44 Mineral Nutrition of Plants 



categories, and the relative rate of possible transformation is indicated 

 roughly by the width of the connections. 



Forms of potassium in relation to availability categories 



To serve as an example, the forms of potassium which make up 

 the various categories of this element are given in the diagram. In this 

 case, the exchangeable form makes up nearly all of the readily availa- 



THE THREE AVAILABILITY CATEGORIES OF PLANT NUTRIENTS IN SOILS 

 J 



\ 



READILY 

 AVAILABLE 



T 



MODERATELY 

 AVAILABLE 



1 



SLOWLY 

 AVAILABLE 



Figure 4. Schematic illustration of availability categories of plant 

 nutrients in soils (exemplified with the element potassium) and trans- 

 formation from one category to another. Arrows indicate the direction 

 of transformations, and the wider the connection the more rapid the rate 

 of transformation. 



ble category. Only small and rather insignificant amounts exist as 

 water-soluble salts, such as nitrates and sulfates. 



Moderately available potassium (extracted with dilute acid) exists 

 in the so-called "fixed state" as hydrous mica (illite) and biotite. It 

 appears that at high levels of exchangeable potassium, some of this 

 potassium gradually becomes transformed to a nonexchangeable or 

 fixed form. Possibly in this transformation the montmorillonite which 

 holds the exchangeable potassium goes over to hydrous mica or some 



