best for S is by X-ray fluorescence, for NO3 you can use NO3-sensitive electrodes or ion-chromatography. Check our papers in RG!

Dear Dr. Premnath,

Principles/ outlines of the most commonly used methods for analysis of total S and nitrate N in soil are briefly presented below together with references. 

Total S analysis in soil: A rapid and precise method of determining total sulfur in soils is described by Tatabai and Bremner (1969). The soil sample is heated with alkaline sodium hypobromite solution to oxidize soil sulfur compounds to sulfate, and the sulfate thus formed is reduced to hydrogen sulfide by a modified Johnson-Nishita procedure and determined colorimetrically as methylene blue. The method gives quantitative results with pure organic and inorganic sulfur compounds, including amino acids, sulfonic acids, organic sulfates, sulfoxides, thioureas, sulfates, sulfites, sulfides, and elemental sulfur. It gives quantitative recovery of organic and inorganic sulfur added to soils and soil extracts, and its results with soils agree closely with those obtained by other methods proposed for total sulfur analysis of soils.

[Reference: Tabatabai MA and Bremner JM (1969) An Alkaline Oxidation Method for Determination of Total Sulfur in Soils. Soil Sci. Soc. Am. Proc. 34(1) 62-65].  

Soil Nitrate N analysis (KCl extraction/ Cd-reduction method): This method involves the quantitative extraction of nitrate nitrogen (NO3-N) from soils using 2.0 M KCl.  Nitrate is determined by reduction to nitrite (NO2-N) via a cadmium reactor, diazotized with sulfanilamide and is coupled to N-(1-Napthyl)-ethylenediamine dihydrochloride to form an azochromophore (red-purple in color) measured spectrophotometrically at 520 nm. The method is readily adapted to manual or automated techniques.  The procedure follows that of Keeney and Nelson (1982) for determining nitrate nitrogen with a modification in which 25 mL of KCl and 5.0 g of soil are used instead of 100 mL and 10 g soil.

[Reference: Keeney DR and Nelson DW (1982)  Nitrogen - inorganic forms.  In A. L. Page (eds.) Methods of soil analysis, Part 2.  Agron. Monogr. 9, 2nd ed. ASA and SSSA, Madison, WI.  p. 643-698].

I hope you find this input interesting and useful.