14 BULLETIN 699, U. S. DEPAETMENT 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 other 

 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. 



iMich. Agr. Expt. Sta., Bui. No. 43 (special), (1908). 



2 Science (n. s.) 85, 390 (1912). 



•Wis. Agr. Expt. Sta. Research Bui. No. 29, pp. 273-321 (1913). 



