Why can oats develop manganese-deficiency symptoms in alkaline soil that contains manganese?

Oats can develop manganese-deficiency symptoms, such as the disease known as gray speck, even when manganese is present in the soil because of the influence of soil pH on nutrient availability.1

In alkaline soils, an elevated pH level causes the manganese to react with oxygen, resulting in the formation of manganese dioxide. This specific chemical form of the nutrient is insoluble, meaning it cannot be absorbed or utilized by the plant roots. Consequently, despite the presence of manganese in the soil, the plant experiences a deficiency because the element is chemically bound in a state that is unavailable for uptake.1