Re-using potting mix.

[hawkeyes]

Trawled through these posts, couldn't, find any posts on sterilizing mix. Would this be OK. Thought of placing a small plastic bag in microwave with some in it. Any thoughts.

[NAHamilton]

Do you need to sterilise it?

I just sieve it and reuse. I also dry it out for a while in the hope of killing any weeds in there although that doesn't appear to help.

Cheers,
Nigel

[FruitFly]

I've been meaning to ask this question too.

Thanks for posting.

[Jarad]

What's in your potting mix?

I know that people on here reuse the inorganic parts of their soil mixes like diatomite, zeolite and pumice. Whereas things like pine bark lose all of their nutrients and good stuff so it's better to just chuck those bits.

[tgward]

I usually soak it for a couple or 3 days( to nullify seeds) then spread to completely dry then sieve to remove the fines -mostly inorganic material is left-mix in with already prepared mix--

[kcpoole]

I Just wash and then dry on a screen, then sieve out fines
I have so many weeds here that whether new or old mix makes no difference.

Ken

[toshtony]

Sorry this is slightly off the topic but when adding a new different mix like kanuma for azaleas which has been growing in potting mix do you reuse the mix in the root ball or do you need to wash it all off. Would you also mix some of the existing soil type with the kanuma for the 1st time to ensure a smother transfer. I'm thinking of changing all my azalea and crabapples into a more acidic substrate next repot.

Thanks
Tony

[kcpoole]

@ ToshTony
For me it depends on what the original soil is like
If nice and open then i will just scrape out some and repot with Kanuma, but if solid and compacted then i will spray with hose and remove it all, then root prune and repot into Kanuma and poke it in the root mass with a skewer
Note that Azalea get very dens root mass anyway so you most times have to reuce it fairly much anyway

Ken

[hawkeyes]

Back to my post, it was hijacked. The mix is diatomite, zeolite, purlite and pine bark. This mix is from plants that have died, I just thought any pathogens (?) could be hiding. So giving a good soak in water, would work?

[shibui]

This mix is from plants that have died, I just thought any pathogens (?) could be hiding. So giving a good soak in water, would work?

Spot on Hawkeyes. This is the biggest risk reusing mix. If a plant has died there may be pathogens present. Do you really want to risk spreading that to other plants? Is the mix that valuable? Many growers get away with it without any problems for years then suddenly the diseases build up and devastate a collection. Once started fungal disease can be very difficult to get rid of.

If you really need to reuse potting mix it should be sterilised. An old electric frypan can treat small amounts. Add enough water so that the whole lot heats evenly to boiling point. 72C is pasteurisation temp but to ensure that all mix is heated it is better to go to boiling.
Microwave can also treat mix. Make sure the mix is damp, dryish mix will catch fire in the microwave (not sure if that was stone or charcoal in the mix that overheated but be warned)
I have an old microwave in the shed because the cook has forbidden potting mix in the kitchen microwave You'll need to work out a time/temp for different amounts of mix with the power of your machine. I generally nuke 4 litres of mix in an old icecream container for 15 minutes to be sure but you could check the temp with a thermometer to make sure it gets above 72C in the centre.

[twoshay]

Would 72 degrees be sufficient? I am a tattooer by trade, we need to steam our instruments at 121-132 degrees for a set amount of time. Soil may be different, just putting it out there though.

[shibui]

Good question twoshay.
72C is Pasteurisation temp. It kills most pathogens but not everything which is why medical equipt is autoclaved to much higher temps. Maybe plants are not as important as people?

[twoshay]

Interesting, good to know, maybe we are just more susceptible to infections

[shibui]

Many infections common in medical situations have been bred tough by inadequate controls so they are now tougher to kill. Same thing has happened in sgriculture with weeds and pests becoming resistant to some chemicals and treatments - Warning to bonsai growers who don't sterilise properly.

[hawkeyes]

Thanks to the feedback fellow panel members, invaluable ideas supplied. The risk of reusing potting mixes is too great. Tipping it out or sterilising is the best way to look after your valuable plants. Shibui, as usual v. good advice. Unfort. my microwave in kitchen just keeps going on, and on. Might look for an old one in the Op. shops.