The second primary nitrogen relationship that we have to consider is the process of ammonification. The ammonifying power of soil fungi was first demonstrated by Muntz and Coudon,[46] and by Marchal[40] in 1893, the former showing that _Mucor racemosus_ and _Fusarium Muntzii_ gave a larger accumulation of ammonia in soil than any of the bacteria tested; and the latter that _Aspergillus terricola_, _Cephalothecium roseum_ and other soil fungi were active ammonifiers, especially in acid soils. Shibata,[62] Perotti,[49] Hagem,[26] Kappen,[31] Löhnis,[39] and others, have observed that urea, dicyanamide and cyanamide are decomposed with the liberation of ammonia; and Hagem[26] has recorded the same process for peptones, amino acids, and other organic nitrogen compounds in plant and animal remains in the soil. The latter author considers soil fungi more important ammonifying agents in the soil than bacteria, a conclusion in which McLean and Wilson,[44] and perhaps most later workers concur. McLean and Wilson[44] found large differences in the ammonifying powers of various soil fungi, the _Moniliaceæ_ being the strongest ammonifiers, the _Aspergillaceæ_ the weakest. Generic and specific differences have been confirmed by Coleman,[15] Waksman,[67] and other authors. Waksman and Cook[70] suggested that such variations may be due, not to innate differences in the metabolic activities of the several organisms, but to differences in reproductive times, and that there might be some relationship between sporogeny and the ability to accumulate nitrogen. Kopeloff[35] has carried out experiments on the inoculation of sterilised soil with known quantities of spores and found that, although the amount of ammonia accumulated increased with the number of spores the proportion was not direct but modified by the food supply. After the first five days’ growth, the rate of ammonia production varied markedly in a two-day rhythm which seemed to be due to the metabolism of the fungus rather than to recurrent stages of spore formation and germination in the life history. The amount of ammonia liberated has been shown by recent work[66] to depend upon the available sources of carbon and nitrogen. In the absence of a carbohydrate supply the protein is attacked both for carbon and nitrogen, and since more of the former is required much ammonia is liberated. In addition, however, to the carbon and nitrogen control, the process of ammonification by soil fungi is intimately related to physical conditions. Working with pure cultures, McLean and Wilson,[44] Coleman,[15] Kopeloff,[35] Waksman and Cook,[70] and other students, have shown that the amount of ammonia accumulated depends upon such factors as the presence of phosphates, the period of incubation of the fungi, aeration, the moisture in the soil, the temperature, the degree of soil acidity, the type of soil, and so forth.