There would seem, therefore, no doubt that the decomposition of celluloses and other carbon compounds is of primary importance in the life-activities of soil fungi. NITROGEN RELATIONSHIPS. In this section we shall consider the problems of nitrogen fixation and nitrification, of ammonification, and of the utilisation of nitrogenous compounds by soil fungi. As soil fungi form so large a part of the soil population, the question of whether they can make use of the free nitrogen of the air is of primary importance. During the last two decades many investigators have attempted to solve the problem, often studying allied or identical species; but if one consults some thirty researches published during this period, opinion is found to be about equally divided. Even, however, in those studies where nitrogen fixation has been recorded the amounts are very slight, usually being below 5 mgrms. per 50 c.c. of solution, and often being obviously within the limits of experimental error. Latham,[37] however, working on _Aspergillus niger_, recorded variations ranging from a nitrogen loss of 42·5 mgrms. to a nitrogen fixation of 205·1 mgrms. per 50 c.c. of medium. Ternetz[63] found that different strains of _Phoma radicis_ may fix from 2·5 mgrms. of nitrogen in the lowest case, to 15·7 mgrms. in the highest per 50 c.c. of nutrient solution. Duggar and Davis[20] report that _Phoma betæ_ may fix nitrogen in quantities of 7·75 mgrms. per 50 c.c. of medium. The latter authors, in a very able critique of the problem, indicate certain possible sources of error in previous work, and if one examines the studies in which nitrogen fixation has been recorded in the light of these criticisms, it is difficult not to think that, with the exception of the genus _Phoma_, good evidence for nitrogen fixation by fungi is lacking. _Phoma betæ_ is a common pathogen attacking beets, whilst _P. radicis_ is a mycorrhizal form inhabiting various Ericales. Apart from these exact quantitative studies, which have given a negative verdict, there is a considerable amount of positive but indirect evidence for nitrogen fixation by mycorrhizal fungi,[55] and it is very unfortunate that more of these forms have not been investigated quantitatively. As the evidence stands to-day, one must conclude that the fungus flora does not play any part in the direct nitrogen enrichment of the soil. Equally obscure is the question of nitrification and denitrification by soil fungi, but this is the result of a lack of study rather than of a plethora of indeterminate researches. Direct nitrification or denitrification has not been established, but the work of Laurent[38] and a few other workers appears to show that soil fungi can reduce nitrates to nitrites.