Wood mulch has a lot of carbon but has almost zero nitrogen.
In order to break down,the microorganisms will use all the nitrogen they can get and your plant will be starved of nitrogen.
After it draws nitrogen and becomes verry well composted,it will release all that nitrogen back.
3 months is too little ,depending on climate,what type of wood chips nd the amount of nitrogen in the soil.
If you want them to compost faster,simply add nitrogen over the wood chips.
Ureea or a powerfull source of N,not manure as that doesnt has enough nitrogen to compost itself and sometimes it takes years to decompose a dryed cow dung.
Thanks mate that is very informative, trying to get my head around it all as im only new and dont fully understand how this all works. What would you reccomend i fill the hole with for someone who is new and needs a more simple solution? Im not sure i would be confident in adding the nitrogen on top of the mulch or being able to tell whether i need to add more/less etc. The mulch is not wood chips its more palm fronds etc that are shredded not sure if that makes much difference?
I would use those palm leaves as a mulch ,mostly to keep the soil around the tree shaded and to conserve water.
For mulch you dont need it to compost fast .You want the mulch to compost as slow as possible to make shadow for longer time and keep the soil moist.
In a hole i would only add a bit of biochar ( charcoal with compost and soil mix).
The charcoal doesnt decompose and lasts for centuries .
The charcoal is not a fertiliser but it stays in the soil for long time and improoves aeration.
You will have to read more if they use charcoal as soil amendment for mangosteen.
From.what i know,mangosteen likes acidic soil and charcoal contains a little ash wich is alkaline ( has ph of 9).
If you have a verry acidic soil allready ,you could add a little charcoal.If not then you probably shouldnt use it.
Add regular fertiliser with nitrogen on top.You will see the mulch gets composted faster where you add nitrogen.