Certified GKH Gardening Expert
I recommend a liquid fertilizer for orchids. There are many that are specifically blended for Orchids, and this will remove the guesswork. Rather than feeding them once per month, you can feed them half strength every two weeks, although if these are outside, then there may not be a need to feed them at all.
This collection of articles will give you much more information: https://www.gardeningknowhow.com/ornamental/flowers/orchids