10 



Transformation of Mineral* Substances 

 in Soil by Microorganisms 



Through their roots, plants take up from the soil a number of dif- 

 ferent elements in the form of salts. These are frequently classified 

 as essential and minor or trace elements. The first group includes 

 sulfur, phosphorus, potassium, calcium, magnesium, iron, manganese, 

 and sodium. The second group includes zinc, molybdenum, cad- 

 mium, chlorine, aluminum, boron, copper, silicon, and a variety of 

 others. In the transformation of these elements in the soil, micro- 

 organisms frequently play a direct part, though often their effect 

 upon the various elements is merely indirect. Even such inert ele- 

 ments as silicon may be essential in the growth of certain bacteria 

 and of various algae, notably the diatoms. Molybdenum has been 

 found to play a role in the fixation of atmospheric nitrogen. Other 

 elements are found to have a neutralizing effect upon plant toxins. 

 Still others, like cobalt, may influence the growth of the cell to retard 

 multiplication. Some of the elements, notably phosphorus, are essen- 

 tial constituents of the cell nucleus and its cytoplasm. Still others, 

 like sulfur, may serve as sources of energy to various bacteria and 

 also form essential constituents of certain amino acids. 



Transformation of Sulfur 



Sulfur is one of the elements essential for plant growth. This ele- 

 ment makes up about 0.11 per cent of the earth's crust. Phosphorus 

 occurs in about the same concentration. It appears from analyses of 

 river water that sulfur is removed from the soil faster than any other 

 element, however, since its ions, SO4, tend to dissolve in the water, 

 whereas the PO4 ions tend to precipitate in the soil. 



*■ The term "mineral" is used to designate all elements and compounds not 

 containing carbon or nitrogen. 



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