75 

 3ardsley and Lancaster (1960, 1965) used activated charcoal to remove 

 color from their extracts before determining S turbidimetrically. 



A 0.01 M Ca(H 9 P0 / ) ? -2H 9 extractant (approximately 500 ppra P) and 

 the NH.OAc+HOAc extractant of Bardsley and Lancaster (39 g NH.OAc in 

 1 liter of 0.25 N HOAc) were compared. Both extractants were studied 

 with and without activated charcoal. Since most of Florida soils are 

 low in extractable S, a narrow soil: solution ratio was used (Bardsley 

 and Lancaster, 1960). Ten grams of air-dried, screened soil were 

 shaken for 30 minutes with 25 ml of the extractant in 100-ml extraction 

 bottles on a reciprocating, mechanical shaker. Where charcoal was 

 used, approximately 0.25 g of "Darco G-60" activated charcoal was added 

 after the 30-minute shaking period using a calibrated scoop. The sam- 

 ples were reshaken for 3 minutes. All extracts were filtered through 

 Whatman no. 42 filter paper; sulfate-S was determined turbidimetri- 

 cally. Ten milliliters of the extract were used for sulfate determina- 

 tion. An equal volume of the extracting solution was added to all 

 standards and blanks. Charccal "blanks" were run to determine any 

 sulfate-S extracted from the charcoal. 



Where no charcoal was used and color appeared in the extract, 

 duplicate samples were read on the spectrophotometer when determining 

 S by turbidimetry . Mo BaCl„ or BaSO, seed suspension was added to one 



2 H 



of the samples. Absorbance was read on both samples, and the absor- 

 bance due to color in the extract was subtracted from the absorbance 

 of the sample with BaCl added. From this value, S in the extract was 



Cm 



calculated . 



Air-dried, screened soil from a Myakka fine sand and from a 

 Kendrick fins sand at to 15 cm and 15 to 30 cm depths was-' used for 



