SUMMARY 1 ( .)"> 



The soil minerals do not dissolve as completely and easily 

 as the above statements might indicate. Reactions in the 

 soil are never complete and while potash, for example, may 

 continually dissolve from feldspar it is never completely 

 decomposed. Intermediate compounds are formed. Tem- 

 perature affects the solubility of soil minerals. The higher 

 the temperature, the more active the decomposition, except 

 in the case of carbon dioxide, which is less soluble in warm 

 water than in cold water. Some minerals decompose very 

 readilv; others are more resistant to the action of soil 

 solvents. 



Plant food which is rendered soluble is not altogether 

 leached out of the soil, but is retained by a process called 

 absorption. This process may be either chemical or physical 

 in nature. Chemical absorption is that which takes place 

 because of a chemical reaction, and results in the formation 

 of an insoluble compound. Sometimes the reaction consists 

 merely of an exchange of bases by an insoluble compound 

 and a soluble compound. At other times it consists of a 

 reaction between two soluble compounds, resulting in pre- 

 cipitation. Referring to the first instance it is to be noted 

 that, other things being equal, bases vary in their ability 

 to replace one another. Potassium and magnesium, for 

 example, will replace sodium and calcium. Very frequently 

 reactions which result in the formation of an insoluble 

 compound are the exact reverse of a reaction which results 

 in the formation of a soluble compound, and this is due 

 to the law of mass action, which states that the direction 

 of a reaction depends on the active mass of the reacting 

 substances. Reactions proceed in one way or another until 

 the reacting masses balance one another. This equilibrium, 

 however, is never accomplished in a soil, for the chemistry 

 of soils is essentially dynamic in nature. 



Soluble compounds in the soil are retained by physical 

 absorption because of an attraction which exists between the 

 surfaces of certain solids and substances in solution. This 

 attraction results in a greater concentration of dissolved 

 material immediately about the surfaces of solids. Physical 

 absorption or adsorption, as it is termed, depends on the 



