114 BACTERIA AND SOIL MINERALS. 



changes through which it passes is not yet known. Part of the 

 phosphorus finally assumes an insoluble condition, but a part is dis- 

 solved in the soil water and becomes available as plant food. This 

 solvent action of the bacteria is attributable to the acids that are 

 produced. As we have already seen, the decomposition of organic 

 products always gives rise to certain organic acids and these are 

 capable of dissolving phosphorous compounds that would be in- 

 soluble in water alone. The solvent action resulting from bacterial 

 decomposition is not wholly the result of the acids, for by some 

 means yet unknown the phosphorous compounds may be dis- 

 solved even when no acid is produced. They are not, however, 

 dissolved in sterile soil; therefore the availability of the phosphorus 

 is due to bacterial activities. Such a formation of soluble phos- 

 phorus from decaying organic compounds is going on constantly in 

 the humus, and in soils rich in humus the process furnishes phos- 

 phorus in sufficient quantity for vegetation. 



Sometimes, however, more phosphorus is needed, and it may be 

 supplied by minerals. The rock phosphates are rendered available 

 in much the same manner as the organic phosphates. The phos- 

 phorous compounds in the rock are very slightly, if at all, soluble in 

 water. In ordinary soil small, but sufficient quantities are dis- 

 solved through the agency of the soil bacteria. Hence also the 

 acids produced by decomposition are important agents in dissolving 

 the rock which, though not soluble in water, is soluble in acids. It 

 is a well-known fact that these phosphates are made more available 

 as a fertilizer by being composted for a time in manure, a fact 

 clearly explained by the solvent action of the acids produced by 

 decomposition, as well as by other functions of the bacteria not yet 

 understood. They are also made more effective when plowed into 

 the ground with the plants used for green-manuring, this condition 

 giving rise to rapid bacterial action, resulting in a decomposition 

 which aids in rendering the phosphorous compounds available. 

 Thomas slag is also dissolved by similar activities. In short, while 

 bacteria do not furnish phosphorus, they are the active agents in 

 rendering available the phosphorus from both organic and mineral 

 sources. 



