Theoretically, the ideal method for determining ammonia in soils would give the absolute amount of ammonia present as such and as the ammonium radical, but in the light of present knowledge, it cannot be said beyond all doubt that any conceivable method would give this ideal result. This is true because a large part of the nitrogen of the soil is present in protein and protein degradation products, the extensive decomposition of which gives large amounts of ammonia. The uncertainty regarding just what products are present does not permit the finding of conditions which we can be certain will expel ammonia already present and not decompose any material to give, among other products, ammonia. The problem is further complicated by the well known absorptive and adsorptive powers of the soil.
Potter, R. S. and Snyder, R. S.
"The determination of ammonia in soils,"
Research Bulletin (Iowa Agriculture and Home Economics Experiment Station): Vol. 2
, Article 1.
Available at: https://lib.dr.iastate.edu/researchbulletin/vol2/iss17/1