Manganese is absolutely a true micronutrient. It is necessary in very small amounts relative to other nutrients, but it is still completely essential. It is required for respiration, photosynthesis and nitrogen assimilation as well as pollen proliferation and pollen tube growth. It plays a role in root cell elongation and helps provide resistance to root pathogens. The ability for a plant to uptake Manganese is PURELY pH dependent. If you allow your pH to drift over 6.5, you have little to no uptake of Manganese by the plant. Deficiency appears as tan sunken spots in chlorotic areas between veins. For best uptake, keep pH at 5.5 or below.