[Ian Atwood]

My apologies for the tardiness of my post but I have only recently started gardening. I came across the site when I was searching for soil sterilizers. I liked JayG's idea of using a wallpaper stripper.

I have now built a stripper powered sterilizer which gives me a batch of 16 litres of sterilized compost in just under an hour. I produced my second batch yesterday.

I think I have managed to attach a photograph.

If anybody is still interested in this I would be happy to provide more details.

[coldandwindy]

Any "wet" (steam or boiling water) method would cause leaching & be difficult to control.

Could you spread it out under a black sheet & put it in the sun? Might not get hot enough but if you go to a bit more Heath Robinson-ness could you use a black radiator attached to pipes that snake round the compost?
 It wouldn't need to be very hot. Pasteurisation = more than 63 degrees for 30 minutes, or that's what they say for milk anyway.

[Ian Atwood]

I don't think leaching is going to be too much of a problem (speaking from my position of ignorance). Using steam most of the heat transfer is through release of latent energy as the steam condenses to water and I estimate I am using a bit under 2 litres of water to cook 16 litres of compost up to 80C. Some of this water will stay in the compost - how much depends on how dry it was to start with - and almost all of the rest drips out into the plenum chamber at the bottom of my box. I have been throwing this water away but from what you are saying maybe I should harvest it and use it as a liquid feed.

A black sheet in the sun is probably an efficient way of doing things but I have two objections. First is that I am a lazy gardener and loading up my storage box and plugging it into a steam source sounds a lot easier. My second objection is one of timing. My biggest need for sterilized compost is spring/early summer when I am making up seed trays and potting on. Like you, I am in Scotland and unless we are very lucky the weather at this time of year is unpredictable at best. It can be difficult enough to dry my washing on the line never mind solar cooking compost. Having said that today is gorgeous and sunny.

I am not sure what you are getting at with the idea of a black radiator. Indirect heating is always going to be less efficient.

Temperature we need is a funny one. I am new to this game and have no personal knowledge base but I have seen the figure 80C bandied about. It is always going to be a balance with higher temperatures killing more nasties but possibly degrading beneficial elements of the compost. A bit like the difference between pasteurized and UHT milk.

[coldandwindy]

.

Sorry - wasn't criticizing your method; to be honest I hadn't read it yet when I posted! I was reading my way down the thread & replied to the first post & then everything hit the fan (its lambing time here) & I never finished reading.

I am a lazy gardener

Me too but I am also very mean so black sheet in the polytunnel is probably more for me!

I have seen the figure 80C bandied about
 

That would be more accurate then. 63 is what kills Brucellosis! 

[Ian Atwood]

Me too but I am also very mean so black sheet in the polytunnel is probably more for me!

A man after my own heart.

Only expense I had was just under £5 for plumbing parts to make a 1/4" BSP inlet port. Wallpaper stripper is 25+ years old and rescued from my loft, the box I already had. Other bis & bobs (wire mesh, landscape fabric & timber offcuts) were just lying around. If I was starting off having to pay £30+ for a stripper and £15 for a box it wouldn't have got very far.

Electric worked out at just under 3p/litre compost.

[jezza]

Hello theres electric soil sterilisers about,I use to use one at school it held a barrow load of fine soil  it use to be left on over night during the week at 65 degrees,the sterilizer was once put in the front entrance to the school on a Friday night  so that the caretaker could turn it off Saturday morning,he went away a Friday night and forgot about it the school stank on Monday morning ,if you microwave soil take any stones out as they dont half make a mess of the insides ,my Grandads neighbour  use to have a coal fired copper in his wash house that he used for steam sterilising soil    jezza

[Subversive_plot]

A simple way if you have an outdoor grill or camp stove of any kind and a couple of old all-metal cooking pots with lids, one fitting inside the other. Pots maybe from a "thrift store" or something similar?

1.  Place medium to be sterilized in the small pot, put on lid.
2.  Place small pot inside the large pot, have a couple inches of water (at least) in the large pot, put on large lid.
3.  Get a fire going on the grill, put the large pot on the fire, bring to a boil.  Stay nearby, i would leave it on boil for 20-30 minutes.  Remove the pot from the heat, allow to cool completely.

[Ian Atwood]

Perfectly valid way to do it - if you need a small quantity. Each little half seed tray I use for sowing takes approx 1 litre of compost. It doesn't take many trays to add up to a reasonable quantity and I am already well into my second 16 litre batch.

As I mentioned above, I am a lazy gardener  and it takes the same effort to sterilise 16 litres as it does a potful - unless you can use very large pots. Method I am using also sterilises the compost in a storage box which normally lives under my potting table so I only have to move the compost  twice - once into the box to charge it up and once out of the box and into trays and pots as I need it.