14 BULLETIN 699, U. S. DEPARTMENT OF AGRICULTURE. 
The fermentation of organic matter apparently had little or no 
effect on the solubility of finely ground rock phosphate in water 
and a depressing effect on its solubility in 0.2 per cent citric acid 
solution. In regard to the rock’s solubility in 1 per cent sodium 
hydroxide solution, however, this depression is not so marked if 
indeed there is any. This investigation included a number of othef 
interesting experiments, showing the solvent effect of carbon dioxide 
on natural phosphates and the great increase in the quantity of this 
gas in soils treated with manure. The pot work with raw rock 
phosphate conducted by Truog is discussed elsewhere in this bulletin. 
In summing up the results of his work Truog concludes that the 
“solubility of phosphate as measured by a solvent like 0.2 per cent 
citric acid may be very different from the availability as measured 
by a growing crop.” He thinks that the role which organic matter 
plays in rendering raw phosphates more available in the field is due 
to the increased quantity of carbon dioxide resulting from organic 
decomposition and the better distribution of the phosphate which is 
brought about by mixing it intimately with the organic material. 
Sackett, Patten, and Brown’ in an investigation on the solvent 
action of soil bacteria upon the insoluble phosphoric acid of bone and 
raw rock phosphate found that certain types of soil bacteria have the 
power of converting small quantities of insoluble phosphates into 
soluble form independent of acid formation, but that when bacterial 
growth is accompanied by the formation of acid the amount of phos- 
phate dissolved is considerably greater. 
Tottingham* found, however, that in mixtures of rock phosphate 
and manure both the water-soluble and citrate-soluble phosphoric 
acid were reduced by fermentation. 
In a later and more exhaustive investigation Tottingham and Hoff- 
man showed that the action of fermenting manure on natural phos- 
phates is much more complex than was formerly supposed. These 
investigators claimed that the decreased solubility of the phosphates 
in such mixtures was due to the fixing or absorption of the phos- 
phorus by the manure organisms, but that the availability of the 
phosphorus in the cells of such organisms as measured by a growing 
crop (pot test) was as great as that in acid phosphate. Moreover, 
after fermentation has practically ceased the absorbed or altered 
phosphate is released in forms soluble in carbonated waters. The 
final conclusions reached in this investigation were that advanta- 
geous results are obtained by composting rock phosphate with fer- 
menting manure, but that it is inadvisable to practice the same 
scheme with acid phosphate. 
1 Mich. Agr. Expt. Sta., Bul. No. 43 (special), (1908). 
2Science (n. s.) 35, 890 (1912). 
3 Wis. Agr. Expt. Sta. Research Bul. No. 29, pp. 273-321 (1918). 
