42 

 solutions (Olsen, 1957). Barrow (1967) was able to relate 0.01 M 

 monocalcium phosphate extractable sulfate to the decrease in S uptake 

 by plants grown for 183 days in a greenhouse. The uptake of S was 

 also greatly influenced by the nature of the soil; these differences 

 were also indicated in the total extractable sulfate-S. Both plant 

 uptake and extractable S were significantly modified by the nature of 

 the soil, time of sampling, and previous fertilization. 

 2.2.2 Absorption from the Atmosphere 



Crops growing on soils low in extractable sulfates may not always 

 respond to direct applications of S in the field (Bremer, 19 76: Jones 

 et al. , 1979). A possible explanation for these observations is 

 direct absorption of SO,, from the atmosphere. Fried (1948) first 

 demonstrated that alfalfa plants could take in SO^ through the leaves 

 and convert it into organic S compounds. Olsen (1957) found that the 

 amount of S0„ absorbed by healthy cotton plants was a function of the 

 effective leaf surface. Healthy plants obtained about 30% of their S 

 from the atmosphere whereas S-deficient plants absorbed over 50% of 

 their S from the atmosphere. He concluded that while the S0 ? concen- 

 tration of the atmosphere is inadequate as the sole source of S for 

 plants, the atmosphere could provide an important supplementary source 

 of S to growing plants. 



Sulfur-deficient alfalfa plants exposed to a greenhouse atmosphere 

 during the winter months in Wisconsin were capable of absorbing as much 

 as 73% of their total S from the air (Hoeft et al., 1972). Total S 



collected by PbO candles during the period the plants were grown was 



2 

 1.05 mg S/100 cm for a candle located in the greenhouse and 10.57 mg 



? 

 S/100 cm for a candle located outside the greenhouse. Air movement 



