420 MICROBIOLOGY OF SOIL 



isolation and culture of nitrate bacteria. Heinze and others have 

 noted the favorable action of calcium carbonate on the growth of 

 Azotobacter, while the beneficial influence of calcium carbonate and sul- 

 phate on the development of Ps. radicicola has been repeatedly observed 

 by different investigators. 



Lipman and Burgess found that calcium carbonate stimulates 

 nitrogen fixation by A. chroococcum in solution, but is without effect 

 in soil. Magnesium carbonate is very toxic both in soil and in solution 

 for cultures of A. chroococcum even in concentration of o.i per cent. 

 Calcium carbonate exercises a protective action against the toxic 

 properties of magnesium carbonate. 



PHOSPHORUS 



AVAILABILITY or PHOSPHATES. Phosphorus exists in the soil largely 

 in the form of phosphates of calcium, magnesium, iron and aluminum. 

 A small portion of it occurs in organic combination in lecithin, phytin 

 and other compounds. The soil phosphates possess a very slight degree 

 of solubility and often fail to become available rapidly enough to meet 

 the demands of the growing crop. Fortunately the presence of 

 carbon dioxide generated from decaying organic matter hastens the 

 solution of the inert phosphates, thus: 



Ca 3 (PO 4 ) 2 + 2CO 2 + 2 H 2 O = Ca 2 H 2 (PO 4 ) 2 + Ca(HCO 3 ) 2 



For this reason a maximum supply of available phosphates may be 

 secured by plants in the presence of readily decomposable organic 

 matter. 



Apart from carbon dioxide as a means for making available inert 

 phosphates, bacteria produce organic and inorganic acids that are of 

 direct service. The influence of nitrous, nitric and sulphuric acids, all 

 of them products of bacterial activity, is undoubtedly of some im- 

 portance. The influence of lactic, acetic and butyric acids, as well as 

 of the more complex humic acids, must be of considerable moment. 

 For instance, in the decomposition of bone meal by B. mycoides, 

 Stoklasa found that 23 per cent of the phosphoric acid had become 

 soluble, whereas in similar uninoculated portions of bone meal only 3 

 per cent of soluble phosphoric acid was found. The significance of 

 organic acids produced by microorganisms is brought out even more 

 strongly in the loss of phosphates from acid soils. 



