The Use ofAspergillus Niger(M)* for the Determination of Magnesium, Zinc, Copper and Molybdenum Available in Soils to Crop Plants

Abstract

The fungus Aspergillus niger (Mulder’s M strain) requires the following mineral elements for normal growth ; nitrogen, phosphorus, potassium, magnesium, sulphur, iron, zinc, copper, manganese, molybdenum, and possibly gallium and vanadium. When one of the essential mineral elements is omitted from the culture solution, the growth of the fungus is markedly reduced, and the addition of increasing amounts of the element, from deficiency to sufficiency levels, results in an increase in dry-weight yields and sporulation. The unpurified mineral macro-nutrients and dextrose that ordinarily comprise culture solutions contain sufficient of the micronutrient elements to provide for the normal growth of the fungus. Therefore chemical and ion-exchange methods are used to remove the micronutrient to be determined from the mineral macronutrient and dextrose so that a standard growth series can be prepared for the " test ’’ element, from deficiency to normal levels. A determination of the “ test ” nutrient is made by adding a known amount of material, e.g. soil, extracts of fresh plant tissues, to 50 ml. of culture solution containing all the mineral elements other than the one to be determined. By reference to a standard growth series, using as criteria for assessment the development of spores and dry-weight yields of the fungus, the amount of the element present in the test substance is determined. The method has been used to determine normal and deficiency levels of magnesium, copper, zinc and molybdenum in British soils. The bioassay method for molybdenum is very sensitive, and can be used to determine levels of the element in tissue extracts and in the ash of plants which are not detected by chemical methods.