Hoagland — 154 — Plant Nutrition 



amounts of potassium fixed, in both replaceable and 

 non-replaceable forms. The explanation advanced 

 above would imply a reaction with potassium that 

 would decrease the exchange capacity of the colloid, 

 by an amount equivalent to the potassium fixed in non- 

 replaceable form. 



This decrease in exchange capacity is realized in 

 experiments on bentonitic colloids, and on some col- 

 loids separated from soils, by ascertaining the ex- 

 change capacity of the colloids by means of determina- 

 tions of the total amounts of ammonium ions capable 

 of adsorption by the colloidal preparations, before and 

 after the fixation of potassium in non-replaceable form. 

 Demonstration of this loss of exchange capacity is 

 not always so easily performed on the entire soil. 

 There are in fact many unsolved problems of potas- 

 sium fixation, which belong to a special field of study 

 in soil chemistry. From the point of view of plant 

 nutrition we are more immediately concerned with 

 the relations of the various forms of potassium com- 

 binations to the capacity of the soil medium to supply 

 potassium to plants at a rate commensurate with their 

 physiological requirements. 



The Capacity of the Soil to Supply Potassium to 

 Plants: — What is the relation of the solution to the 

 potentiality of the soil to supply potassium to plants? 

 An earlier lecture presented the point that not often 

 is the total amount of potassium present in the soil 

 solution at any one instant, even in the whole mass 

 of soil accessible to the roots, sufficient to meet the 

 needs of the plant over its entire growth period. On 

 the basis of the soil solution theory of absorption of 

 nutrients by plants, potassium would have to enter 

 this solution from the solid phase of the soil as ab- 

 sorption by the plant takes place, rapidly enough to 

 prevent the concentration of potassium from falling 

 to a critically low value at any time, in relation to 

 physiological requirements of plants at various stages 



