534 PRINCIPLES OF SOIL MICROBIOLOGY 



It has been claimed that manganese 59 and arsenic 60 exert a stimu- 

 lating effect upon nitrate formation. Montanari 61 could not confirm 

 this so far as arsenic is concerned. Heavy metals inhibit nitrate for- 

 mation according to their protein-precipitating properties, Hg and Ag 

 salts being most injurious. Copper, zinc, iron and lead may exert a 

 stimulating effect. 62 Ashby 63 found that, in the presence of iron hydrox- 

 ide, nitrification takes place even in the absence of carbonates; the 

 catalytic effect of iron is very important in the growth and respira- 

 tion of the organisms. The injurious influence of dicyanodiamide 

 upon the activities of the nitrate forming bacteria is illustrated in 

 figure 34. 



Influence of soil gases upon nitrate formation. A liberal supply of 

 oxygen was found 64 to be very favorable to nitrate formation. The 

 mere stirring of the soil was found to greatly stimulate the process; 

 this stimulating effect was looked upon 65 as due to better aeration 

 (fig. 35). It is known, however, that nitrate formation takes place 

 as rapidly in compact clay soils as in coarser grained soils, when the 

 available water is the same in both cases. 66 This would tend to indi- 

 cate 67 that the amount of oxygen necessary for nitrate formation need 

 not be abundant, so long as it is sufficient for the normal respiration 

 of the organisms and the moisture supply is favorable. The optimum 



69 Olaru, D. A. Role du manganese en agriculture. Son influence sur quel- 

 ques microbes du sol. Paris. Bailleres. 1920. 



60 Greaves, J. E. Some factors influencing ammonification and nitrification 

 in soils. The influence of arsenic. Centrbl. Bakt. II, 39: 542-560. 1913; 

 Biochem. Bui. 3: 2. 1913. 



61 Montanari, C. Azione degli elementi oligodinamici sui batteri della nitri- 

 ficazione. II. Staz. sper. agr. ital. Modena, 50: 69-72. 1917. 



62 Lipman, C. B., Burgess, P. S. The effects of copper, zinc, iron and lead 

 salts on ammonification and nitrification in soils. Univ. Cal. Publ. Agr. Sci., 

 1: 127-139. 1914. 



63 Ashby, 1907 (p. 394). 



64 Warington, R. Lectures on investigations at the Rothamsted Experi- 

 mental Station. Exp. Sta. Rec, 3: 894-903. 1892. 



65 King, F. H., and Whitson, A. R. Development and distribution of nitrates 

 and other soluble salts in cultivated soils. Wisconsin Agr. Exp. Sta. Bui. 85. 

 1901; Bui. 93. 1902. 



66 Schloesing, Th., Jr. Contribution a l'etude de la nitrification dans les sols. 

 Compt. Rend. Acad. Sci., 125: 824-827. 1897; Fischer, H. Versuche iiber 

 Stickstoffumsetzung in verschiedenen Boden. Landw. Jahrb., 41: 755-822. 

 1911. 



67 Gainey, P., and Metzler, L. F. Some factors affecting nitrate-nitrogen 

 accumulation in soil. Jour. Agr. Res., 11: 43-64. 1917. 



