Manganese is a micronutrient because it is

Manganese is a micronutrient because it is required by plants in small quantities.
Micronutrients are essential to plant nutrients that are found in trace amounts in tissue but play an imperative role in plant growth and development. Without these nutrients, plant nutrition would be compromised leading to potential declines in plant productivity.
Manganese (Mn) is an important plant micronutrient for plant growth and development and sustains metabolic roles within different plant cell compartments.
It is an essential cofactor for the oxygen-evolving complex (OEC) of the photosynthetic machinery, catalyzing the water-splitting reaction in photosystem II (PSII).