664 PRINCIPLES OF SOIL MICROBIOLOGY 



free from lime. 52 The following reactions are involved in these proc- 

 esses: 



1. CaC0 3 + (NH 4 ) 2 S0 4 = CaS0 4 + (NH 4 ) 2 C0 3 



2. (NH 4 ) 2 S0 4 + 40 2 = 2H N0 3 + H 2 S0 4 + 2H 2 

 CaC0 3 + H 2 S0 4 = CaS0 4 + H 2 + C0 2 

 CaC0 3 + 2HN0 3 = Ca(N0 3 ) 2 + H 2 + C0 2 



3. Ca(H 2 P0 4 ) 2 + CaC0 3 = 2CaHP0 4 + H 2 + C0 2 



4. CaC0 3 + C0 2 + H 2 = Ca(HC0 3 ) 2 



The use of fertilizers which are directly acid or which lead to the for- 

 mation of acids in the soil, such as ammonium sulfate, superphosphate 

 and sulfur, leads to a depletion of the calcium content of the soil. 



Calcium is also used directly as a nutrient by various microorganisms 

 and small amounts of it may be assimilated. However, its chief value 

 lies in the neutralization of organic and inorganic acids in the soil and 

 in replacing injurious soil bases (sodium in alkaline soils), thus pro- 

 ducing a medium more favorable for the growth of plants and micro- 

 organisms. Calcium salts also neutralize to some extent the injurious 

 action of soluble magnesium salts, as shown by various studies 515 on 

 the influence of the lime-magnesia ratio upon crop growth. This 

 ratio is not of great importance in bacterial action but the concentra- 

 tion of magnesium in solution and its relation to the concentration of 

 the other constituents are of great importance. 54 



The amount of available calcium in the soil can be determined by 

 adding 5 to 10 grams of soil to 50 or 100 cc. of mannite solution free 

 from calcium, inoculating with Azotobacter and determining the rela- 

 tive amount of growth. 55 



Transformation of magnesium in the soil. Next to calcium magnes- 

 ium is the most common base in humid soils with which the zeolites and 

 humates are saturated. Magnesium is also introduced in considerable 

 quantities into the soil in the various inorganic materials (dolomitic 

 limestone, rock phosphate) and in the organic matter added to the soil. 



42 Hall, A. D., and Miller, N. H. J. Effect of plant growth and of manures 

 upon the retention of bases by the soil. Proc. Roy. Soc. (London), B., 77: 1-32. 

 1905. 



83 Lemmermann, O., Einecke, A., und Fischer, H. Untersuchungen fiber die 

 Wirkung eines verschiedenen Verhaltnisses von Kalk und Magnesia in einigen 

 Boden auf hohere Pflanzen und Mikroorganismen. Landw. Jahrb., 40: 173- 

 254. 1911. Kelley, W. P. The lime-magnesia ratio. Centrbl. Bakt. II, 42: 

 519-526, 577-582. 1915. 



84 See Greaves, J. E., and Carter, E. G. The action of some common soil 

 amendments. Soil Sci., 7: 121-160. 1919. 



66 Christensen, 1923 (p. 730). 



