654 



PRINCIPLES OF SOIL MICROBIOLOGY 



to 3.8 per cent of CO2. Czapek 31 ascribed the most important role 

 in bringing the soil minerals into solution to carbon dioxide. 



Carbon dioxide interacts with the different phosphates in the follow- 

 ing manner: 



Ca 3 (P0 4 ) 2 + 4H 2 + 4C0 2 = Ca(H 2 P0 4 ) 2 -H 2 + 2Ca(HC0 3 ) 2 

 2(CaHP0 4 -2H 2 0) + 2C0 2 = Ca(H 2 PO*)2-H 2 + Ca(HC0 3 ) 2 + H 2 

 Ca 4 P 2 9 + 5H 2 + 6C0 2 = Ca(H 2 P0 4 ) 2 + 3Ca(HC0 3 ) 2 



One kilogram of soil containing 0.13 per cent P2O5, when extracted for 

 twenty-five days with distilled water (five repeated extractions of five 

 days each), yielded 0.002 gram P 2 5 , while 0.0085 grams P 2 5 was 

 extracted in the same period by water containing carbon dioxide. 32 



TABLE 71 

 Solubility of different phosphates 



The various organic acids (butyric, lactic, acetic, propionic, formic, 

 valerianic, gluconic, citric, oxalic, fumaric, etc.) formed by soil organ- 

 isms interact with the phosphates, carbonates and silicates present in 

 the soil, to give lactates, acetates, butyrates, citrates, etc. These 

 are usually further decomposed by different microorganisms to carbon 

 dioxide and carbonates. Table 71 shows the amounts of various 

 phosphates made soluble when extracted by dilute solutions of organic 

 acids (for 500 hours), by water saturated with carbon dioxide and by 

 the growth of Azotobacter chroococcum, the particular phosphate being 

 the only source of phosphorus in the medium. 



31 Czapek, 1920-1921 (p. xv). 



32 Stoklasa, 1911 (p. 649). 



