal particles. As 

 we shall see, this omission has far-reaching consequences. 



In this paper we shall assign the adsorbed ions to the solid phase. 

 Although they are surrounded by and bathed in water molecules, they 





