REDUCTION AND OXIDATION PROCESSES 249 



microbes only indirectly, as the metabolic products exert their 

 effects as agents of solution, precipitation, oxidation, and reduc- 

 tion. A few illustrations may indicate the nature of these effects. 



Phosphorus is frequently introduced into the soil in the form of 

 rock phosphate or as insoluble tri-calcium phosphate. In this 

 form it can be used by green plants to only a very limited extent. 

 When the phosphate is acted upon by the various organic acids 

 which are formed in the soil by fungi and bacteria (gluconic, 

 citric, oxalic, fumaric, lactic, butyric, formic, acetic, valerianic, 

 or by the carbonic acid, which is always present in abundance), it 

 is changed from an insoluble to a soluble form, which is more readily 

 available to plants. 



Of the various acids produced by the microorganisms, the 

 carbonic and the organic acids exert much weaker effects as solvent 

 agents than some of the inorganic acids, because of their relatively 

 low ionization. Much more pronounced effects are exerted by the 

 inorganic acids formed by microbes, such as nitric, resulting from 

 the oxidation of ammonia, and sulfuric, formed from elementary 

 sulfur. Under natural soil conditions, relatively small amounts 

 of organic and strong mineral acids are produced by microor- 

 ganisms. On the other hand, carbon dioxide is produced in a con- 

 tinuous flow of greater or less intensity wherever living organisms 

 are present. Its effects upon more resistant soil minerals would 

 not be pronounced except over comparatively long periods. The 

 strong mineral acids, even if formed in small amounts, can liber- 

 ate some potassium from its combination with the zeolitic portions 

 of the soil and even from the more resistant soil minerals. 



As a result of the decomposition processes which take place 

 in the soil, the reaction changes either to more acid or to more 

 alkaUne, and frequently one way, then another. The decomposi- 

 tion of urea, for example, leads to the formation of ammonium 

 carbonate, which tends to make the reaction of the soil more 

 alkaline; the ammonium carbonate is then oxidized to nitric acid, 

 which tends to make the reaction more acid than it was initially. 



Reduction and Oxidation Processes. — Numerous other 

 reactions take place in the soil as a result of the activities of micro- 

 organisms, especially the processes of reduction and oxidation. 

 The following may be mentioned as suggestions of these reactions: 

 the reduction of nitrates to nitrites, to ammonia, to oxides of 

 nitrogen, or elementary nitrogen ; the reduction of sulfates to sul- 



