532 PRINCIPLES OF SOIL MICROBIOLOGY 



monia, at a pH 9.5, the injury to nitrate-formation was equal to 70 

 per cent. Most inorganic alkali salts have only a slightly injurious 

 effect, usually only at a concentration greater than 0.3 N; alkali earths 

 are more injurious, 0.1 N causing an injury of 60 per cent (p. 394). 

 The injurious effect of cations of alkali salts depends to a large extent 

 on the OH concentration. Meek and Lipman 53 found that nitrifica- 

 tion takes place in solution in the presence of 10,000 parts per million 

 of NaCl(0.1 N), but not in higher concentrations. Na 2 S04 was not 

 injurious even in concentrations of 30,000 parts per million (0.42 N). 

 Na 2 C0 3 was found to be most injurious. 54 However, the nature and 

 amount of the nitrogen source were found to modify the injurious effect 

 of the salt. 55 The ratio of the calcium to magnesium is not of great 

 importance to the activities of the nitrate-forming bacteria, but the 

 total concentration of magnesium in solution and its relations to the 

 concentration of the other constituents are of great importance. 59 



According to Greaves and associates 57 chlorides, nitrates, sulfates 

 and carbonates of Na, K, Ca, Mg, Mn and Fe exert a toxic effect 

 upon nitrate-formation in the soil, depending on the specific salt and 

 not on the electro-negative ion. The quantity of a salt which can be 

 applied to a soil without decreasing the nitrate-nitrogen accumulation 

 in the soil varies with the salt. In the soil under investigation, the 

 order of decreasing toxicity was found to be as follows: Na 2 S04, Na 2 C0 3 , 

 CaC0 3 , K 2 S0 4 , K 2 C0 3 , Fe(N0 3 ) 3 , NaN0 3 , MgS0 4 , Fe 2 (S0 4 ) 3 , Ca(N0 3 ) 2 , 

 KN0 3 , KC1, Mg(N0 3 ) 2 , MnC0 3 , MnCl 2 , MnS0 4 , Fe 2 (C0 3 ) 3 , MgCl 2 , 

 Mn(N0 3 ) 2 , FeCl 3 , MgC0 3 , NaCl, CaCl 2 and CaS0 4 . Those com- 

 pounds which become toxic in lower concentrations are not necessarily 

 most toxic in higher concentrations, as the toxicity of some salts increases 

 more rapidly than the toxicity of others. The common alkali salts are 

 very toxic, including CaCl 2 , Na 2 S0 4 , Na 2 C0 3 , and the less common 

 Ca(N0 3 ) 2 . All the salts, except Na 2 S0 4 , Na 2 C0 3 , CaC0 3 , K 2 S0 4 , K 2 C0 3 

 and Fe(N0 3 ) 3 , act as stimulants in some concentrations; the amount of 

 stimulation depends on the salt, CaS0 4 and CaCl 2 being most efficient. 



"Meek and Lipman, 1922 (p. 529). 



64 Lipman, C. B. Toxic effects of "alkali salts" in soils on soil bacteria. II. 

 Nitrification. Centrbl. Bakt. II, 33: 305-313. 1912. 



66 Kelley, W. P. Nitrification in semi-arid soils. Jour. Agr. Res., 7: 417- 

 437. 1916. 



86 Kelley, W. P. The lime-magnesia ratio: II. The effects of calcium and 

 magnesium carbonates on nitrification. Centrbl. Bakt. II, 42: 577-582. 1914. 



57 Greaves, J. B., Carter, E. G., and Goldthorpe, H. C. Influence of salts on 

 the nitric-nitrogen accumulation in the soil. Jour. Agr. Res., 16: 107-135. 1919. 



