Boiling to Sterilize Soil and Sawdust?

I have an area that was once a sheep pen. It has tons of beautifully rotted organic manner. But the area was neglected before I moved here, and it's infused with invasive weed seeds that persist up to 7 years.

I want to use this material, but not if it spreads nasty seeds and creates a make-work project. So, my thought is to add water and boil it in small batches as I'm making char.

I would use the same technique for raw sawdust and bark that's free from a local firewood company but may contain insect pests that would ruin my spruce trees.

How long of a simmering boil would be long enough to fully kill nasty seeds and insect eggs? 15 minutes perhaps?

I have no idea how long one would need to heat the water.  Some thoughts on that is to average the mass and temperature.  If you are taking 8 pounds of soil at say 70 degrees, and a gallon, or eight pounds of water at say 200, the weighted average would be 135 degrees, theoretically.  That will take a lot of water and heat, as soil is dense.  Pouring a gallon of water over a pound of soil may not be enough to kill seeds, especially since the mixture is not going to be homogeneous and equally distributed.  A steamer over the top of the soil might give you the transfer of energy you need; but to what depth?

I would recommend the a less intensive way or sterilization if you must.  I would irradiate it.  Spread a few inches of dirt on a large tarp of other surface.  Maybe rack it around once a day.  Air and UV rays will kill everything in the soil that is designed to grow in a low oxygen low light environment.  

I would like to point out that sterilized soil is not soil.  It is just dirt.  If you hope to use it productively after sterilization, you will need to recharge it with life, bacteria, viruses, microbes, and earthworms.  One has to rebuild the biome of life for it recover as usable soil.  That may take a season or longer.  There are easier ways to deal with a seed bank.  Let the weeds grow, cut them before seed; or for rhizome, till frequently.  Let the seeds exhaust the bank by letting them sprout, but not live to sexual maturity.  Just a thought.

As far as the bark and sawdust, if you are concerned, I think adding it to water in small batches, so the mass does not reduce the temperature below you target point would work.  

I agree, sterilization in the manner I suggest is the nuclear option, but I think it's the only option left to me. I will mix it into a liquid slurry in old enamel water bath canners that will sit above my charcoal kiln, utilizing the substantial amount of waste heat generated.

As for other methods, I have been agressively taking all possible steps for five years and invasive seed is still sprouting every year. I have a lot of sweat equity invested in this and I intend to get value out of it.

You're quite right to say it will be dead dirt, but spreading it thinly on top of living soil and compost piles will bring it back to life soon enough. The same is true of the raw sawdust/bark mix. In fact, I may cook them together.

It adds an extra step in the short term, but I wonder if using the sterilized 'dirt' as a base in a compost pile will further destroy any seeds as well as allow microbial life to re-colonize.

In fact, any thought about skipping the boiling step and just getting it dug up into a large composting system?

Chris Sturgeon wrote:It adds an extra step in the short term, but I wonder if using the sterilized 'dirt' as a base in a compost pile will further destroy any seeds as well as allow microbial life to re-colonize.

That's a sensible precaution, and I think I will designate one composter for this duty. I am not taking any chances after five  years of hand-to-hand combat -- I'm cleaning up somebody else's mess and my patience is at an end. Though I really doubt that any seed would survive being brought to a boil and then simmered over heat for 15 minutes in a covered pot.

If it was a small volume, I would force it anaerobic for a season in a plastic barrel, drain the stinky water on tree driplines, and bring it back to aerobic. In my experience, no noxious or invasive weed seed survives a long anaerobic fermentation.

My composting is typically on a two-year cycle, given that it's frozen solid for six months of the year. It runs a bit cooler than I like, but I'm doing the best I can by hand, dealing with a fair amount of raw material. I put an old tarp at the bottom, which holds some moisture to prevent mold and keeps quackgrass from invading. The situation may change now that a handy little tractor with loader has arrived on scene.

Anyway, folks, thanks for indulging me as I rant and rave. I appreciate your tolerance.