CHAPTER XXV 



Transformation of Minerals in the Soil 



Nature of mineral transformation by microorganisms. Among the 

 mineral elements of plant food which are subject directly or indirectly 

 to the action of microorganisms, the following may be included : phos- 

 phorus, sulfur, potassium, calcium, magnesium, iron, sodium, manga- 

 nese, also chlorine, aluminum, zinc and silicon. The transformation of 

 the elements and compounds of carbon, nitrogen, hydrogen and oxygen, 

 both in organic and inorganic combinations, are considered in detail 

 elsewhere. The above minerals are transformed in the soil by different 

 processes : 



1. Mineral elements (S, Fe, Mn, etc.) or their inorganic compounds may be 

 used by certain bacteria as sources of energy. 



2. Certain salts (nitrates, sulfates, etc.), rich in oxygen, may be used, under 

 conditions favoring anaerobiosis, as sources of oxygen. This usually leads to the 

 reforming of substances which have been acted upon in the processes of oxidation. 



3. The transformation of minerals present in the soil in the form of complex 

 organic compounds. When the bodies of plants, animals and microorganisms 

 are decomposed or mineralized by the soil microorganisms, a part of the minerals 

 is liberated in the form of inorganic compounds and a part may be reassimilated. 



4. The assimilation of minerals by microorganisms. In the presence of 

 available energy, simple inorganic salts are converted into complex organic com- 

 pounds; this is especially true of phosphates, potassium salts and sulfates and 

 leads to a temporary removal of the soluble salts. Most of these materials are 

 again made available upon the death and decomposition of the microbial cells, 

 as described in the third process. 



5. The indirect transformation of minerals in the soil by products of the 

 metabolism of microorganisms. The action of carbon dioxide, organic and 

 inorganic acids upon carbonates, phosphates and silicates is largely due to the 

 change in the H + or the OH - concentrations of the soil. This is even true of 

 the interaction between insoluble phosphates and the "humic acids." 1 



Phosphates, sulfates and salts of potassium, calcium and magnesium 

 (iron salts in smaller amounts), are the most important compounds in 



1 Baumann and Gully, Mitt. K. Bayr. Moork. 4: 31-156, 1910; Niklas, N. 

 Untersuchungen ilber den Einflusz von Humusstoffen auf die Verwitterung der 

 Silikate. Diss. Miinchen 1912. 



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