Hello all, greetings from Hawaii. I don't have my profile set up yet but I'm at about 800ft and 180inches rain.
I have about 6-12 inches of soil so I planned on making flat circular mounds and surrounding it in vetover grass.
My question is: should I use say 20% compost 80% fill dirt, or just amend the planting hole? If I use more than 20% will the compost break down into nothing and make the mound smaller?
Are you planning on using mandala style planting beds or just building mounds of soil?
Since your soil there is volcanic the mineral profile is super but as you have noted, you are going to need to add both organic matter and microorganisms.
Su ba lives in the Islands and I hope they come and give you advice since it will be local specific.
I would shoot for around 25% to 35% added organic matter (compost and mulch on top) if you have good compost it will add the microbiome for you.
The trick there is probably more about amending and then giving things some time and water so the organisms can become well established prior to planting. (I think two weeks would be sufficient)
If you aren't familiar with the mandala, look up these beds, they work very well and ultimately give lots of planting space with easy access for the gardener.
I was planning on using mandala beds for my garden area. These will be 5' diamerter 2' tall mounds that I was going to use to plant out trees that need more drainage and soil than I currently have on site.
For trees I would make the hole about 3 times the diameter of the root ball and mix in around 25% organic matter (do a mix of leaf litter and compost, that will save on compost) then plant as usual with the mix.
For even better end results you could make the planting hole about 4 times the root ball, that gives the roots plenty of good soil to establish in before they get to the natural soil.
Mycorrhizae will be needed for all trees (any of the local trees should have some of these necessary fungi strands so try and lift about a shovel full per tree you are going to plant).