678 Journal of Agricultural Research voi. xii. no. 10 



in the solution of 27.9 p. p. m. of phosphoric acid, or 12.2 p. p. m. phos- 

 phorus. By comparing these data with the rate of solution reported by 

 Hopkins and Whiting from solution cultures, it will be seen that, while 

 the oxidation of i pound (454 gm.) of nitrogen was accompanied by the 

 solution of 2.033 pounds (922 gm.) of phosphorus in their experiments;^ 

 in these experiments with soil cultures the maximum amount of phos- 

 phorus made soluble was only 0.156 pound (70.8 gm.) per pound of 

 nitrogen oxidized. Therefore, the oxidation products of ammonium 

 sulphate were approximately 13 times as effective in dissolving trical- 

 cium phosphate in solution cultures as in this soil. 



It is interesting to note that the addition of calcium carbonate tended 

 to lower the solubility of tricalcium phosphate wherever applied. 



Sand series. — Ammonium sulphate, when added alone, underwent 

 almost no nitrification in the sand series (Table III) until the last period 

 of the experiment, during which small amounts of nitrate were formed.^ 

 The presence of calcium carbonate, however, promoted very active 

 nitrification of ammonium sulphate. In this case the concentration of 

 nitric nitrogen reached its maximum (92 p. p. m.) in 28 days. On the 

 other hand, the effects resulting from the addition of tricalcium phos- 

 phate only began to be manifested by the fifty-sixth day. Later the 

 nitrate content slowly increased until the close of the experiment, when 

 33 p. p. m. were found. Nitrification of ammonium sulphate in the 

 proportions containing both calcium carbonate and tricalcium phosphate 

 was not so pronounced during the first 28 days as with calcium carbon- 

 ate only, but later the effects were almost identical. 



The nitrification of dried blood in sand proved to be especially inter- 

 esting in that the intermediate formation of nitrite proceeded much 

 more rapidly than the oxidation to nitrate.^ When dried blood alone 

 was added, 42.1 p. p. m. of nitrite nitrogen were found after 28 days and 

 only I.I p. p. m. of nitric nitrogen. Later no further accumulation of 

 nitrite took place, but the formation of nitrate set in slowly with the 

 result that 29 p. p. m. had been formed by the close of the experiment, 

 but even then 5 p. p. m. of nitrite still remained. 



> It should not be inferred that Hopkins and Whiting claim that the products of nitrification will dis- 

 solve rock phosphate at the same rate in soil as in solution cultures. They pointed out (j. />. 405), for 

 example, that nitrous acid may combine with calcium sihcate, calcium carbonate, and other compounds 

 in soils as well as with tricalcium phosphate. Nevertheless they hold that the nitrite bacteria are important 

 agents in bringing about the solution of rock phosphate in field soils. 



2 Appreciable amounts of nitrate were formed in the portions which were intended to be free from com- 

 Kined nitrogen. The nitrate in these instances probably originated from organic matter held in suspen- 

 sion in the soil infusions that were added. The amounts formed, however, were small and consistent ia 

 every case, increasing steadily from an average of 7.3 p. p. m. at the 28-day period to a maximum of ii.i 

 p. p. m. at 98 days. 



5 Data showing that nitrites may accumulate in nitrification experiments have previously been reported 

 (4), but this is a condition not commonly met in the field. The accumulation of nitrites indicates, of 

 course, that some factor in the medium was abnormal, but more favorable for the nitrite bacteria than 

 for the nitrate bacteria. It is known that the nitrate bacteria are more sensitive to adverse conditions 

 than the nitrite bacteria. 



