I22 BACTERIA AND SOIL MINERALS 
the phosphorus being, of course, freed from its relations. The 
entire series of changes through which it passes is not yet known. 
Part of the phosphorus finally assumes an insoluble condition, but 
a part is dissolved 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 decompo- 
sition of organic products always gives rise to certain organic 
acids and these are capable of dissolving phosphorous compounds 
that would be insoluble in water alone. The solvent action re- 
sulting from bacterial decomposition is not wholly the result of 
the acids, for by some means yet unknown the phosphorous com- 
pounds may be dissolved 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 phosphorus from decaying organic compounds is going 
on constantly in the humus, and in soils rich in humus the process 
furnishes phosphorus in sufficient quantity for vegetation. 
Sometimes, however, more phosphorus is needed, and it may be 
supplied by minerals. The rock phosphates are rendered avail- 
able in much the same manner as the organic phosphates. The 
phosphorous compounds in the rock are very slightly, if at all, 
soluble in water. In ordinary soil small, but sufficient quantities 
are dissolved 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 phos- 
phorous compounds available. Thomas slag is also dissolved by 
