Soil, Changes Produced By Micro-organisms. 35 
ing the material at hand into forms which plants can use. It is 
this phase of the question which will be taken up in the next few 
pages. 
Phosphorus. 
Soil phosphates as they occur in Nature, are but slightly solu¬ 
ble in water, and during the growing season, when the demand is 
heavy, they often fail to go into solution rapidly enough to satisfy 
the needs of plants. In soils where the phosphoric acid is low, the 
deficiency is usually made up by applying some form of commercial 
fertilizer known to contain a considerable quantity of phosphorus. 
For this purpose, either superphosphate of lime, ground bone, phos¬ 
phate rock, or Thomas slag may be used. With the exception of 
the superphosphate, the phosphorus in the other materials is in an 
insoluble form, and therefore of no immediate value to plants. As 
has been noted before, during the decomposition of organic mat¬ 
ter, a number of organic acids are formed by soil micro-organisms. 
It will be remembered, also, that quantities of carbon dioxide are 
liberated during sugar and starch fermentation, and that this unites 
with water to form carbonic acid. Now, while all of these acids are 
weak, relatively speaking, they nevertheless play a very prominent 
part in bringing the insoluble phosphates into solution. 
Some time ago the writer conducted a series of experiments 
to determine the value of these acids as solvents for the phosphate 
of ground bone and phosphate rock. The results of this work* 
indicated that when bonemeal was added to milk, and the milk was 
allowed to sour, 25.31 per cent, of the phosphate was made soluble 
by the lactic acid which developed in the milk. Again, when ground 
bone was added to beer wort which was undergoing both alcoholic 
and acetic fermentation, 53.55 per cent of the phosphate became 
soluble in 15 days. Employing dilute lactic acid, o. 1 per cent., 3.13 
per cent, of the bone phosphate was rendered soluble in five days; 
while the same strength of acetic acid gave 4.12 per cent. These fig¬ 
ures will suffice to show that the dilute organic acids, though weak, 
are nevertheless potent factors in bringing insoluble compounds into 
solution. Other experiments conducted at the same time lead us to 
believe that acids are not the sole solvents, for, under favorable con¬ 
ditions, certain soil bacteria, independent of acid formation, are 
capable of converting small quantities of insoluble phosphates into a 
soluble form. In one case, 1.19 per cent, of the insoluble phosphate 
* The Solvent Action of Soil Bacteria upon the Insoluble Phosphate of Raw Bone Meal and 
Natural Raw Rock Phosphate. Sackett, Patten and Brown. Special Bulletin No. 43, Michigan 
Experiment Station. 1908. 
