PLANT NUTRITION 259 



ion. Na+ and K+ carry a large number of water molecules; Ca++ and 

 Mg++ carry fewer. The H+ ions also carry water as they do at all 

 times, and in newer chemical nomenclature they are properly called 

 hydronium ions. In addition to water of hydration carried by ions, 

 water molecules are carried on the faces of these particles and by the 

 spaces and channels between the thin sheets which make up the 

 colloidal plates. 



The composition of the clay colloidal materials varies greatly and 

 depends upon such climatic conditions as temperature and rainfall 

 during their formation, as well as on the composition of the parent 

 rock. The ratio of silica to sesquioxides in soil colloids varies and 

 seems to have an important effect on the properties of these particles. 

 A soil containing a high ratio of silica to sesquioxides shows greater 

 negative charge, dispersibility, viscosity, swelling, heat of wetting, ad- 

 sorption of bases, and rate of base exchange. These colloids retain 

 their negative charge in both acid and alkaline solutions. If a soil has 

 a low silica-sesquioxide ratio the mineral colloids show an amphoteric 

 behavior, for they become electropositive in acid solutions and will 

 then absorb anions such as chloride ions and sulfate ions instead of 

 positive ions. 



Base exchange. The fixation of a positive ion by a colloidal particle 

 of the soil is accompanied by the release of one or more previously 

 held positive ions. Such an exchange of ions is commonly called base 

 exchange or, more properly, cation exchange. An illustration of cat- 

 ion exchange is found in the gradual acidification of soil through the 

 decomposition of organic matter and subsequent leaching of calcium. 

 As organic matter decomposes in the soil, carbon dioxide is produced 

 and combines with the soil water to form carbonic acid. The hydrogen 

 ion of this acid will replace positive ions in the outer layer of the col- 

 loidal soil particles. If the particles contain a large amount of ad- 

 sorbed calcium, the following change takes place: 



Ca++ 



Colloidal 

 particle 



H+ 

 + 2H2CO3 -^ 



H+ 



Colloidal 

 particle 



+ Ca(HC03)2 



Since calcium bicarbonate is soluble in water, leaching will remove this 

 compound from the topsoil. The loss of calcium in this manner will 

 gradually increase the acidity of the soil. 



The following principles have been well established for base- 

 exchange reactions of soils: 



