THE SOIL AS A SOURCE OF MINERAL SALTS 



139 



in the form of relatively insoluble substances from which ions do 

 not readily exchange. Montmorillonite minerals for example, 

 contain large amounts of potassium in a non-exchangeable form, 

 and apatite consists of insoluble calcium phosphates. Such sub- 

 stances are slowly transformed under natural conditions by the 

 complex sequence of changes known as "weathering", and this leads 

 gradually to increased solubility and exchangeability of ions. The 

 rate of weathering varies greatly for different minerals, and is 

 affected by pH, soil moisture content and temperature. Contact 

 between plant roots and soil increases the rate at which ions are 



Table 14. Cation Exchange CAPAcrriEs (C.E.C.) and the Amounts of Major 



Exchangeable Ions in Various Soils. 



(From Wiklander, 1958). 



rendered more exchangeable, both by mechanical effects which 

 cause the break-up of larger soil particles, and by chemical effects 

 of carbonic acid, and organic acids, produced by the roots. 



It is well known that many clay soils are able to supply sufficient 

 potassium for the indefinite growth of natural vegetation, if not of 

 crops. This is possible because an equilibrium exists between non- 

 exchangeable potassium, exchangeable potassium and the soil 

 solution. If a soil becomes low in dissolved or exchangeable 

 potassium, non-exchangeable ions are gradually released and become 

 available to plants. Conversely, some fixation of potassium in non- 

 exchangeable forms occurs when potassium-containing fertilizers 

 are added to potassium-depleted soils. 



