650 PRINCIPLES OF SOIL MICROBIOLOGY 



insoluble phosphates, within 50 to 60 days. Normal soils contain 

 0.025 to 0.3 per cent of phosphorus, very little of which is soluble in 

 pure water, but quite appreciable quantities are soluble in water con- 

 taining carbon dioxide. In fertile soils, the phosphorus is present 

 partly in the form of organic phosphorus compounds. 18 A part or even 

 all the organic phosphorus found in the soil may be in the form of bodies 

 of microorganisms. 19 The total phosphorus brought into the soil by 

 a two ton crop of green manure (including roots) may amount to 20 

 to 50 pounds P 2 5 per acre. The P 2 5 content of straw is 0.15 to 0.30 

 per cent, of clover and timothy hay 0.50 to 0.55 per cent, of fresh horse 

 manure (without straw) 0.34, cow manure 0.21, sheep manure 0.40, 

 chicken manure 0.83 per cent. 20 



The following analysis of the dry matter of a few typical bacteria 

 show that these organisms can store away considerable quantities of 

 phosphorus, 17 (table 70). Seventy-nine to 81 per cent of this phos- 

 phorus was found to be in the form of nucleic acid and 7.6 to 8.6 

 per cent as lecithin. The ash of yeasts may consist of 60 per cent 

 P2O5. The ash of fungi, however, contains a lower concentration of 

 phosphorus than yeasts and bacteria, depending largely on the phos- 

 phoric acid content of the medium. A more or less constant nitrogen- 

 phosphorus ratio (N:P 2 5 = 4.2%: 2.0%) was found 21 in the dry 

 mycelium of Asp. niger. A similar ratio is found in the cells of other 

 organisms and in the soil organic matter, pointing to a definite c/p 

 (organic) ratio in the soil. 



The presence of phosphorus is so important for the growth of micro- 

 organisms that a direct correlation has been found between the amount 

 of available phosphate in the medium and the growth, mannite de- 

 composition and nitrogen fixation by Azotobacter (see p. 577). 



Decomposition of organic phosphorus compounds by microorganisms. 

 A large number of microorganisms including various heterotrophic bac- 

 teria, fungi and actinomyces, are capable of decomposing organic 

 phosphorus compounds. 



Lecithin contains 9.39 per cent P 2 5 , 1.6 per cent N and 65.36 per 

 cent C. It contains two fatty acid radicals, usually palmitic and stearic 

 or oleic, which are rather poor sources of carbon for microorganisms. 



19 Gortner, R. A., and Shaw, W. M. The organic matter in the soil. V. Some 

 data on humus-phosphoric acid. Soil Sci., 3: 99-111. 1916. 



20 Thome, 1914 (p. 429). 



21 Schmi'icke, R. Der Phosphorstoffwechsel einiger Pilze, mit besonderer 

 Berucksichtigung von Aspergillus niger. Biochem. Ztschr., 153: 372^423. 1924. 



