Manganese (Mn) plays a vital role in photosynthesis, forming the core of a key enzyme responsible for breaking down water at the chloroplasts. A byproduct of this process is the oxygen we breathe. While manganese is an essential micronutrient enzyme activator, high concentrations are quite toxic.
Other enzymes that are built with manganese form lignin, an important building block enabling a healthy cannabis plant to bulk up a thick, tough, almost woody stem which provides an effective physical defense barrier against pathogens. Since a manganese deficiency can result in an increased overall susceptibility to fungal disease it stands to reason that maintaining adequate amounts of this micro-nutrient helps reduce the need to apply fungicides and enhances your chances of harvesting residue-free finished product. Manganese also contributes to the enzymes which produce waxy substances guarding against heat and drought stress which means less irrigation, saving you some water.
Because manganese is not very mobile in cannabis plant phloem, an extreme lack of the micronutrient will be expressed by newer leaves as an interveinal chlorosis symptom — easily mistaken for iron deficiency. Later on, the chlorotic areas will go tan, whereas in an iron deficiency they turn nearly white. Foliar feeding can provide a quick fix to this fairly uncommon malady; but for a long-term solution, consider adding a small amount of a product called “manganese greensand” to your cannabis growing medium. I’ve also seen this material on Alibaba selling by the metric ton for not much more than what you’d pay for a few fifty pound sacks at retail. Greensand itself contains the mineral glauconite, which is iron potassium silicate; manganese greensand is greensand coated with manganese oxide and is used in the water softening industry as a filter medium.
Symptoms of manganese deficiency often appear because of nutrient lock-up. Most decent soils, pre-mixes and fertilizers have some soluble manganese, such as manganese chelate or manganese sulfate, but at a pH above 6.5 plants can’t absorb it. Considering the pH sweet spot for growing cannabis is 5.8 to 6.5 you might easily get out of bounds without careful monitoring. At a pH below 5.5 cannabis plants can take up too much manganese and older leaves might show signs of toxicity such as brown tips or edges. Too much uptake of manganese can also result in symptoms of iron deficiency as the roots get “fooled”.
Any advice and opinions about the cultivation of cannabis offered by Bruce N. Goren are his own and do not represent the University of California or the Master Gardener Program.