[granjan]

Professor Stefan Buczacki swears by putting 1 kg in a covered dish in a microwave on full setting for seven minutes 

[fatcat1955]

But what is the point?

[DD.]

To run your electricity bill up?

I know it won't, but assuming that a litre weighs a kilo, it would take over 4 hours of continual nuking to do a 40L bag.

[mumofstig]

IIRC didn't Harry Dodson light a fire in the garden in the Wartime Kitchen and Garden to do this.

an old thread with a useful link here 
http://chat.allotment-garden.org/index.php?topic=83033.msg926296#msg926296

[compostqueen]

I don't think it's worth trying to sterilise large amounts of compost for seed sowing either.  By all means put a trayful in a warm oven (you can leave the door ajar if you wish) but to do enough to be self sufficient is not really practical. 

If you make loads of compost in your daleks, or whatever, that should provide everything you need for soil improving and save you lots of money on bagged stuff.  In the scheme of things though you don't use tons of compost for sowings as it's mostly in shallow trays.

I am not surprised that you have slugs in your compost bin! You will get worms too    The do a good job and churn everything up for you.  In summer the bin will heat up til it's too hot for you to keep your hand in, and the slugs will probably clear off    You can't expect a weed, slug free home made compost. It's alive I tell you 

[simonwatson]

I use my homemade compost sometimes, mixed with spent compost from toms, to make a seed/potting mix. I put the seeds in a pattern in the middle of the pot. Anything that comes up outside this pattern is easy to spot and pull out straight away. It's a variation on planting radish seeds alongside slow to germinate things like parsnips.

[gavinjconway]

Have you ever thought of buying in compost from the garden centre. 4x50 litre bag's for £12 Try heating that lot up and see how much it cost's.

Ditto to my point above - so much easier..

[mumofstig]

Have you ever thought of buying in compost from the garden centre. 4x50 litre bag's for £12 Try heating that lot up and see how much it cost's.

Ditto to my point above - so much easier..

The OP was trying to find a way to do it with no heating costs involved - a worthwhile project, surely 

[simonwatson]

If you're not looking to make much, how about placing small amounts in black plastic bags in the greenhouse in full sun. Even at this time of year if we get a day of sun they should heat up enough to cook any seeds present.

[JayG]

As I mentioned earlier in this thread I have been known to sterilise compost in the microwave, but I've never measured the quantities, temperature, or timed how long it took, so today I thought I'd have a "play" (any excuse!)

A medium sized Tossco tie-handle freezer bag (low-density polyethylene) was filled with used compost from a bag outside, the handles tied (but not too tightly) and put in the 850W microwave.

It took a total of 20 minutes in 5 minute bursts (turning bag over each time) to get the compost up to 85C. 30 minutes later the temperature was still 75C, so sterilisation should be complete (anything over around 85C risks the production of plant toxins, although not everyone agrees about that.)
When cool, I will securely tie the bag and it should stay sterile until I use it.

The full bag weighed 5lb, was only 2C when I started, and a little wetter than I would have liked, although it does need to be moist.

20 minutes worth of 850W microwaving costs about 4-5p, but would have taken much less time with warmer and less wet compost.

If you want to try it I recommend "double-bagging" to avoid mishaps, and to use oven gloves (when "cooked" it's too hot to touch.) Don't tie the handles too tightly because there will be quite a bit of steam produced, especially if you overdo it!
There was no smell that I could detect, although even using tie-handled bags I certainly wouldn't guarantee that would be the case if you had added chicken poo to the compost first!

So, it's not difficult, messy, or expensive to do, but it would be very time-consuming in large quantities - whether it's worth it is down to individual preferences (as ever.)

[aelf]

Thanks for doing the experiment JayG - I guess that makes you the kitchen Boffin 

I do all my seed-sowing at the plot, which is about 3 miles from home. Anyone got a long extension lead that I could borrow for the microwave? 

Seriously tho, I don't think it's worth the bother - I do get some unwanted seed germination in my home-made compost but never enough to really worry me - the weeds just get pulled or rubbed out when I water/inspect/pot on etc.

That said, the mad scientist in me has been awoken  I'm thinking a large metal sheet held up on brick pillars with a wood fire underneath. Just spread a couple of shovelsfull of soil on the sheet, let it warm up a few minutes and then douse with fine water spray. The steam produced will kill off seeds and the soil will dry quickly. Srape it off into a sack and put more soil on the sheet. A couple of people in a production line could get quite a lot of soil sterilised this way in an afternoon 

[simonwatson]

If you're determined to do it I can't help but think that a solar furnace would be the greenest approach. It should also satisfy the mad scientist too!

[JayG]

I do all my seed-sowing at the plot, which is about 3 miles from home. Anyone got a long extension lead that I could borrow for the microwave? 

Hmm.........now let me think about this - the power loss of a 240V extension cable is X volts per mile assuming a wire cross-section of Y millimetres. ................  


         


(Actually I wouldn't dream of microwaving any of my dalek compost just to kill weed seeds, but it could be useful on a small scale for making sure sowing compost is bacteria and fungus free as well as seed-free.)

[DD.]

Your power loss is actually I²R 

[JayG]

Amp currently positively resisting the urge to tell you watt I think of that (most of my power loss is due to a mis-spent youth!)