Try it out and see for yourself rather than we advising you from a distant.

Dear Sir. Concerning your issue about whether the detergent NP-40 interfere with Berthelot test  for ammonia. Ammonia is extracted using 2M KCl and determined colorimetrically (NH4-N by Berthelot colorimetry, NO3-N by Cd reduction and NED colorimetry) . Analysis is carried out on fresh samples, to avoid losses of NH4-N on drying. Results are reported on a dry weight basis however, by calculation using %moisture of the soil (measured as dry matter). Also, another procedure : sample can be hydrolyzed by the action of the urease to produce ammonia and carbon dioxide. The ammonia reacts with hypochlorite and phenol in the presence of nitroprusside to form indophenol, which in alkaline medium gives an intense blue colour. The intensity of the colour formed is directly proportional to concentration of urea in the sample. To avoid interference you can the above methods. The following below links may help you in your analysis:

http://www.hill-laboratories.com/file/fileid/33832

http://pdf.usaid.gov/pdf_docs/Pnadf783.pdf

Thanks