Lecture 7 — 153 — Potassium Nutrition 



At first, in the researches in California on the 

 potassium problem chief attention was given to the 

 fixation of potassium in the replaceable form. At the 

 time it was supposed that at least the rapid fixation 

 of potassium was primarily in this form. But soon 

 it became clear that some of the potassium added to 

 certain soils could also undergo fixation of such a 

 nature that the potassium could not be easily replaced 

 by another cation. This fixation may take place quickly 

 and thus not require long-time weathering processes 

 in the field. The potassium entering this form has 

 been termed "non-replaceable," on the basis of criteria 

 provided by chemical procedures commonly used in 

 the laboratory for estimating replaceable potassium. 

 It is not to be inferred that a sharp line of demarcation 

 can be drawn between replaceable and non-replaceable 

 potassium, but a valuable differentiation of degree of 

 resistance to chemical reagents can be made. 



Many studies have been conducted in recent years 

 by various investigators on fixation of potassium by 

 soil colloids in the non-replaceable, or difficultly re- 

 placeable form, but not all the questions have been 

 satisfactorily answered. The fixation of potassium 

 by this mechanism is generally increased by wetting 

 and then drying the soil. Sometimes repetition of the 

 wetting and drying treatments augments the fixation 

 in non-replaceable form. One tentative explanation 

 of this phenomenon given by several investigators 

 rests on a property of certain types of soil colloids 

 that have lattice structures capable of expansion and 

 contraction under different degrees of hydration, as 

 is true of colloids of the montmorillonite type (see 

 illustrations in Lecture 1). According to this hypo- 

 thesis potassium ions would react with the colloid 

 and the lattice would contract, especially when drying 

 occurred, preventing the subsequent replacement of 

 the potassium ions, although according to our experi- 

 ments the bases calcium, magnesium and sodium are 

 first released in amount almost equivalent to the total 



