Fast drainage and sufficient soil depth are key. I'd likely add some additional grit to the soil mix and make sure you have a good 12" of loose soil in the raised area. Also, make sure the soil pH is right around 6.0 to 6.5.

Thanks for your reply gardengal.

What kind of grit would be good? It seems like there are more fine grit materials: sand, decomposed granite. there's perlite. and then there's coarser materials like pumice and lava rock.

I was watching some a lecture on avocado growing by Laguna Hills Nursery, and the owner there thinks pumice is the best thing to use . But I haven't found a place to get pumice in bulk here and the bags are extremely expensive.

Also what proportion of gritty material would be good to add? Do you think it would be better for the avocados if I replace the 15% microbark with 15% gritty material? Could the bark in the mix decaying and turning anaerobic over time threaten the health of the avocado roots?

Can't find a local source for the poultry grit. Would need to get it mail order, which would really raise costs. But I discovered it's made from granite, and I can get decomposed granite here, both in bulk and in bags from HD. Would that be a good substitute?

Also, I'm wondering would modifying my raised bed mix in that way - 50% screened topsoil; 20% decomposed granite; 15% micro bark; 15% peat - be good for fruit trees generally, since it would increase drainage? Or do you only recommend it for avocados?

When I plant my avocados I'll also be planting other things - citrus, apple, cherry, and some subtropical fruit trees - so I could just use that as my general mix for everything if you think it would be generally beneficial for fruit trees.

Really?? There are no feed stores in your area and no one raises poultry?? Even Chewy.com carries it (under $10 for a 7# bag) and orders over $35 ship for free.

DC is NOT the same as poultry grit and I would not recommend substituting DC for grit. Unless washed and screened, DC contains about 50% fines (i.e. dust) - that's what provides its stability when compressed. And makes it incompatible in a soil mix.

I'm not in an agricultural area. There may be some people raising chickens here, but there are strict regulations about raising them and the number one can have is limited, so probably not as common as other places, which may explain the lack of feed stores. I'll call around on Monday and see if anyone can point me to a place.

I could consider ordering from chewy but I'm probably going to need quite a lot of it, so I'd prefer something I could buy in bulk.

Is there anything else you'd consider substituting for it? How about pumice? Pumice I could probably get in bulk, but I anticipate it might be pricey. I could see what price a supplier quotes me and compare if with ordering the cherry stone from chewy.

Probably a stupid question, but is there a reason these trees can't be planted in the ground? Perhaps your yard is paved over or something and you don't have a patch of earth? It would make life a lot easier.

floral, I tried planting in the ground in the past and everything either died or simply never grew. I face a few challenges: dense clay soil, and aggressive gopher and vole populations that devour the roots of anything I plant. I grow in the beds so I can plant raised above the ground, and so that I can use hardware cloth to protect the roots as the tree establishes itself. I'm actually hiring a landscaper to come in and make a very long raised bed just so I can plant fruit trees in a row. Two of those trees will be avocados. So that's why I'm looking into the best mix for the avos.