TRANSFORMATION OF MINERALS IN THE SOIL 657 



the nitrite-forming, nitrate-forming, and sulfur-oxidizing bacteria. The 

 continued oxidation in normal soils of ammonium salts to nitrous and 

 nitric acids results in the formation of appreciable quantities of these 

 acids, 132 parts of ammonium sulfate giving, on complete oxidation, 

 126 parts of nitric and 196 parts of sulfuric acids. Even organic com- 

 pounds rich in nitrogen, like urea, finally lead to a high acidity. 



CO(NH 2 ) 2 + 2H 2 0-+(NH 4 ) 2 C0 3 + 40 2 -^2 HN0 3 + C0 2 + 3H 2 



The acids may, under proper conditions of reaction, interact with 

 the tri-calcium phosphate added to the soil and make it soluble. 



Ca 3 (P0 4 )2 + 2 HN0 2 = 2 Ca HP0 4 + Ca(N0 2 ) 2 

 Ca 3 (P0 4 ) 2 + 4 HN0 2 = Ca(H 2 P0 4 ) 2 + 2 Ca(N0 2 ) 2 

 Ca 3 (P0 4 ) 2 + H 2 S0 4 = 2 CaHPO, + CaS0 4 



Theoretically 188 parts of nitrous acid mixed with 310 parts of pure 

 rock phosphate should give 234 parts of acid phosphate and 264 parts 

 of calcium nitrite. As the average of thirteen tests in liquid culture, 

 Hopkins and Whiting 38 found that 115 parts of phosphorus and 211 

 parts of calcium were made water soluble for every 56 parts of nitrogen 

 oxidized by the nitrite forming bacteria. No further increase was 

 obtained from the action of the nitrate bacteria, since the oxidation of 

 the nitrite to nitrate does not bring about any further increase in 

 acidity. 



However, the reactions taking place in the soil are not similar to 

 those observed in liquid cultures, due to the fact that the nitrous acid 

 combines in the soil with calcium and magnesium carbonate and salts 

 of organic acids liberating rather weak organic acids, as shown in 

 table 73. 39 



The acids resulting from the activities of the bacteria neutralize 

 the carbonate in preference to the tri-calcium phosphate in the soil. 

 When considerable acidity is produced, as in the presence of ammonium 

 sulfate, some phosphate goes into solution. But the degree of acidity 

 necessary for the transformation of the tri-calcium phosphate into 

 soluble phosphates is so high (p. 659) that it may become distinctly 

 injurious to crop growth. The amount of phosphate that goes into 

 solution, as a result of the activities of the nitrifying bacteria is, there- 



58 Hopkins, C. G., and Whiting, A. L. Soil bacteria and phosphates. 111. 

 Agr. Exp. Sta. Bui. 190. 1916. 



39 Kelley, W. P. Effect of nitrifying bacteria on the solubility of tricalcium 

 phosphate. Jour. Agr. Res., 12: 671-683. 1918. 



