Manganese deficiencies are often caused by a pH that is too high, or if the plant is getting too much iron.
I think I got MG def in my little GT girls, they are rather healthy, but stunted and wont grow past a foot tall, but making some MEAN buds tho for such little plants lol.
I blame the high iron in the soil
Just realized the ENTIRE reason I have MG deficiency at my one spot
I used the same spot 3 years ago, and I used a lot of blood meal in my soil mix. I think the excess blood meal poisoned the soil with excess iron (high iron already in native soil). Because now that I think about it, before hand, that spot was lush with very green johnsongrass. And 3 year later, it has very light green, almost yellow, sparse unhealthy looking johnson grass, i dug the holes anyway.
The soil is poisoned damnit! Anyone know how to rid the soil of excess iron? Lol probably impossible.