Trapper, assuming the nursery's topsoil is decent, I'd be inclined to make the mounds out of straight topsoil then, after planting, top dress with 2-3 inches of compost annually. I used a mix and now regret it as this imposes additional irrigation requirements. In any event, you'll need to watch your moisture levels carefully because of the differential between the mounds and the underlying soil. Welcome aboard. Misterbaby.

Thank you, Misterbaby. And thanks for the welcome, too.

The nursery also offers:

Sandy Loam Topsoil:
Good base for lawns or gardens. Needs amending with organic material for gardening.

So (assuming the nursery's topsoil is decent), you'd suggest using that. Should I actually mix some compost into it when I'm getting set up (since the description suggests a need for amending), or just top dress it annually?

If you do any mixing, Trapper, mix the nursery topsoil with the existing clay underneath. When you fertilize after planting, the nutrients will leach down to the roots. Be sure to have a soil test run to see if you need to adjust the pH or add nutrients this fall, especially phosphates as these are difficult to get absorbed later on. Misterbaby.

I would recommend a soil test as well. If it looks good just using the native soil should work fine (the Santa Clara Master gardener program here in San Jose changed from recommending mix w/native soil to just native because they found planting the trees in their test garden that the native soil only trees years later were stronger/healthier). We have heavy clay here in San Jose and a lot of people just plant into the native dirt. A raised bed/mound is a good idea.

Melike, I agree that the native soil should work fine but have concerns about standing water where the mounds came from. Misterbaby.

Thank you Misterbaby and Melike. I appreciate your advice.

I did have the native soil tested last year, and the results were a little discouraging. Nitrogen was 0 ppm. I'm kind of new at all this, but I didn't know 0 ppm was even possible.

I'm not sure if 0ppm is possible either and I've been working with plants full time my whole life- it doesn't make sense on the face of it and I wonder who did the test. On the other hand, N is poorly measured in a typical soil test because an acid wash doesn't record much of the native N being released through biological processes.

Are there other kinds of trees growing in the soil you have- either there or nearby? If plants that don't only thrive in swampy areas are making it in your soil, so can fruit trees, if they are on the appropriate root stocks. On raised beds even with native soil only, root stocks shouldn't even be a concern.

Use plenty of arborist wood chips for mulch- 3-4" thick pulled away from the trunks and the clay will not harden into cement when it dries.

I have planted orchards in hundreds and hundreds of sites and have only encountered soil impossible for tree growth a couple of times. However, in fair to poor soil, ample compost (say a quarter yard) well blended with the top few inches of native soil in a radius of at least 5' can help fruit trees establish more quickly and won't have the negative affect of planting in a completely different soil on top of another. Planting on berns seems to have positive affects on most soils.

Generally it is easier to establish bare root trees in heavy soils so roots can be spread nearer to the surface.

Thank you, Harvestman. That's encouraging.