76 

 extraction. Three rates of S — 0, 4, and 8 ppm — were added to each soil, 

 The soil was moistened and allowed to dry and screened before extrac- 

 tion. The results are given in Table 9. All values are the mean of 

 two determinations. 



None of the methods were comparable in the amounts of S extracted 



from the soils. The 0.01 M Ca(H„P0. ) _ • 2H„0 without the charcoal 



— 2 4 2 2 



(Method A) produced the most consistent results. Considerable amounts 

 of S ware extracted from the charcoal (Methods B and D) . The amount of 

 S in the charcoal blank was subtracted from that in the sample. The 

 amount of S was quite variable in the charcoal blanks, and this vari- 

 ability probably accounted for the differences in the samples extracted 

 using charcoal. The NH,0Ac+H0Ac extractant (Method C) removed slightlv 



less sulfate-S than the Ca(H PO. ) 9 • 2H extractant since acetate is not 



c. Si 

 as effective in replacing sulfate as the phosphate anion. The decision 



was made to use the 0.01 M Ca(H„P0. ) „ ■ 2H„0 extractant. In those sam- 



— 2 4 2 2 



pies where color remained in the extract, a blank was run when S was 

 determined turbidimetrically . 



The 1979 paper by Hue and Adams on the indirect method of sulfate 

 determination on soil extracts offered another alternative for sulfate 

 determination on colored extracts. Sulfate sulfur in the 174 soil 

 extracts in the study of S distribution in Florida soils was determined 

 by this method and is discussed in section 6.2. 

 5.2.4 Estimation of Total Sulfur in Soils 



The Mg(N0,) ,,/HNO., procedure used for plant tissue digestion was 

 used for estimating total S in soils. Some modifications were needed 

 and these are described in Appendix B. Some preliminary studies and 

 iustif ication for using this techniaue are described beloxj. 



