If you've fertilized those plants with any sort of commercially formulated fertilizer, whether organic or straight chemical, it will almost surely have the manganese component - listed under trace elements, or way down on the list of nutrients. My orchid fert bag lists manganese at .05%, same on the bags of fruit tree fert and acid-lover's stuff I bought for my (nearly defunct) blueberries and my azaleas.
Stands to reason that the manganese would also be present in compost made from your plants that had been fertilized, since it is an immobile nutrient. Or in any compost made from grass and weeds in a fertilized garden.
Surprisingly, although there is manganese in the MSU fert we discussed elsewhere, I notice it seems to be lacking magnesium, which is a pretty important component for orchids. My orchid fert bag lists magnesium at 1% and I augment that with some extra in the sprayer during the cold months - about a tablespoon per 2gal.
