CHANGES IN INOEGANIC CONSTITUENTS 421 



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 undoubtedly species whose phosphorus requirement is greater 

 than that of other species. Indeed, conditions may arise that favor the 

 rapid assimilation of soluble phosphates by bacteria. In that case the 

 microorganisms would act as competitors to the higher plants. Among 

 the species favorably affected by an abundant supply of phosphates 

 Azotobacter is quite prominent. Hence nitrogen fixation is in a meas- 

 ure dependent upon a proper supply of phosphorus compounds. 



Fred and Hart have shown that the potassium ion does not mate- 

 rially influence ammonincation; soluble phosphates cause large increases 

 in the number of bacteria, ammonification and carbon dioxide produc- 

 tion. By applying soluble phosphates to the soil crop production is 

 increased, and it is due, in part, to the promotion of bacterial activity. 

 The increased bacterial activity results in a more rapid decomposition 

 of the organic matter, thus making available for the growth of crops 

 larger quantities of nitrogen and probably of minerals. 



SULPHUR 



SULPHUR COMPOUNDS IN THE SOIL. Sulphur occurs in the soil in 

 the form of sulphates and in that of organic compounds. In ill- 

 aerated soils the reduction products of sulphates, viz., sulphites, sul- 

 phides and even elementary sulphur, may be present in small amounts 

 as a transition stage. According to Berthelot and Andre the protein 

 compounds of the soil humus are quantitatively more important than 

 the sulphates. However, this is not true of arid and semi-arid soils 

 in which sulphates represent a larger store of combined sulphur than 

 is contained in organic substances. 



Sulphur-phosphate Composts. In the composting of sulphur, 

 ground rock phosphate (floats) and soil certain soil bacteria oxidize 



