LETHAL GASES 191 



and of the air of a greenhouse shows that greenhouse concentrations are 

 approximately 90 per cent of atmospheric when ventilators are partly 

 open, 60 per cent when ventilators are closed. The fact that the many 

 plants gro^vn throughout the year in the Institute greenhouses are con- 

 sidered comparable to plants grown in areas where there is no sulphur 

 dioxide, is an indication that exposure to sulphur dioxide in the prevailmg 

 concentrations and durations has no unfavorable effect on plant life." 



Sulphur Dioxide of Atmosphere as a Sulphur Source for 



Plant Nutrition 



Setterstrom, Zimmerman, and Crocker ^'^ found that sulphur deficiencies 

 in the soil for growth of alfalfa could be supplied in part by fumigating the 

 air with non-marking concentrations of SO2. The Cruciferae, which are 

 very rich in organic sulphur, gave negative results. As we have already 

 seen, Thomas and associates ^^ found later that sulphur deficiency for 

 aKalfa could be supplied in part by SO 2 fumigation. 



Available sulphur is deficient ^-^ in some soils for maximum yield of 

 protein-rich legumes like alfalfa. Indeed, it is so deficient in some soils in 

 Washington, Oregon, and Idaho that sulphur additions will increase alfalfa 

 yields as much as five-fold. As measured by crop needs in general, sulphur 

 supply of soils is about equal to phosphorus supply, and as measured by 

 the needs of high-sulphur crops like certain legumes and crucifers, the 

 sulphur supply in the soil is below the phosphorus supply. Moreover, 

 sulphur in the form of sulphates is leached from the soil in large amounts, 

 w^hereas phosphorus is held rather tenaciously. In the United States 

 enormous amounts of sulphur, largely in the form of SO2, are released into 

 the atmosphere by smelters and 'by industrial and home consumption of 

 coal and other fuels. This finally reaches the soil by being washed down by 

 ram or absorbed directly by the soil or vegetation. This averages many 

 pounds per acre per year for the w^hole country, and of course is high near 

 industrial and population centers and very low in remote rural sections. 

 This replenishment of sulphur in the soil may be an important factor in soil 

 fertility around big population centers. Together with the later use of acid 

 phosphate, which is about half calcium sulphate, this may account for the 

 fact that gypsum ^ was very effective on clover in colonial days but has far 

 less effect in eastern United States today. Also it is possible that fumes 

 from the Trail Smelter may be improving and not injuring the yields of 

 alfalfa in the upper Columbia Valley in the United States. This is made 

 probable by the fact that soils in neighboring valleys both east and west are 

 deficient in sulphur for maximum alfalfa production. 



Effect of Sulpbtur Dioxide on Animals 

 Men and animals, like plants, ^^ are subjected to sulphur dioxide fumes. 

 In the atmosphere of smoky cities sulphur dioxide sometimes reaches con- 

 centrations of 10 ppm. Workers engaged in the manufacture of refrigerants 



