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. 
