ACTION ON MINERALS 91 



the form of an insoluble silicate, which also contains alum- 

 inum. A soil may contain thousands of pounds of this 

 potassium compound to an acre and yet may respond to 

 the application of soluble potassium salts. It is believed 

 that the by-products of bacterial action have an effect 

 upon ihc insoluble compounds, tending: to bring some of 

 the potassium into soluble form. For example, the bicar- 

 bonate of lime is supposed to react with the aluminum- 

 potassium salt, forming potassium bicarbonate. It is 

 probable that stronger organic acids formed in the decom- 

 position of organic matter exert a solvent action on the 

 potassium compounds of the soil. 



It is essential to have soluble potassium compounds not 

 only for the green plants but for some of the important 

 classes of bacteria in the soil. 



Sulphur. Sulphur is an essential constituent of every 

 plant or animal cell. The green plants derive their sup- 

 ply from the sulphates of the soil. The amount of sulphur 

 needed is so small that in most soils there is an abundant 

 supply for the needs of the crop. Some soils, however, re- 

 spond to the application of sulphur. It is probable that 

 the favorable action of superphosphate is sometimes due 

 to the sulphur it contains. 



The sulphur of organic matter appears as a sulphide 

 when the material undergoes decomposition. When the 

 decomposing material is high in sulphur, as in the case of 

 the egg, the odor of this gas is very apparent. The odor 

 may also be noticeable in sewage, in which the formation 

 of hydrogen sulphide can be easily shown by adding a 

 little ferrous sulphate. The iron is precipitated as iron 

 sulphide, which is black in color. 



Hydrogen sulphide contains much energy and forms a 

 source from which certain bacteria derive the energy for 

 growth. They oxidize the sulphide to free sulphur, which 



