CHANGES IN INORGANIC CONSTITUENTS. 287 



influence of calcium carbonate and sulphate on the development of Ps. 

 radicicola has been repeatedly observed by different investigators. 



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 + 2 CO 2 + 2 H 2 O= Ca 2 H 2 (PO 4 ) 2 + Ca(HCQ 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 importance. 

 The influence of lactic, acetic and butyric acids, as well as of the more com- 

 plex 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 un- 

 inoculated portions of bone meal only 3 per cent of soluble phosphoric 

 acid was found. The significance of organic acids produced by micro- 

 organisms is brought out even more strongly in the loss of phosphates 

 from acid soils. 



In so far as the organic phosphorus compounds are concerned bac- 

 terial activities are important in that the processes of decay restore the 

 phosphorus to circulation. Hence, it will be seen that microorganisms 

 are directly concerned in the migration of phosphorus from the soil to the 

 plant and from the plant back to the soil. 



RELATION OF PHOSPHORUS TO DECAY AND NITROGEN-FIXATION. 

 Just as bacteria influence the transformation of phosphorus compounds 

 in the soil, so phosphorus itself affects the growth and activities of bacteria. 

 As one of the essential constituents of living cells it reacts on the growth 

 of microorganisms and influences species relationships. There are un- 



