For the synthesis of proteins the plant requires nitrogenous compounds. To a small extent these are supplied in inorganic form from the atmosphere. During thunderstorms considerable quantities of oxygen combine with the nitrogen of the air. Thus by the aid of atmospheric moisture nitrous and nitric acids are formed, which reach the soil in rain. Nitrogen in the form of nitric acid, HNO3, is easily assimilated by plants, while free nitrogen is absolutely useless to most plants. In addition to this inorganic source of nitrogen, vegetation has another, not less important, organic one in the subterranean associations of nitrogen-fixing bacteria, fungi, and algae. (Braun-Blanquet 1932:235)
Ammonium Formation (Ammonification).—Nitrates are the most effective nitrogen compounds for the nourishment of plants. For all higher plants not living in symbiosis with the tubercle bacteria, nitric acid forms, if not the only [Nota de rodapé 1: Cf. Schreiner, A., The organic constituents of soil, Science 36, 1912. According to the investigations by Ziegenspcck (1922), mycotrophic plants do not assimilate nitrogen in the form of nitrates but in that of ammonium compounds (or amino acids). Upon strongly acid soils where no nitric acid is formed, these plants, therefore, have the advantage over autotrophic plants. ] , at least by far the most important nitrogenous food substance. The nitrogen compounds present in the soil must generally be converted into nitric acid, HNO3, or its salts in order to fulfill their physiological function. But this transformation can take place only with the cooperation of nitrifying bacteria. It proceeds step by step. The undecomposed organic matter first goes  through an ammonifying process wherein both bacteria and fungi participate (Mucor, Aspergillus, Fusarimn, etc.). According to Marchal (1893), Bacterium mycoides plays a major rôle in ammonification, especially in cultivated soils. It is an intensively oxidizing organism, which, given a sufficient air supply, unites oxygen with the complex organic compounds and releases their nitrogen in the form of ammonia, NH3. Numerous other representatives of the soil flora react similarly. The end product of this first transformation in the soil is ammonium carbonate, (NH4)2CO3. Ammonium carbonate is also derived from the decomposition of urine and excreta under the influence of urinefermenting bacteria (B. coli, B. vulgare, B. fluorescens, and species of Micrococcus, Urococcus, etc.). (NH4)2CO3 is the initial substance in the formation of nitrates in the soil, that is, in nitrification. (Braun-Blanquet 1932:237)
BRAUN-BLANQUET, Josias. 1932. Plant sociology: the study of plant communities. (Trans.: George D. Fuller; Henry S. Conard) New York: McGraw-Hill Book Company.