178 TRANSFORMATION OF MINERAL SUBSTANCES 



these soluble compounds in the vicinity of the developing plant 

 roots may be of particular importance in their nutrition. 



Thus, under aerobic conditions, iron may be brought into solu- 

 tion provided the reaction becomes more acid. Under anaerobic 

 conditions, the solution is greatly accelerated by reason of the fact 

 that iron becomes reduced to the ferrous form and in this condition 

 is much more soluble than in the ferric form at reactions close to 

 neutrality. Solution is further favored by the fact that anaerobic 

 conditions lead to the formation of organic acids which exert con- 

 siderable dissolving effects. 



Most of these changes in the amounts of iron in solution follow 

 changes in the environmental conditions by microbial activities. 

 In general, the development of anaerobic conditions or the forma- 

 tion of acids results in the solution of iron and usually its reduc- 

 tion. Either a change from anaerobic to aerobic conditions, or a 

 decrease in acidity, or both, lead to oxidation of iron and may cause 

 its precipitation. The fact that iron is readily oxidized and 

 reduced greatly increases the capacity of microorganisms to mod- 

 ify its condition in the soil. 



Transformation of Potassium by Soil Microbes. — Potas- 

 sium is considerably less susceptible to effects of microorganisms in 

 its relationships to plant development in soils. It belongs to a 

 large group of elements in soil which are not affected in a great 

 variety of ways, on account of the fact that they do not enter into 

 organic combination to the extent of such elements as nitrogen or 

 sulfur. Its entrance into organic compounds is more generally 

 confined to replacing the hydrogen of the acid groups where it 

 forms the salts of these acids. In plant residues a large portion 

 of the potassium is present in inorganic form, occurring in the 

 various fluids of the cells. Upon ignition of the organic substances 

 the potassium remains in the ash residue. Another factor appre- 

 ciably limiting the capacities of microorganisms to affect the 

 element is its stable character. It is not oxidized and reduced 

 and consequently cannot be carried through the extensive series 

 of changes common to nitrogen, sulfur, and iron. The influence of 

 microorganisms on such elements as potassium is confined to caus- 

 ing its solution from organic and inorganic substances and its 

 assimilation in growth of the microbial cells. 



Considerable potassium becomes added to soils in the form of 

 various organic and inorganic compounds. Stable manures, green 



