176 TRANSFORMATION OF MINERAL SUBSTANCES 



into di- and mono-basic phosphates, which are more soluble and 

 consequently more readily available to plants. In view of the 

 fact that the formation of the inorganic and organic acids takes 

 place in the soil constantly as a result of the decomposition and 

 oxidation processes, insoluble phosphates tend to become gradu- 

 ally soluble, especially when accompanied by processes of active 

 decomposition of organic matter and oxidation of ammonia and 

 incompletely oxidized sulfur compounds. When the soil contains 

 free carbonates and the reaction is neutral or alkaline, the acids 

 will be neutralized by the carbonates in preference to the phos- 

 phates, and the latter will not go into solution so readily. 



The amount of phosphoric acid made available to plants at any 

 given time is thus found to be a result of various complex biological 

 and chemical processes which tend either to bring the phosphoric 

 acid into solution (when the decomposition processes are pre- 

 dominant) or take it out of solution (when the synthesizing micro- 

 bial processes are active and the amount of available phosphate is 

 limited). The amount of available phosphorus in any given soil 

 can be readily and simply determined by microbiological pro- 

 cedures as discussed in Chapter IX. 



Transformation of Iron by Soil Microbes. — Iron is seldom 

 added to soils in fertilizing materials and is contained in organic 

 residues in very limited quantities, but it is one of the elements 

 present in the soil in very large amounts. In spite of the great 

 abundance of the element in various compounds in soils and in 

 spite of the fact that it is required by plants in very small amounts, 

 it is so slightly soluble in alkaline soils that the supply of plants with 

 soluble compounds of iron is a serious problem under such condi- 

 tions. Iron is of considerable importance in the physical compo- 

 sition of soils, and in extreme cases its solution in the upper layers 

 and precipitation in localized deeper regions of soils may be so 

 extensive as to play an important part in the formation of imper- 

 vious layers called "hardpans." 



Microorganisms are associated with the movement of iron in 

 the soil in various ways. Among these changes may be included 

 solution and precipitation of iron compounds; these reactions are 

 frequently accompanied by reduction and oxidation processes. 



In the precipitation of iron from soluble inorganic compounds 

 of the element, the iron bacteria have been considered to play a 

 preeminent part. These organisms are so named since iron in the 



