SOIL FORMATION 8i 



Now, these various nitrogen and carbon acids readily change the 

 phosphorus into soluble phosphorus. Actual experiments show 

 that about one pound of phosphorus and two pounds of calcium 

 are made soluble by the oxidation of one pound of nitrogen. 

 Twelve out of twenty of the bacteria obtained from soil by one 

 worker exerted a definite solvent action on difficultly soluble 

 plant-food. Some of these acted by means of carbon dioxid, 

 others by means of acids. Most of these bacteria are the common 

 ammonifiers. The nitrogen-fixing organisms produce many 

 acids, and under good conditions they can generate one and three- 

 tenths times their own weight of carbon dioxid during twenty- 

 four hours. This dissolves in water and attacks even ordinary 

 quartz rock; granite, and rocks related to it are rather quickly at- 

 tacked, with the liberation of potassium and other elements. It 

 is also certain that the carbonated waters would change the in- 

 soluble phosphates into an available form. 



Sulfur. — Sulfur is an essential element for all plants. The 

 quantity required by plants is small, and although the quantity in 

 some soils is small yet determinations at the Illinois Experiment 

 Station show that about forty pounds are brought to the soil an- 

 nually in rain water. This is acted upon by bacteria, with the 

 production of sulfuric add which would liberate plant-food. 

 The sulfur of the soil is mainly in the form of sulfates or 

 organic sulfur. 



Bacteria act on sulfur compounds in three ways: (i) They 

 decompose organic compounds with the formation of hydrogen 

 sulfide or other ill-smelling gases; (2) they oxidize sulfur com- 

 pounds of the soil; and (3) the true sulfur bacteria act on hy- 

 drogen sulfide and other sulfur compounds with the production 

 of metallic sulfur, sulfuric acid, and eventually mineral sul- 

 fates. The majority of the common putrefying bacteria pro- 

 duce hydrogen sulfide. Many of the disease-producing bacteria 

 produce this gas when growing in an appropriate medium. Not 

 a few soil organisms will reduce the sulfates to hydrogen sul- 

 fide. 



The two classes of the true sulfur bacteria act opposite to 



