The current thinking is that unamended soil is best for tree planting Read some of the articles in the link about tree root balls, compost, etc.

The best technique is to mulch with compost, not mix it into the soil.

Does the dump give away "mulch" or "compost"? "Compost" means the stuff has been broken down to some degree; mulch might be simply ground or shredded material that has yet to compost. A good compost process will have killed all weed seeds.

Here is a link that might be useful: horticultural myths

Unless you are a city resident you can't get free compost. I have not used Zanker, but have gotten good stuff from U-Save and also from Guadalupe Mines Landfill, which is only open during the week...

I agree with hoovb, clay soil is excellent for trees as long as it is well drained. Most clay soil will drain very well. It is only when it is under laid with a hardpan that drainage is a problem. Al

Yes actually Zanker is a private dump so I have to buy their compost. It's about $18/yard which is cheaper than if I go to summerwinds or other nurseries.

I am just going to top my clay soil with compost. (I'm thinking of topping the compost with dyed hard wood mulch also, but not sure if that's an overkill). So I am not really going to "amend" by mixing the soil with compost. But just topping it. Hope that clears any confusion.

@chandinlg
How was your experience with the compost from Guadalupe Mines Landfill? Did it have a lot of weed?

Mulch has more than one goal. It will be effective as a weed control if thick enough until the microbes have broken it down, also to limit the loss of moisture. When purchasing mulch most of us are looking for material that will be effective over the long haul. Hardwood chips will work well for this. Finished compost if thick enough will also work well for the first year. Both mulch and compost will feed soil micro organisms as well as help support a good worm population. Fears of lowering the soil nitrogen by the degrading wood chips is not really an issue in most applications. What ever you decide will be a positive if enough is used. Al

I live on a street where the average homeowner lives in their home for 25 years so I know most of my neighbors. When one is having their big trees trimmed they hire a company that brings a 10 yard dump truck with a big chipper behind. I just get the tree company to dump the chipped load in my driveway and use it for mulch.

I then let other neighbors come by and get what they want. Many people want one yard or a pickup truck load but most people don't need 10 yards all at once. I've had 8-10 yards dumped 3 times this summer and used about half each time.

The tree company wins because they don't have to dump before going to the next customer. We win getting lots of mulch for free. You do have to use extra because the leaf parts will compost quickly. You do have to make sure they aren't chipping palm trees.

- pearlygate, I got only a few 'weed' sprouts from the compost from either site. Many gardeners in my area have used the Guadalupe Mines products. My only beef with them was being closed on weekends making self-pickup an issue... It's the delivery cost which gets you on all these sources...
I have a lot of fruit trees and they grow similarly in the amended vs. un-amended soil areas. Amending does wonders for vegetables etc. After the initial soil treatment, re apply a mulch every year to keep up the organic matter (free tree chippings).
Chad

I have a somewhat different take than some on this post Some of us with really heavy clay simply need to amend our soil. I also live in the bay area and my soil is way to heavy for most plants to survive. I have tried to plant citrus and other plants in the native soil and they have done extremely poorly.

What you don't want to do is to dig a hole in the clay and add top soil or something else that drains readily. Water will just collect in this hole at the interface between the two soils. This will reduce aeration to the roots and they will die from lack of oxygen.

When amending my soil I try to dig it in so that there is a gradient. At the surface there is 100% amended soil. 1 foot down or so there is 50% clay/50% amended soil. 2 feet down it is 80% clay/20% amended soil. This way there is not a sharp interface between the two. With this technique I have had very good luck with most everything I have planted.

Good luck.

-Brett