Inorganic substances, especially aluminum, in relation to the activities of soil microorganisms

Soluble salts of aluminum have been found by a number of investigators in acid soils which have been treated with soluble salts of other metals. Aluminum in soluble form is found in soils where sulfur is being oxidized. Sulfuric acid and sulfates are present and act on the aluminum. Calcium carbonate has been found by all these investigators to inactivate the toxic aluminum, iron, and manganese when it has not been used in excessive quantities. Phosphates have likewise been found to prevent the injurious action of aluminum. Aluminum associated with hydrolyzable and highly ionizable salts has been shown to exert a toxic influence on plants aside from the acidic effect of the associated acid radical. Aluminum phosphates have been demonstrated to be excellent sources of phosphorus for a number of agricultural plants in spite of the aluminum present. Soils that have not been treated with salts show practically no soluble aluminum in clear water extracts. The form of aluminum first extracted by normal potassium nitrate in aqueous solution is non-diffusable and appears to be a colloidal form of aluminum hydroxide. Practically all acid soils which have been studied contain quantities of easily replacable aluminum when brought in contact with salt solutions. Certain acid soils contain large amounts of soluble manganese and this element appears to support nitrification. Soluble manganese occurs in soils as a result of sulfur oxidation. There is available to agronomists information which indicates that the use of moderate amounts of liming materials and ample quantities of phosphates will precipitate the toxic forms of aluminum, iron, and manganese in acid soils and at the same time permit the establishment of pH values in keeping with the requirements of soil organisms, the growth and feeding power of crops, and their response to feeding. It has already been shown that attention to the above considerations makes possible the production of maximum quantities of nitrates and a large symbiotic nitrogen fixation.