Avocados above all need good drainage but at the same time need plentiful water. Loam on the sandy side is best if you are not on a hill. California commercial groves are typically found on slopes to ensure quick drainage. Avocados can put out quite a root system so eventually those roots will be reaching into whatever native soil you already have.

https://www.davisenterprise.com/features/gardening/don-shor-growing-your-own-avocados/

https://www.crfg.org/pubs/ff/avocado.html

Thanks for comment. I will be planting on ground that has a moderate slope. When you say loam on the sandy side, is 40% enough?

have a soil test done.. and FIND OUT if you need to amend the soil for nutrients ... and after you have actual knowledge.. act accordingly ... IF there is any need at all ... in 99

% of situations.. we recommend avoiding adding much of anything to the planting hole ... unless the soil test indicates such.. and even then.. its usually NOT fertilizer [nitro] .... that can come later .. our usual goal at planting is getting over planting stresses ... when the tree is not hungry.. but stressed ...

most profession field growers ... prefer clay soil ... it is not only usually nutrient rich ... but also .. when dealing with trees ... allows them to dig rootballs with soil attached.. so they can sell larger plants ball and burlap ....

you are stuck on percentages .... of which... i dont see how you define or apply such ... practically speaking.. whats the effective difference between 30 or 40% sand??? .. and who is going to sell you such a highly defined soil ... except perhaps a snake oil salesman ....???

to help you understand ..... digging a hole in bad non draining clay.. and filling it with draining soil ... really gets you nowhere ... as the hole.. still doesnt drain ... so the usual idea.. is to plant high ... as you have noted ... and allow the tree .. to put its roots into the bad soil ... trees have evolved to do such ..... and are much better at it.. than we ever will be ...

also understand.. that the root mass of a tree can end up.. two or 3 times the size of the tree you see above ground ... in a mostly pancake shaped form ... so it will quickly outgrow any raised bed you provide ... this happens in tree time ... so again ... what you are providing ... is a temporary setting.. so the tree can settle down.. and grow out of the stresses associated with planting ...

you dont mention the size you are starting with ... smaller transplants get settled.. and start growing to norm faster than huge transplants.. because.. well done.. it can get over the stresses ... and get to work ...

the answer you want .. has already been provided ... you need drainage ... at planting ... so the tree can get past the first year or two .. and then take care of itself ... also.. drainage can be manipulated.. by how you water ... e.g. if you are going to turn it into a bog by improper watering ... the type of soil you use.. isnt going to make much of a difference ...

welcome to the forums.. see link below for general primer on planting trees ... a tree is a tree is a tree .. dont get to wrapped up in thinking each type has wildly different planting rules .. there may be some minor tweaks ... but all in all its all the same ... e.g. this MI boy has never grown an A outside of rooting a seed in a glass of water .. but ... imo ....my general trees guide.. stands ...

the base bottom line.. plant it.. water it like a tree [sec 12 at link] ... and try not to love it to death.. and you ought to be well on your way .... gardening is more of an art.. than a science.. so dont get too wrapped up in numbers .... deal with the concepts ...

welcome to the forums ....

ken

this guide used to have a section on planting in clay.. i dont know why brandon changed it at some point ....

https://sites.google.com/site/tnarboretum/Home/planting-a-tree-or-shrub

I add 1/4" lava pebbles (no fines) to soil for raised beds. Sometimes I add a lot (50-50 mix). As GG notes, for avocados you might then need to fertilize periodically. Too cool here to get them to fruit reliably so I've never tried them.

Unless you want to make beds of imported soil something like 35 ft. across what would accommodate plants the size of vegetable crops will not be big enough to house the long term development of avocado trees. Probably better instead to find out what can be done with your existing soil to get these latter to grow for you. As would usually be done with a commercial planting.

If you are located in a state such as California where these trees are a commercial crop your County Extension Service web site should have plenty of information about involved particulars. Or links to other parts of the Extension network that do.

Based on comment from gardengal48, I have found a quality garden mix from a nursery that I am told drains well. The blend is a quality topsoil, aged forest humus, premium garden humus, work castings, aged mushroom compost, rice hulls, poultry litter and all natural pumice stone. At $65 per yard, this stuff better be good! Thoughts anyone?