From where I sit, it looks like you have a sandy soil that has had fill added over the sand. The layers look very separated and distinct.

Is the soil in your area quite sandy and are you near the coast?

Are you concerned about something being wrong with your soil below 12"? 12" is a fantastic depth for a topsoil layer; the paler colored subsoil layer below that lacks organic matter, which is why it's so pale and sickly looking. But that's perfectly natural for a subsoil.

Do not under any circumstances engage in any "double digging." That's something people used to think was a good idea decades ago, but most have since come to their senses. Soil layers sort themselves out much better when left as undisturbed as possible; you should not be tearing everything up and mixing the subsoil on top. It's fine where it is.

Are you having some specific problems with plant growth that you are trying to address? If you are preparing for a soil test, you should follow the directions about how deep to take your samples. The subsoil isn't something you need to worry about, barring some very specific issue with it like a drainage issue. Plant roots live in the topsoil layer, where the oxygen, organic matter, and soil life resides.

Topsoil is defined as the top 4 to 12 inches of soil. Subsoil is anything below that. Sometimes the topsoil will appear to be a dark brown, almost black, color and subsoil might be lighter. Soils with adequate amounts of organic matter in them will appear dark brown in color, but that does not always mean there is adequate amounts of organic matter in the soil. The pictures you posted do not appear to have any organic matter in the jars and that is a problem.

The soil I see appears to be sand, when rubbed between your fingers is there a gritty feel or a smooth greasy feel? Gritty is sand, smooth greasy is clay.

Double digging is usually to thoroughly incorporate an added material through a root zone.

What's the purpose behind your inquisitiveness? Information? Looking towards a specific project?

In general terms, a soil could be sand, silt, or clay, and any combination thereof. Imagine a triangle with each leg labelled with one of those three tags, and hash marks representing percentages. I bet you could find such a thing online.

That last jar looks like some sand, a huge amount of silt in the middle, and a little clay on top. Which is kind of a loam, actually a great soil. It should drain well but not too awful fast like pure sand would, and does not retain water like a clay.

I'd do as others suggest, leave it alone and concentrate on testing and improving the topsoil. 12" is plenty and a lot of people will envy your foot of dark topsoil (glass shards notwithstanding).

Apologies for the delay on following up on this, and thank you to everyone who responded.

I have since refilled the hole pictured above, with the differing soils at their original depths.

I hesitate to counter some of the points brought up, please realise I only mean to initiate discussion...

Regarding the concept that roots only grow in the top 12" of soil, an immediate logical fallacy is apparent - specimens planted at depth below the surface would, by the 12" theory, have significantly smaller root systems than plants grown at the surface. Asparagus are planted at 10" depth - does that mean their roots only penetrate 2" downward?

Clarifying to say the first foot of root growth of the plant, regardless of planting depth, contains the largest number of roots, would make more sense.

But we also need to address the elephant in the room - root systems aren't only one foot deep. There is significant root mass beyond that depth, albeit in smaller numbers, due to reduced branching in the increasingly compact soil. Asparagus roots typically surpass 10' in depth.

Research carried out by an ecologist and a botanist from University of Nebraska backs up these two points, and the resulting book is a very interesting read and available free online here: http://soilandhealth.org/wp-content/uploads/01aglibrary/010137veg.roots/010137ch6.html

Now I don't mean to say that TILLING to that depth is beneficial, detrimental, or of neutral value. This is a topic I hope to explore.

On another point brought up...

If it is topsoil that was added on top of the native soil, it would have been done at least 35 years ago, prior to development of this lot. I've been thinking along the same lines, even before digging the hole, based on the sheer volume of glass and rusty nails coming to the surface, which seemed indicative of a contractor cutting costs and using cheap fill.

Some research of the county deed database revealed this lot used to be occupied by several large commercial greenhouses until about '84, a logical source of the glass, which to date has been rather uniform in composition, vs variation one might expect to see in "cheap fill"

Either way, a 'big picture' question comes to mind: If the current topsoil is of overall decent quality, does that mean soil will maintain it's state and not degrade over time?

If an area was good for growing crops 300 years ago, will it likely still be so today? (Assuming undisturbed and naturally overgrown with vegetation)

(EDITED, accidental submission before finishing!)

It should remain good quality soil, with a little maintenance if you are removing nutrients (i.e. growing vegetables). It's a good idea to add a little compost every year, and/or use degradable mulches like wood chips, shredded yard waste (both of these often free), leaves, grass clippings etc.

Have you sent a sample off for analysis? It might be good to do that just to know where you stand on all the nutrients, minerals, organic matter and pH.

I hate to bring it up but due to the age of prior uses and the glass (which suggests demolition), if I was growing vegetables in this or I had kids, I would spend a couple extra bucks on lead testing.