You might be looking at soil that contains a large amount of the by-products of composting; acetic, oxalic, butyric, humic and fulvic acids. That implies that the compost was unfinished. Try testing it to see if it's finished by attempting to sprout something like carrot seeds. Or, just wait to see if the pH starts to rise on its own.

Don't amend with lime until it has stabilized. You didn't say were you lived or what the base soil's natural pH is. If you are in the Northeast, you might have naturally acid soil. But if you live where the clay is naturally alkaline, you would be best to just let it sit the way it is. Some of the humic/fulvic acids will leach into the clay and make it fertile.

I'm not sure what you mean by "electronic tester." However, if you use this instrument to test 5% white vinegar from the grocery store, it should indicate 3.5, approximately. Did you have part of the probe in the clay, and part of the probe in the top layer?

Your soil is probably OK

pH of 0 would be purified acid so your meter is malfunctioning.

A lab test would be great but you could try one of the cheap test tube kits at your garden center in the mean time.

Hey thanks all for replying! I use this electronic soil tester from Ferry Morse:

http://www.backyardstyle.com/shop/index.php?page=shop-flypage-28312

It's probably not a very accurate instrument, lol. I think I'll wait a bit and let the soil stabilized as suggested by idaho_gardener, and see how it goes. Meantime I'm gonna get one of those test tube kits. Thanks again!

How about asking the person who provided the soil to test it for you? If they sold you unfinished compost wi/o telling you so, that is not good customer service ... a 'mistake', no doubt. Give those sweet guys a chance to correct their mistake.

And you know what they say about removing clay and filling in with lighter material ... water will not percolate beyond the lighter material, and you may end up drowning you plants.

borderbarb, I did ask the contractor but he didn't seem very knowledgeable about the soil (tho he did a great job with the hardscaping and the rest). I think I'm gonna find out more at the nursery. And yes! I was afraid of the water not able to permeate the clay layer. I think I''ll still have to dig down deeper for individual plants. Thanks for the reminder!

I generally brag about my clay soil that has morphed into lovely loam, due to many years of amending and double-digging, etc. That is VERY labor intensive. [I was working thru divorce anger...very effective therapy...didn't kill anyone]

But I'm sure I read somewhere on these forums about a man who used power post hole digger to dig very deep holes to plant his tomatoes. The idea of making short work of a lot of hand tool labor is very attractive. I can imagine that digging and amending those holes, adding earthworms, growing plants with clay-busting roots ... after a few years, you too can brag about your beautiful clay-based loam.

There is a "stand & twist" hand tool that works really well for mixing compost into clay soil. Its designed to avoid bending over, for people with back problems, and it does that, but it is also faster than a shovel for mixing compost with packed clay. This tool will do the top 6 or 8 inches of soil, if you want to go much deeper, then you might have to break out the shovel.

You're entitled to bragging right if you morphed those clay soil into loam! I had occasion to amend a big patch of clay soil also at my last house, but didn't get to stay long enough to enjoy the next season because I'd to relocate. It sure was labor intensive as you noted; and playing with dirt does have its therapeutic effect - I enjoyed every minute of it more than I enjoy pushing paper in an office that's for sure =)

ericwi, thanks for the suggestion. I saw the contractor crew used something similar to dig holes for planting fence posts. Look very effective!

I strongly think your meter is malfunctioning. Only strong, pure mineral acids (sulfuric acid, nitric acid, etc) have a pH of 0. You should get a soil test.

Anything with a pH of 0 would be so corrosive that it would eat through anything it contacted so that pH meter you have is not reliable, few that cost less than several hundred dollars are.
What any clay soil that is compacted needs is organic matter and what you have created in your backyard, at least for a time, is a bathtub. Water will not be able to drain out very easily and may well flow into that area since water can move through that new soil much more easily then your native clay.

Thanks all for your suggestions! It has to be a malfuntioned meter. I took a sample over to the nursery and they did a quick litmus test and showed up green; so that solved the mystery of the episode. Actually I don't mind the soil being more acidic since I'm planting mostly acid heads like bamboo, camellias, etc. Although 0 pH is a bit much! lol.

The layer beneath isn't as bad as the hard compacted clay above; my intention was to mix in organic materials gradually and let them decompose overtime. I'm also thinking of bringing in some worms to speed it up. I did that with the last patch of bed I worked on tho I didn't stay long enough to notice any result! It's going to take time for sure; meantime it gives me an excuse to play with worms! lol. I might also plant some "cover crops" to work the soil. I'm beginning to suspect I enjoy working the soil more than I enjoy planting lol.

"But when I ran a soil ph test the needle swung all the way down past 0!"

That is chemically impossible.

Pour out a bit of milk and test that ... it should be very close to 7.3