Here are a few ideas for purifying soil:
For adsorbing toxins from soil that will be used for alkaline vegetables, which is most of them, soil can be limed with calcium carbonate. A few fruits need liming as well, such as muscadine grapes.
For adsorbing toxins from acidic soil that will be used for most fruits, either dry pine needles or leaves may be used for their carbon content (these are fine for veggies too).
Bio-char from a campfire, wood stove or fireplace also contains carbon.
Excessive rains which are predicted to accompany Px in some areas might flush toxins out of soil, possibly leaching minerals along with it and establishing both good and bad fungi.
Watering with nettle or comfrey compost tea can kill unwanted pathogens (didn't Marshall mention a Px flyover bacteria specifically by name in one of his books?).
As a drastic measure, solarizing soil plots with black plastic can be done, and hopefully beneficial microbes will recolonize the soil on their own afterward. Feeding leftover milk to soil will expedite the process of probiotic recolonization - we do this with cherry trees which are prone to leaf spot in rainy years.
Covering the soil until the fly-by is complete might be a preventive measure, though if the event is expected to happen during the growing season, might want to use a fine netting so that some light, air and water can get to field crops.
As an unconventional use of sand, it could be placed ahead of time as a thick top layer above the soil, allowing contaminants to be captured as they percolate down. This might be heavy and unwieldy to implement though, unless sand is readily available on-site or nearby.
Potting and moving plants indoors temporarily might work, protecting the immediate soil around the roots of plants and allowing nature to take its course with un-potted soil left outdoors.
Trusting the chlorophyll of plants to neutralize toxins naturally might be the simplest method, albeit the lazy way out with fewer guarantees. This might work best with non-edibles, forests, and grass if any is left. Sowing a fast-growing cover crop such as nettles around more valuable crops might be an efficient way of purifying soil. Bottled seltzer water, which itself contains carbon and makes soil nutrients readily available to plants, could be used to draw contaminants into cultivated plant root systems for uptake to leaves and even more expedited chlorophyll detoxification.
Finally, we gardeners might choose to work in tandem with post-Px massive environmental changes, and see what will grow in the new conditions, selecting plant specimen that demonstrate adaptability.