Transformation of K, Ca, Mg, Fe 241 



inorganic ionns, notabU' salts of sulfates, chlorides, and phosphates; 

 in insoluble inorganic form, known as marl; and in the form of stal)le 

 maniues and plant residues. The K_.0 content of plant residues 

 ranges from 0.5 to 2.0 per cent. Fresh manure contains 0.288-0.504 

 per cent K^.O. Its concentration in the ash content of bacterial cells 

 is 4.0-25.6 per cent, and in that of fungus mycelium, 8.7—39.5 per 

 cent. The presence and activities of microorganisms influence greatly 

 the a\'ailabilit\- of potassium in soil to plant growth. Microbial ac- 

 tivities may lead to an increase in the available potassium, as when 

 organic matter is decomposed by microorganisms and when acids 

 interact with the zeolites, liberating the potassium. Orthoclase inter- 

 acts with certain microbial products to give soluble potassium salts: 



AUOs-KoOOSiO, + 4H2S()4 - Al2(S04)3 + K2SO4 + ()Si02 + 4H2O 



AlaOa-KoOCiSiOo + ra(H(X)3)2 = AL^Oa-CaO GSiOs + ^2KHC()3 



Potassium compounds present in the soil or in culture are assimi- 

 lated by bacteria and fungi and stored away in the cell material. 

 When the latter is decomposed, the potassium again becomes avail- 

 able. The potassium replaces in the soil the zeolitic bases Ca, Mg, 

 and Na. The concentration of available potassium in the soil is thus 

 controlled not only by the total concentration of the element, but 

 also by the form in which it is present in the soil, by the degree of 

 saturation of the soil zeolites, by the soil reaction, by the available 

 organic matter, and finally by the activities of various groups of 

 microorganisms. 



The Azotobacter method has often been used to determine the 

 concentration of available potassium in the soil, which was found to 

 vary between 2 and 30 per cent of the total, depending upon the 

 fertility of the soil. 



Calcium and viagnesium are also essential in the nutrition of soil 

 microorganisms. In addition, they play important parts as buffering 

 substances for neutralizing the organic and inorganic acids formed in 

 the soil. 



Iron may undergo in the soil a variety of transformations through 

 the activities of microorganisms. It is essential for cell synthesis. 

 Certain bacteria are capable of oxidizing ferrous salts to ferric com- 

 pounds and of utilizing the energy liberated for the assimilation of 

 carbon dioxide, in a manner similar to the action of nitrifying and 

 sulfur-oxidizing bacteria, according to the following reaction: 



2FeS04 + 8H2O + 2Ca( O3 + O = ^2Fe(OH)3 + !2CaS04 + 2CO2 



