This is a rather interesting and fairly complicated situation, but simple in some ways. I have heard many things about subsoil in gardens and how to deal with it. Before I wrote this I did some research and most of the articles that came up where from the 1800-1920 time frame, not saying they are wrong but many agricultural practices have destroyed farm land around the globe. I didn’t use any information I found on how to treat subsoil but here are some things to think about and you can make your own decision.
Here are some basic ideas (facts).
(Image taken from https://www.britannica.com/science/ecological-succession)
In the beginning of the Earth it was bare rock, cooled magma and lifeless, there was no topsoil. The beginning is not the only time this bare rock landscape exists, it can come from modern lava flows or from glaciers as they moved along the Earth’s surface and scraped everything organic off the top of the bedrock.
In the above image it shows pioneer species like lichens growing on the rock, along with weathering, the roots of the lichens get in the cracks of the rocks and between freezing and root growth the rocks crack in to smaller pieces, eventually the lichens die leaving behind organic matter and smaller rock particles. This process continues for thousands (or millions) of years building top soil. As the quantity of top soil grows other larger plants begin to grow in that area doing the same thing, deciduous trees loose leaves every year adding to the topsoil layer also.
The above picture shows the bedrock going lower while the top layer stays at the same height therfore the amount of top soil gets deeper.
Keep in mind I am shortening the description of what is happening here.
It doesn’t take a volcano or a glacier to remove topsoil, we have bulldozers and erosion from poor management practices that do just fine in destroying the natural process of soil formation.
If you are forming a garden in an area that used to be a lawn much of the top soil may have been hauled away. In my 10 years of landscaping all new subdivisions had the top soil removed all the way down to the subsoil, we would have to come back and add a few inches to put sod down. Sounds crazy but the building contractors were paid to take the soil off and dispose of it but in reality they sold it (to us) who would resell the topsoil to the home owner who had just paid for the same soil to be taken away! Many of the subdivisions went from having 2-3 feet of topsoil to having 2-3 inches of topsoil and they paid for it twice.
If your are more rural that scenario may not have happened but if it was previously a field, it is common for a field to loose 5 tons of organic matter each acre every year through erosion alone.
My situation involves forest that was clear cut before I bought it and then I had a bulldozer come through and clear the remaining trees off to make fields. As much as I hate clear cutting it does help create soil if the stumps are left, the trees die, their roots are deep in the subsoil, they rot leaving behind organic material and a path for future roots to penetrate the subsoil and continue the process of creating topsoil.
The problem with bringing in a bulldozer to clear is that it rips up the stumps and roots and generally they are pushed into a pile and burned to send all the carbon up into the atmosphere where it isn’t supposed to be.
So back to the garden, if your subsoil is fairly close to the surface your plants will have a hard time growing, there won’t be as much moisture retention, the roots won’t have as much area to grow, and they will have a harder time accessing some of the minerals.
Permaculture principles would involve planting plants that would break up the subsoil for you, when it comes to food forests it helps to plant support species that grow deep roots and mine minerals, some trees like black locust also add nitrogen to the soil. At some point the support trees are cut, die, rot, and increase organic material in the soil. The problem with this and trying to plant a more regular garden is that then you have roots to worry about and get in the way if you are trying to till. Planting support trees is still the best way to break up the subsoil if possible.
Pay attention to what is growing if you have time, I try very hard to be patient with weeds, more than likely they are helping somehow. Dandelions are very good at breaking up compacted soil, so are polk weed plants. Think back to the above picture and think about what you see growing in poor soil conditions and what grows in good soil, rarely does one plant grow in both situations, you never see lichens growing in good soil and and you never see an oak tree growing in a lot with no topsoil unless the soil was removed from around the tree. If I have the time I let the plants go crazy, sometimes I will cut the heads off of dandelions so they don’t reseed but almost never kill the plant, their root goes much deeper than many of the plants I plant.
If we don’t have time to let nature take it’s course, you can insert a pitchfork or something similar into the subsoil and wiggle it a bit, not enough to break your tool but enough to make holes that moisture and top soil can get into. Do this every foot or so in your beds and it will help accelerate the transformation from subsoil to top soil.
After the loosening of the subsoil is complete your bed will look a bit disheveled but that is okay, you will probably need to add soil amendments as well.