i'll be testing few trees in 5 gal SIP set up this yr. what is the best soil mix for it? i'm thinking 75% peat and 25% perlite with cup of lime, a cup of slow release fertilizer, and cup of worm casting. handful of iron.. 

I am also testing some soil this year with SIPs and am curious to hear what others have to say. My usual soil mix is one part sand, one part hard wood mulch and one part compost. I know that Bill uses straight compost in his pots and it seems to work well although I can imagine the pots are really heavy. 

That seems like a very light mix, Pete.  Do you get much wind in NC?  I always had trouble with wind bowling my trees over and my mix wasn't that light.  I started adding some compost just to make it heavier. lol..

not too windy here. tho, it doesn't take much to knock down my trees. i usually tied them up to the rail and so on. 

Funny you posted this topic.  I was about to do the same.  What I'm planning for my covered SIPs at this point, is

• Pro-Mix BX
• dose of lime
• dose of worm castings
• dose of osmoscote

i used to top dress all the containers. but with idea behind pushing the trees further along, i think mixing them might have better results. i mean, after 3 months or so, i'll have to top dress again but initial 3 months with things mixed in might kick start the trees. 

Pete,
A 75% peat mix will work with a properly designed SIP. I've used that ratio in the past successfully.  Most commercially available peat based mixes will work, but stay away from the moisture control products, they will waterlog the soil. Also always test the SIP with the mix before planting. For a simple test you will only need to fill the SIP 1/2 full with mix, fill the reservoir and observe the saturation and aeration of the mix over a few days.

I actually plan on using a less porous mix (larger particles and more aeration), but have not been able to test the mixes due to the weather (freezing cold and almost 18" of snow cover).

A simple trick that I learned a few years ago is to make and use an 85% peat based mix in the soil wick and the bottom 1 to 2 inches of the SIP to compensate for the lack of wicking ability of the potting mix.

Good Luck.

Question.. When ya'll are talking about peat based mix...  are you talking about peat moss or peat humus?

s. peat moss.

Frank, I also have a large bag of Promix-BX and have been looking for a way to use it.  The components in this mix are below.  One thing to note is that it already has some lime added so I'm assuming you wouldn't need to add much more for figs.  Also it has a wetting agent.  I'm not sure if this is a negative for using it in SIPs.  My impression with "moisture control" mixes is that they have a component that holds water so maybe the wetting agent (being different) would be ok in SIPs.  I am thinking about adding some pine bark fines, additional perlite and maybe some coir.  Maybe try a 5:1:1:0.5 mix of Promix-bx/pine bark/coir/perlite ?

Ingredients

• Canadian Sphagnum Peat Moss
  (75-85% by volume)
• Perlite - horticultural grade
• Vermiculite-horticultural grade

• Dolomitic and Calcitic limestone
  (pH adjuster)
• Wetting Agent
• Mycorrhizae – endomycorrhizal innoculum
  (Glomus intraradices)

So what I used last year is the sand-based mix shown below.  Figs loved this mix once they got adjusted to being bare-rooted in order to transplant into it.  The only problem I had (which may be an issue with all SIPs) is that there was a lot of green wood late the fall.  They are slow to get the message to go dormant, though this could have had something to do with fertilizer.  Some of these never lignified and I suspect I will have die-back on some of these figs.  But the other issue is it is a very heavy mix.  It's ok for 5 gallons but as I expand into larger SIPs I will need to change to another mix.  Nullzero who suggested this mix to me recommended changing it to 30% coir, 20% perlite, 10% mesquite charcoal, 30% pine bark, 10% pumice.  I am also considering peat based mixes but have heard that peat breaks down more quickly than coir in SIPs.

30% play sand (washed)

10% mesquite lump charcoal - not in briquet form; crush with hammer to dime size or smaller

20% pumice - use Drystall horse bedding from Southern States

20% coconut coir (wetted and expanded)

20% pine bark - use Kambark pine fines from Behnke’s

steve, what does charcoal do for the soil? i have seen CP growers using that before, but never figured it out.

Ha, Nullzero where are you?  I seem to remember that it is good for the beneficial microbes in the soil mix and water reservior but am not sure.

Here's the post I was looking for that got me interested in trying a sand-based mix:

http://figs4funforum.websitetoolbox.com/post/show_single_post?pid=1278426759&postcount=5

The charcoal is supposed to be good for the mycorrhizal colonies, improves aeration, and raises ph.

hmm... charcoal sounds good.

Thank you for that info, Rewton!

Danny (dkirtexas) uses a heavy mix with sand. Can't recall the ingredients but have it somewhere in my email. I haven't had a chance to try it yet. But he gave me a 2gal Smith tree in that. Nearly gave me a hernia when I picked it up. But the tree was undoubtedly very happy. It stayed green far longer than any of my trees. And I didn't have to water it much.