Not sure scraping off the top soil will remediate the issue:
- in a nuclear war, most of the nuclear power plants are likely to meltdown or release large amounts of radiation from spent fuel pools.
- Wind and rain will continue to diffuse contaminated dust for years. Even if you use non-contaminated soil it becomes contaminated over time just from airborne dust and rainfall
- In addition to radioactive contamination, destroyed cities are likely to burn for many months, if not years. After 9/11 the Trade Center buildings continue to smoke for more than a month even with firecrew pouring water into the wreckage.
- During heavy rains water is likely to wash contaminents from nearby land that wasn't prepared. Perhaps if you scrape land on top of hill since water will flow away.
My thoughts on how to grow food safely:
Use greenhouses to isolate your food crops from the contaminated environment.
You need to use deep ground water to have a process to decontaminate rainwater. The only process I can find is to use an ozone water treatment to convert soluble contaminates into non-soluble contaminates so they can be effectively filtered out. Only a very high pressure reverse osmosis system can remove soluble contaminates.
Protect fruit/nut tree prior to the nuclear war with a non water permeable barrier and with a irrigation system below to provide water.
pile up top soil prior to the nuclear war and protect using a non water permeable barrier. This way you have a source of non-contaminated top-soil. Use raised beds with the non-contaminated soil so any runoff water from heavy rain does not contaminate it. Use crop cover hoops to help protect the raised beds from getting contaminated from rainfall and dust.