Compacted soil is that which has been compressed or weighted for some reason - excessive foot traffic, repeated use of equipment, vehicles driving over it. Even consistent and heavy rainfall can result in compaction. Compacted soil has very few pockets, aka pore space, that allows for the natural penetration of oxygen and water, essential to good plant health. Compacted soils also frequently result in poor drainage issues. Compaction also makes it difficult for roots to spread and develop, resulting in stunting and toppling in winds and weather.

Clay-based or heavy soils are generally more prone to compaction by virtue of their structure but any soil can become compacted if care is not provided to avoid. Soils underneath lawns are nearly always compacted due to foot traffic, kids' play and the use of lawn mowers, lawn tractors, fertilizer spreaders, etc. This is why annual core aeration is recommended to counteract this effect.

If it's hard to dig in, assume they won't like it. Undisturbed soils on natural sites often quite soft and loose. Roots grow in the air between the soil particles, quite a large proportion of plant species can't take soils that aren't well aerated. Common garden examples are heath family (Ericaceae) plants such as rhododendrons, azaleas, and heathers. When planted in heavy soils failures of these are likely, although toughest kinds such as hardy hybrid rhododendrons may persist for years in unsuitable soils, sometimes when dug up later turning out to have hardly rooted out of the original soil root ball at all.

So if I purchased a tree with this characteristic, would the soil it's planted in need to be kept like a garden bed (no walking on it)? And if so, how far out would it need to be loosened/aerated? The dripline?

Thanks!

If you have compacted soil, one tree that will still do well despite it is Alnus cordata (Italian Alder).

Otherwise, it is possible (tho' expensive) to get soil de-compacted; do a google search for "Terravent" plus your city or state name, to find local operators.

Resin

Or dump loose dirt on top and plant in that.

Still looking for a practical definition. gardengal gave us a working definition, but still looking for a definition that means something to someone with nothing more than a shovel.

And sorry, hard to dig with a shovel does not work. There are numerous other factors, rockiness of the soil, moisture content, roots and other materials, and just what kind of soil you have (there are approximately 43,000 kinds of soil found in the U.S.).

True, most plants do not like compacted soil. It's kinda like wearing shoes a couple or a few sizes too small. It can be done, but you're not gonna be happy about it.

Of course, if we only planted if we had truly optimal conditions, we'd hardly plant anything.

There are meters that can be obtained to help you measure the level of compaction. There are methods of subsoiling and other means of abrogation that can be considered. However, in practical terms, none of these are probably necessary.

Ironically, one of the best combatants of compaction have recently obtained great notoriety as being "invaders".

We can assume we no longer live in a pristine world, that everything has some level of disturbance, including compaction. We can also assume plants have a much greater ability to "deal" with less than ideal conditions that we assume. How many times have you seen a tree or other plant clinging to a cliff, or growing out of a crack in concrete, asphalt, or creasote soaked railroad tie?

Odds are, you are ok to plant just about anything you want. Other properties of your soil (porosity, fertility) will have greater impact upon the plants you choose than compaction level.

Just because I'm curious, wouldn't dumping a pile of soil on the ground and planting in that just be an inverted form of amending?

bboy, that might create a perched water table though.