Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Use your garden soil if it has not been exposed to pesticides, including herbicides, insecticides, and fungicides.
The soil from deep down might be OK, but here is some background that might help you decide.
Some chemicals do not cling to the soil very well, and with every rain they get washed lower and lower into the soil.
If there is bedrock under the soil these chemicals might stop there. Sometimes these chemicals include toxic levels of heavy minerals.
Many garden chemicals used to not break down in the soil and similarly, might end up on top of bedrock.
So, if digging you piers you hit bedrock, I think I would take the soil that was higher up, perhaps about the middle depths of the piers.
Here are a couple of things you could try:
Make a slurry of soil and water. Test the water for all the aquarium tests you have. If anything seems bad, then you might be able to lock it up when you mineralize (especially ammonia, nitrite or nitrate), or maybe you do not want to use that soil.
Put some of that soil in a pot or box and grow some radish seeds. Radish plants seem to show soil chemical problems easier than many other plants. If the radish plants grow well, then the soil is probably safe. Especially if you mineralize it.