SOIL ORGANISMS 405 



Since the sulfur added to the soil at Ithaca, New York, 

 amounts to only 65 pounds of SO3 yearly to the acre, other 

 sources of sulfur assume considerable importance in fertility 

 practice. It seems probable, however, that the Judicious 

 use of fertilizers carrying sulfur in conjunction with farm 

 manure, green-manure and crop residues, will adequately 

 care for the sulfur needs of the average soil (see par. 264). 



223. Factors influencing sulfofication. — The sulfofying 

 activities of the soil flora are greatly influenced by conditions 

 within the soil. Brown ^ has found that the addition of farm 

 manure and green-manure greatly stimulates sulfofication, al- 

 though carbohydrates alone seem to exert a depressing influ- 

 ence. Lime, unless applied in very large amounts, encour- 

 aged the transformation of the sulfur compounds, increas- 

 ing the amount of sulfates present in the soil. The reason 

 for this influence is evident from the reactions already quoted. 

 The partial oxidation of hydrogen sulfide or of free sulfur 

 produces sulfurous acid (H2SO3), which exerts a retarding 

 influence on further action, unless a base, such as calcium or 

 magnesium, is present to form a salt of this acid. 



Brown's results also indicate the preponderant influence 

 of aeration, moisture, and organic matter on sulfofication. 

 Optimum conditions for crop growth, as far as these factors 

 are concerned, seem also to be optimum for the transforma- 

 tion of sulfur compounds in the soil. These same conditions 

 also favor satisfactory reactions within the carbon cycle as 

 well. 



^ Brown, P. E., and Kellogg, E. H., The jDetermvnaiion of the Sul- 

 fofytng Fower of So^ls; Jour. Biol. Ghem., Vol. XZI, No. 1, pp. 73-89, 

 1915. 



Brown, P. E,, and Johnson, H. W., Studies in Sulfofication; Soil Sei., 

 Vol. I, No. 4, pp. 339-362, 1916. 



Brown determines the sulfofying pow«r of soil by adding .1 gram of 

 NaaS or free sulfur to 100 grams of fresh soil, adjustmg the moisture 

 content to optimum and incubating from five to ten days. The sulfates 

 are then determined by shaking the soil with water for seven hours, 

 filtermg and precipitating the sulfates with barium chloride. The 

 amounts of sulfates are estimated in a sulfur photometer. An untreated 

 sample of soil should be run as a check. 



