438 SOIL FUNGI 



have properly been assumed to account for soil acidity. Hagem 

 (1910) concluded that inorganic soil constituents containing such 

 minerals as calcium, magnesium, and phosphorus are dissolved 

 bv these organic acids and thereby made available for green plants. 

 Soil fungi are therefore to be regarded as important in soil fertil- 

 ity. Much remains to be determined, however, concerning the 

 indirect role of fungi in making available iron, sulphur, and the 

 many other elements that green plants require in small amounts. 



.Many soil fungi, as grown on artificial media or on sterilized 

 soil, should be studied intensively to increase our knowledge of 

 their biochemical activities. Similarly two or more species, if 

 grown in association in the same culture, might yield valuable 

 data. The application of these findings in explaining the activities 

 of fungi in normal soils would require the exercise of incisive 

 thinking and well-balanced judgment. Success would be most 

 likely attained if such studies were undertaken by a corps of work- 

 ers, including microbiologists, chemists, and physicists, working 

 in collaboration. 



Means for measuring soil fertility continue to be sought because 

 in the future an adequate supply of food and feed crops will come 

 more and more to depend upon a better knowledge of soil fertility. 

 Partly for this reason the use of Aspergillus niger to test the soil- 

 potassium needs of a given crop, as was proposed by Mehlich 

 etal. (1933), has intriguing possibilities for application to require- 

 ments for other minerals. 



Undoubtedly soil fungi perform an important role in produc- 

 ing growth-promoting substances that are utilized by green plants. 

 It is a well-established fact that crop plants do not grow as well 

 on infertile soil if the fields are enriched with mineral fertilizers 

 as if they are enriched with manure or organic material containing 

 equivalent amounts of minerals. The relationship of soil fungi to 

 the production of growth regulators should be further elucidated. 



The results of researches on soil fungi, if viewed in perspective, 

 emphasize that soils are not static, but dynamic. The ever-chang- 

 ing balance between each kind of soil microbe and the mineral 

 and non-living organic content of soils still remains largely un- 

 known. A concise summary of these subjects, together with an 

 excellent bibliography, is to be found in a paper by YVaksman 

 (1944). 



