672 Journal of Agricultural Research voi. xii. No. 10 



A number of investigators have shown that the solution of tricalcium 

 phosphate may be efifected by the biochemical oxidation of sulphur. 

 Recently Lipman, McLean, and Lint (5) found that large amounts of 

 rock phosphate may be made soluble by the sulphur bacteria in fer- 

 menting mixtures containing elemental sulphur. 



The effects of the nitrifying bacteria on the solubility of tricalcium 

 phosphate have recently been investigated by Hopkins and Whiting (j). 

 They found that the nitrite bacteria (Niirosomonas spp.) have the power 

 of oxidizing ammonium sulphate in solution cultures containing trical- 

 cium phosphate but no carbonate or free base, and that under these con- 

 ditions the nitrous acid and sulphuric acid, formed from the ammonium 

 sulphate, attacked the tricalcium phosphate and rendered notable 

 amounts of phosphorus and calcium soluble in water. Similar effects 

 were found when ammonium nitrate was substituted for ammonium sul- 

 phate. By calculation they found, as an average of 13 tests, that for 

 every 56 pounds of nitrogen oxidized, 115 pounds of phosphorus and 211 

 pounds of calcium were made soluble in water. They also found that no 

 change in the solubility of tricalcium phosphate takes place as a result 

 of the action of the nitrate bacteria (Nitrobacter spp.). As pointed out 

 by them, the oxidation of nitrite to nitrate does not necessitate an in- 

 crease in acidity, but is merely a matter of adding an atom of oxygen to 

 the nitrite. 



Hopkins and Whiting have discussed their views regarding the prac- 

 tical bearing of these experiments at considerable length, and have as- 

 signed special importance to the nitrite bacteria as agents in promoting 

 the solution of rock phosphate in the field. In commenting on these 

 investigations Davenport ^ even suggested that the importance of the 

 nitrite bacteria as agents in the solution of rock phosphate, is on a par 

 with that of the legume bacteria in nitrogen fixation. 



Since the formation of nitrous acid is commonly considered to be an 

 essential step in the nitrification process and as nitrification is generally 

 active in productive soils, it is at once apparent that relatively large 

 amounts of phosphate will be made soluble by this group of bacteria, 

 provided the reactions that take place in soils be similar to those found in 

 solution cultures. 



The practical importance of phosphates in agriculture and the general 

 interest in the several phases of the nitrification process justify further 

 investigation of this problem. Accordingly, the writer has made some 

 studies on it at the University of California Citrus Experiment Station. 

 In these studies both soil and sand cultures have been employed. The 

 formation of nitrate and the solubility of calcium and phosphoric acid in 

 water were used in this investigation as measures of the biochemical 

 action of the nitrifying organisms. 



• Foreword to the publication by Hopkins and Whiting (3). 



