I'm not a fan of totally replacing native soils in raised beds. Often the "soil" you can get from a supplier is mostly compost with maybe some sand. Over time this combo will settle. The compost decomposes and then you mostly have a sand layer over the original clay, which can cause water and drainage issues. Anytime you have a definite layer unmixed like that you will have drainage issues eventually. So anything you add needs to be mixed well with the bottom layer. No layer cake please, ;)

Better than that is to take that compost/sand mix and incorporate it into your native clay soil to make the raised bed. The only problem with clay is the drainage issue, other than that clay is very rich in minerals, it takes fertilizer well, and it will hold water for much longer than sandy soil.

By mixing the compost/sandy loam mix into the clay thoroughly, at about a 50/50 rate (50% mix to 50% clay) you will improve the drainage of the clay and have a soil that will stay better drained for much longer. I also like to add about 1/4 amount of fine bark as the bark has a longer staying power than the compost.

Also, just the act of mounding up the clay into a raised bed helps the drainage of the clay physically. I've seen successful raised beds of clay where the only thing done is the physical raising it up from the surrounding soil. Gravity does the rest. They do top off these beds with some compost as a mulch also, but if you are in a very rainy area, use mulch with caution. If a dry area weather wise, lots of mulch is great.

I had originally planted the trees in a pseudo mound, about 8 inches above the surface. Now they have sunken and are only an inch maybe above the yard grade and my mulch has made this compacted crust like mess. This is my first endeavor in dooryard citrus.. still learning.

Thanks for your helpful comment. I am thinking of buying soil sold for roses and adding some of my clay. The issue with our soil here is that it contains tons of limestone rocks, so pH can be a concern.

Best

Mick

I am thinking of mixing small gravel, compost, pine bark and native soil for the raised bed. Thoughts? Would you use pea gravel or vermiculite? I am afraid of the pea gravel causing compaction later on.

Just as background when we first moved in our yard flooded terribly. I installed a french drain to help move the water. So far that has worked great to prevent flooding. I still have no idea what affect that has on moisture in the soil. I assume it remains saturated and the drain just moves the free water away.

Best,

~Mick

I'd lean more towards pea gravel, vermiculite can hold a lot of water. Perlite is better for drainage, but I've seen when used in the ground it tends to float and end up all at the top.

Can you get small sized crushed lava rock or pumice? That works well because it has fine pore in it so you get more air in the soil but it's heavy enough to stay in place. It would be my preference even over sand.

Also I've heard Turface works well, but it can be hard to find. Medium to small sized crushed gravel of any rock may work better than pea gravel because of the more angular surface, and it's fairly easy to find at rockeries and material suppliers. It's often used as base rock for pathways.

I'm pretty sure french drains do help with the moisture issues. By draining the free water away, it reduces the overall moisture content of the soil and allows the soil to take in more air. It's the lack of air in saturated soil that really the problem more than just the water. Plants need both air and water in the soil and saturated soil has little to no air in it, so basically the plants suffocate. The drains will definitely help.