How much soil are we talking about?

We recently dug a well in our back garden; we had one already but thought it was best to have a back up. There is no public water supply where we live. It was interesting to see the soil as it came out. The first 2-3 meters was nitrogen rich and dark brown in color, then as they dug deeper the soil became more mineral until at 15 meters it was bright pink (mainly iron and manganese, no nitrogen). It is unusual to find nitrogen in any quantity at depths of more than 5 meters. I don't know what you plan to do with the excavated soil. It is virtually useless in the raw state, even if the structure is good. We filled an old water container (about 100 liters/ 250 gallons) with high nitrogen leaf compost and mixed it in with that. After about 9 months it was looking reasonable, although before I use it I'm planning to add some potash. - Ian.

Yes I bet what you have is subsoil and that's why nothing's growing in it. Amend with compost. If it's not a huge amount of soil you might try adding some sand too, often subsoil has much less sand in it that the topsoil above it because the clay particles and mineral ions wash down and collect in the subsoil.

From the Encyclopeadia Brittanica,
"subsoil, Layer (stratum) of earth immediately below the surface soil, consisting predominantly of minerals and leached materials such as iron and aluminum compounds. Humus remains and clay accumulate in subsoil, but the teeming macroscopic and microscopic organisms that make the topsoil rich with organic matter spend little time in the subsoil layer. Below the subsoil is a layer of partially disintegrated rock, and underlying bedrock. Stripping topsoil while clearing land for crop growth or commercial development exposes the subsoil and increases the rate of erosion of soil minerals."
Subsoil is anaerobic so it has no life in it, although it may contain some organic matter. Most often your subsoil will be composed of the same mineral particles as your topsoil. You can make subsoil into topsoil, best, by adding organic matter to it. Then you can use it anywhere.

Thank you all for responding. It's not a lot of soil - maybe 5'x6'x4' deep. It came out of the ground looking pretty much like clay. It was under cement, so it hasn't seen any plant life in many years.

So in about a month, I'll mix in some compost and sand, as mentioned by reg_pnw7 - it's the cheapest and easiest way of fixing the soil.

My other question is: I planted some tulip bulbs in the soil - should I dig them up first, then add compost and sand and replace the bulbs?

It'll make it easier to mix in the amendments if you do. Otherwise the bulbs will get trashed. But they've probably started root growth already so you may have to pot them up until you can replace them.

If it was under cement then that contributes to the lack of normal soil microbial activity and organic matter. No air or water getting down there. Compost will help that.

The bulbs that you planted have everything they need to blossom next spring, except adequate moisture. What they need good soil for is to develop the flowers that will grow next year. Putting some compost on the soil will allow the Soil Food Web to move the nutrients in that compost where it can do the most good, the root zone of the bulbs about when they need it.

"Subsoil is anaerobic"

Simply nonsense.

Soils near here and over vast areas of the nation and world are aerobic far down into the subsoil. Several hundred feet is not too unusual. Several thousand feet in some exceptional places.

Physically or chemically, though, even shallow subsoils may differ starkly in terms of plant growth compared to uppermost layers. I have commonly seen fields where the spoil from six-foot deep ditches was spread roughly 10-15 feet into the adjacent fields and supported visibly stunted corn. I have seen it too many times for it to have been a fluke (say a missed fertilizer application in outermost rows, and the soil was clayey so overdrainage near the ditch was unlikely). With some subsoils, growth might be improved though. I have seen horizontal lines of tree seedlings growing in the exposed Bh horizon of spodozols (~2 feet down) in the moderate upper slopes of a large excavated pond.

I have also found that a good idea with bare clay is to sow winter rye in it in fall. Turn under in spring.