Please have a look at this link and PDF attachment.

https://dl.sciencesocieties.org/publications/sssaj/abstracts/47/4/SS0470040833

I have used these methods in lab but I think getting ammonium-free distilled water is an issue.

Soil incubation studies require adding water on regular basis in order to maintain moisture in soils. A little of ammonium in distilled water defeats the purpose.

Dear Arvind Ji

The link provided by you for downloading is not accessible to me. Please send me the pdf file.

Check the web of the University of California (Riverside)

Dear Dr. Naik,

I give hereunder the summarised method for colorimetric determination of NH4- and NO3- N in KCl extracts of soils:

KCl extracts of soils are often coloured due to organic or colloidal matter. A dialyzer is used to remove these interferences and reduce the sensitivity of the method to attain the optimum range in concentration for the simultaneous determination of nitrate, nitrite and ammonium at an analysis rate of 100 samples per hour. Ammonium is determined by the Berthelot reaction, through the formation of a blue coloured compound apparently related to indophenol from the reaction of ammonium and sodium phenoxide, followed by the addition of sodium hypochlorite. Sodium nitroprusside is added to intensify the blue colour. Nitrate is reduced to nitrite at pH 7.5 in a copper‐cadmium reductor coil. The nitrite produced reacts under acidic conditions with sulfanilamide to form a diazo compound that couples with N‐1‐Naphthylethylenediamine dihydrochloride to form a reddish‐purple azo dye measurable at 520 nm.

I hope you find the information very useful and easy to follow.

Ref: Tel DA and Heseltine C (1990) Comm Soil Sci Plant Anal 21 (13-16):1681-1688.

Thanks Dr. Kundu Sir