Phosphorus and Bacteria 283 



mineral phosphates are affected in their solution by 

 the presence of carbon dioxid produced by bacteria 

 from the humus. They are also affected in their solu- 

 bility by other substances produced by bacteria, notably 

 organic acids. 



Organic phosphorus. — The other class of phosphorus 

 compounds — the organic — exist in the humus and have 

 a double origin. By far the greater part of the phosphoric 

 acid in the humus is locked up in an insoluble condition 

 in the decaying roots and stubble. It came originally 

 from the soil-water and was withdrawn from the latter 

 by the plants. Another portion of the phosphoric acid 

 in the humus is also derived from the phosphates dis- 

 solved in the soil-water, but in this case it is changed 

 into insoluble modifications either by being used by the- 

 bacteria themselves, or by combining with some of the 

 substances produced by bacteria. In other words, the 

 soil bacteria help to produce soluble phosphates in the 

 soil. They also use up some of the latter by changing 

 them into combinations not immediately available to 

 crops. 



As the soil-humus is broken down by the attacks of 

 various microorganisms its inert phosphorus may again 

 become available. Indeed, there is evidence to show 

 that the organic combinations of phosphorus in the 

 soil probably play a more or less important part in the 

 supply of this element to higher plants. It is clear, 

 therefore, that bacteria bear an intimate relation to 

 the supply of phosphoric acid for crops not only by help- 

 ing to make soluble the mineral phosphates in the rock 

 particles, but, also, by making available again the 



