In the welter of scattered observations on the utilisation of nitrogenous compounds, it is difficult to trace any clear issue. That proteins, amino-acids, and other complex organic compounds are readily broken down to ammonia by soil fungi is clear, and, on the other hand, it is also clear that soil fungi utilise extensively ammonia and nitrates as sources of nitrogen. On which side the balance lies it is yet impossible to say. MINERAL RELATIONSHIPS. Heinze[27] and Hagem[26] have stated that soil fungi make the insoluble calcium, phosphorus, and magnesium compounds in soil soluble and available for plant food; and Butkevitch[12] has used _Aspergillus niger_ in determining the availability of the mineral constituents, but practically no work has yet been carried out on these problems. A further matter on which sound evidence is greatly to be desired is the part played by soil fungi in the oxidation processes of iron and sulphur. A point which may be mentioned here, as it is of some considerable practical importance, is the large quantity of oxalic, citric, and other acids formed by certain common soil fungi. Acid formation is partly dependent upon the species of fungus--even more the physiological race within the species--and partly upon the substratum, particularly the source of carbon.[5],[54] It is interesting that as a group _Actinomycetes_ do not form acids from the carbon source but alkaline substances from the nitrogen sources.[69] CONTROL OF SOIL FUNGI. In the preceding sections an attempt has been made to sketch rapidly the chief outlines of the widespread relationships of soil fungi and of the fundamental part that they play in the biochemical changes occurring in the soil. It will be evident, even from this survey, that their occurrence is of the utmost agricultural importance, both when helpful as in mycorrhizal relationships or as agents in making complex organic materials available as plant food, or when harmful as when causal agents of disease in plants. It is clear that could the soil fungi be controlled to human ends by the encouragement of the useful forms and the elimination of the harmful, a valuable power would be placed in the hands of the grower of plants. Certain aspects of this control, the cruder and more destructive perhaps, are already practicable, whilst the finer and more constructive aspects remain possibilities of to-morrow.