Studies in soil and plant manganese - II. The relationship of soil pH to manganese availability

In a series of field experiments the relation of soil pH to water-soluble manganese was found to be incompatible with the theory that non-availability of manganese is the result of the formation of insoluble higher oxides at high pH values. Laboratory investigation showed that raised pH is accompanied by a reduction in the level of water-soluble managanese within the space of two hours, thus excluding the possibility that biological oxidation of manganese is the cause of reduced availability. The relationship between pH and the negative logarithmic formulation of water-soluble manganese (pMn) was shown to be curved, whereas the formation of higher oxides would imply a linear relationship. It is suggested that the formation of complexes of managanese with organic matter in the soil may account for the observed relationship of manganese and pH, and that other micronutrients probably behave similarly.