Sterilising seed compost

[Spartan]

I have tried to sterilise compost in the oven (200'C 30 minutes) and in the microwave (800W 10 minutes), and I end up with a mix that does not want to "wet"at all.

i.e.
When you water from above, the water forms a blob that does not want to penetrate the compost.

Watering from below, it takes forever (or never) for the top surface to become moist.

What's going wrong, am I over cooking the compost ?

Would baking it at 70'C for an hour be better ?

Thanks.

[Trevor]

I cook my seed compost for about 90 minutes at around 220C..but I cover it with aluminium foil to help trap some of the moisture...and I sometimes moisten the mix prior to the cooking.
I still sometimes end up with a dry dusty mix, and have seen a little of the wetting problem you're encountering. Try using a spray mist bottle to wet the surface for sowing and then stand the pots in water (or as I do, a fungicide water mix) until the surface is truly wet. I think once you get the mix fully wet once, your problem will go away.
What type of a mix are you using ? I'm using a seed raising mix, propagating sand, vermiculite and perlite in around a 4-4-1-1. Could your mix be high in peat or something that isn't taking the cooking well ?

[David_L]

Yes I think that temerature's too high, you are probably breaking down the compost structure and damaging the organic component. Oven 60-70 C for an hour or so depending on the volume of the compost --- it's got to reach that temperature throughout, just like cooking ---- would probably be better. Personally I don't usually bother to sterilze my seed compost. I just use a fresh small bag of JI seed each time and all the other ingredients are mineral so if desired can be baked at 200 C. I just used boiled cooled water to thoroughly wet the compost from below and give a spray of Cheshunt compound and/or 1% Armillatox outdoor cleaner after sowing and before poping the seed containers into plastic bags.

[Roy]

I sterilize the compost for 10 minutes in the microwave because I use the "baggie" method of raising seeds. Rearing seeds in a sealed container is an ideal method but unfortunately those ideal conditions are also ideal for propagating nasties if you don't sterilise the compost. The seeds themselves are sterilised by resting on the treated compost or the final spray over the top that I always administer before sealing the pot.
The compost can indeed be very dry when you take it out of the microwave and I use a hand spray with a Chinosol solution to dampen the compost slightly before potting it up, then I find the compost will take up water when applied to the bottom of the pot quite easily.
I would never use Cheshunt Compound, as advised in the older publications, as it has been shown to inhibit germination and I am very surprised that the BCSS main page seed germination tips is still advising people to use it.

[Spartan]

Ah, it seems that I am baking the compost dry, I should moisten it first and let the STEAM do the sterilising. So no need to go much above 100'C

I also like to use the baggie & chinosol method.

[Guest]

Roy, interesting comment re Cheshunt Compound which I have always used. Can you expand on that? e.g. is there anything it particularly inhibits?

[Bill]

Well on my first real go at seed raising I planted up quite a few lots of Haworthia seeds in October using a method like David's and it failed miserably as I have lost a third to half of the seedlings to damping off.

Not sure if did I something wrong, but methinks it's back to the drawing board.

[iann]

Yes, moist soil in the microwave for 5-10 minutes. Clearly the soil won't get much above 100C. Not strictly sterilisation since there are microbes that will survive that treatment, but the full baking in an oven seems to lead to other problems. I imagine even the microwaves themselves are pretty fatal to many things. I microwave the soil in place in the trays or pots and sow as soon as it cools down.

[Susanne]

Does anybody know, if normal Cactus Compost from the garden centre is all right for sowing?
Any experience?

[Roy]

Cheshunt compound is used to stop damping off, for which it is effective. So compared to using nothing at all it will give better results but when compared to using other chemicals to achieve the same effect the germination was found to be not as good. In practice I find this is the case .