176 BOTANICAL GAZETTE [MARCH 
kind and quantity of each depend upon the specific organisms and 
upon the substance on which they are acting. These substances 
are sure to come in contact with some insoluble plant food which 
may be rendered soluble, for they have a high solvent power for 
the insoluble phosphates (32). The resulting salts of calcium 
would be attacked further by bacteria with the formation of calcium 
carbonate (5). 
Whether these processes will give rise to an increase in the 
water-soluble plant food of the soil will depend upon whether the 
products of the second, the analytical reactions, exceed the products 
of the first, the synthetic reactions. We must not lose sight of the 
fact that although many of the organic phosphorus constituents 
may not be soluble in pure water, they may be more available to 
the living plant than are the constituents from which they at first 
were derived through bacterial activity. This being the case, 
variations may be expected in the results reported from laboratory 
tests. STOKLASA (33) found that bacterial activity rendered the 
phosphorus of the soil more soluble, whereas SEVERIN (27) in his 
early work found the opposite to be true. Others have found that 
the solvent action of bacteria for insoluble phosphates is in direct 
proportion to the acid secreted by the organism (26). 
In a later work SEVERIN (28) obtained different results. He 
used three soils, one sterile, a second sterilized and inoculated with 
pure cultures of Azotobacter, and a third sterilized and inoculated 
with cultures of B. radicicola and Azotobacter. The solubility of 
the phosphorus increased 8 to 14 per cent over that in the sterile 
soil. The acid-producing organisms, due to the acid secreted and 
their intimate contact with the soil particles, possess the power 
of dissolving silicates. Moreover, arsenic greatly stimulates nitro- 
gen fixation, and there is a relationship between this increased 
bacterial activity and the form and quantity of phosphorus found 
in such a soil (7). 
Although the metabolic activity of Azotobacter gives rise to 
large quantities of phosphate solvents, these organisms transform 
phosphorus into organic phosphorus compounds less rapidly than 
do the ammonifiers (35). There are cases, however, in which 
bacterial activity has decreased the water-soluble phosphorus of 
