Fungi are active also in the decomposition of organic nitrogen compounds and the liberation of ammonia but, on the other hand, they assimilate available nitrogen compounds in the soil and so compete with crop plants. It has been suggested from time to time that soil fungi can fix atmospheric nitrogen but with the exception of Phoma sp. and possibly of Orcheomyces sp. this has yet to be proved.
An aspect of the soil fungi that is being increasingly recognised as of fundamental importance is the mycorrhizal relationship of numerous forms with higher plants. The inner meaning of this symbiosis is not yet clear but there seems little doubt of its vital importance to the life of many host plants, both herbaceous forms and forest trees. On the other hand it is also being increasingly recognised that the soil is a reservoir of vast numbers of disease causing fungi, some of the most destructive parasites of crop plants such as species of Colletotrichum, Fusarium, Helmintho sporium, Rhizoctonia, Phoma,Macrosporium and Alternaria, Bot rytis, Thielavia, Sclerotinia, Phytophthora and Pythium, Clado sporium, Verticillium, Aphanonryces, Ozonium, Rhizopus, Cer cospora, Rosellinia, Actinomyces and numerous other genera being present in abundance. Certain other important parasites
such as Synchytrium endobioticum, Urophlyctis alfalfae, etc., find their home in the soil although there is no evidence, as yet, that they carry on any vegetative existence apart from the host-plants. Certain slime moulds such as Spongospora subterranea, Plas modiophora brassicae, Cystospora batata, Molliardia triglochinis and species of Ligniera occur in the soil and there is evidence that they may live vegetatively in this habitat.
The soil fungi are thus so diverse in their activities that it is quite impossible to assess their value in the profit and loss account of the agricultural ledger. The two outstanding facts are their vast importance and the comparatively slight attention they have received in soil science. (W. B. B.)