Manganese deficiencies do not occur that often and are practically always linked with iron and zinc deficiencies. A lack of manganese will show up in young leaves as they start to turn yellow and exhibit small, brownish necrotic areas in the middle of the leaf.
The veins of the leaf will generally remain green. The outline of the leaf will become dark green. A surplus of manganese will produce an iron deficiency. Manganese does not move across the marijuana plant, making it immobile.
Manganese is important for the breakdown of enzymes and also for the production of chlorophyll and photosynthesis. It also helps make nitrates available to aid in protein production.
An emergency tactic for replenishing the manganese levels is to foliar feed the marijuana plants with a fertilizer that has high water solubility and contains a lot of manganese. This fertilizer can be a special Fe-Zn-Mn fertilizer, a hydro micro, or even a manganese chelate. Add this fertilizer to the water/nutrient solution. Greensand and compost also have high levels of manganese, but absorption is slower.