We're removing a layer of topsoil from a section our yard to lay brick paving. A friend who donated the bricks has expressed an interest in taking this topsoil for their own garden in exchange (they're in a newer neighbourhood without great topsoil).
How should we treat the topsoil before removal to ensure that we don't send weeds/creeper roots, pests, etc. over to our friend's garden?
I would recommend just warning them and not treating the soil at all, unless they request a specific treatment when you ask, or unless they're in another state or you know your soil has particularly bad weeds, pests, pathogens or something.
You could pour boiling water on the soil to kill weeds and bugs, but it will also kill beneficial soil microbes, which may result in smaller cucurbits and give fungi an advantage if there's not a lot of sun on it. If you could do that and then add beneficial microbes, right away, that might be great. I've never tried adding beneficial microbes, unless you count yogurt in a container indoors, but that didn't end well. Mixing your sterilized soil with your friend's poor dirt could possibly colonize it with good microbes, but I don't know for sure.
There might be workable pesticides and herbicides you could use, but your friend might be into organic gardening. Again, I recommend asking your friend before you do anything.
As long as your friends intend to add the cut out soil to their outdoor garden...do NOTHING. If they want to use to fill some sort of a pot/planter...tell them to purchase sterilized soil. Whenever I've had to cut sod out of a site I bust my buns to find a way to use this sod on site. Costs bucks to take it to a proper dump. Otherwise, old sod is SUPER for planting beds. Just turn it over, dump it where you want and then top it off with soil/decomposed organic mulch. Plant beds work better if they are raised. Planting on a flat, compacted soil is not so great. As long as the old grass/weeds are buried a few inches you or your friends don't have to worry about the grass/weeds growing back. They become valuable organic matter for the soil. The beds could be stacked 3-4' high but will become a foot or less once decomposition has taken place (add a bit of nitrogen to hurry up the process). Herbicides/pesticides just aren't necessary in the garden unless you've screwed up royal and need a bandaid. A gardener's goal should be to have as much life as possible within the soil. Beneficials will take care of bad insects/disease if everything is in balance. Using any pesticides will ALWAYS throw this balance off. Are your friends in your neighborhood? Say within 5-10 mile radius? Similar conditions? (Are they in a development where the contractors buried the topsoil or carted it off?) If they are not, keep the sod for yourself. Otherwise, this stuff is the best stuff one could use to build up their soil. Shame to take this to a dump...have you and/or your friends ever done a soil test? Glad you asked!
One option would be to compost it for a year in an existing hot compost pile, if that is available. Prolonged heat will typically kill most weeds, seeds and pests. The sterilization process for most soil bought at home improvement stores entails it passing through industrial ovens before having additional moisture re-added. If you can heat up the soil to 180*f for a prolonged period of time (30-45mins) it should take care of the vast majority of issues. This could be achieved with anything from a normal oven to a fire and a 55 gallon metal drum depending on the amount.
However, if the soil has chemicals such as fertilizer or weed killer it's unlikely that the process would remove it and if your friends application involves a garden where food is going to be produced I would recommend letting caution win the day.
