662 PRINCIPLES OF SOIL MICROBIOLOGY 



mycelium. 47 When this is decomposed the potassium again becomes 

 available. However, only a part of the free potassium salt remains in 

 the soil as such; it also replaces some of the zeolitic bases, such as Ca, 

 Mg, Na. The available potassium in the soil at any given time de- 

 pends not only on the total content of this element in the soil, but 

 also on the form in which it is present in the soil, the degree of satura- 

 tion of zeolitic compounds, soil reaction, available organic matter and 

 activities of various groups of microorganisms. 



The method outlined on page 577 for determining the available 

 phosphorus in the soil can also be utilized for determining the available 

 potassium; 48 30 cc. of water containing 2.5 grams glucose, 1 gram 

 Na 2 HP04 and 0.05 gram MgCl 2 , is added to 100 grams of air-dry soil. 

 The mixture is sterilized and inoculated with Azotobacter. The 

 amount of nitrogen fixed, under these conditions, is an index of the 

 available potassium (allowing that Azotobacter cells contain about 

 2.5 per cent K 2 and 10 per cent nitrogen). Stoklasa thus studied a 

 fertile soil with a total of 0.093 per cent K 2 0, of which 27.4 per cent 

 was available; a soil of medium fertility with a total of 0.27 per cent 

 K 2 0, of which 5.46 per cent was available, and a poor forest soil with a 

 total of 0.137 per cent K 2 of which only 2.18 per cent available. 



Transformation of calcium in the soil. In normal soils, calcium forms 

 the chief base with which the soil zeolites and the organic complexes 

 are saturated. If an alkali salt is added to the soil, the base is absorbed 

 and an equivalent amount of calcium is replaced. The activities of 

 the microorganisms will affect the transformation of calcium in the 

 soil in various ways. (1) Calcium salts, particularly calcium carbonate, 

 may be precipitated in the soil, 49 as a result of the interaction of the 

 soluble calcium salts (of organic acids or nitrates) with carbonic acid 

 produced by the decomposition of organic matter. (2) Calcium car- 

 bonate may be made soluble, as a result of activities of microorganisms 



47 Kyropoulos. Uber die Festlegung von Kali durch Bodenbakterien. Ztschr. 

 Giirungsphysiol., 5: 161. 1915. 



48 Stoklasa, 1925 (p. 021). 



49 Gimingham, C. T. The formation of calcium carbonate in the soil by bac- 

 teria. Jour. Agr. Sci., 4: 145-149. 1911; Drew, G. H. On the precipitation of 

 calcium carbonate by marine bacteria and on the action of denitrifying bacteria 

 in tropical and temperate seas. Carnegie Inst. Washington, Dept. Marine 

 Biology, Papers from Tortugas Labor, 5: 7-45. 1914; Kellermann, K. F., and 

 Smith, N. R. Bacterial precipitation of calcium carbonate. Jour. Wash. 

 Acad. Sci., 4: 40(M02. 1914. 



