We are having some problems with the SunGro #4 mix. Basically, we’ve determined it’s not good for germination. It’ll be fine for transplants, or even containers, with mature plants. Our most current post tonight summarizes our current thoughts with it.

We do, yes! We do build all the soil we use in them, mostly due to cost. You can certainly use regular potting soil. But, at the point of using 30 buckets, you’re definitely in the territory where making your own will be slightly cheaper. We use the Frosty Garden Weapon soil, in this post.

Just to be clear, you didn’t have this problem using the #4 mix for starting seeds? Just this new mix you tried out?

Jeff,
Thanks for both of your replies! Maybe I missed it, but do you have a blog post on how you make/mix the soil for the grow buckets? Can you steer me in that direction, or tell me what you use inside the grow buckets? I’ve just purchased 30 buckets/inserts (and guide buckets) to give this a go this year and I am trying to get everything I’ll need in advance. Trying to figure out the soil needs now.
Thanks!

I think the “expands to 6 cubic feet” is a bit of a marketing gimmick. If you loosely pack the soil, sure, it’d likely take up 6 cubic feet. But, we try to make sure our containers are well packed as that’s beneficial for plants. I will tell you that ProMix is the best soil we’ve ever used and we’d still use it, if we could buy it locally. (Even if it’s $70 for 3 cubic feet!)

And yes, well draining soil is important for GroBuckets. (So lots of perlite, usually, but can also be sand, vermiculite and other materials.) We build our own soil for our GroBuckets, but it’s very similar to a heavy perlite soil mix.

We do buy new soil every year for germination and transplanting. Our soil pile is 100% frozen at this time, so it’s the only option. (It doesn’t thaw until late May/early June) But, we’d still do it even if it wasn’t frozen. Our soil re-use is primarily for our container gardens. In our germination and transplanting environment (indoors), we really don’t want a bunch of other seeds germinating, which happens with re-used soils. Everything we buy eventually ends up in our in-ground gardens or in our container soil, which eventually gets re-used.

As far as I know, these are different “versions” even though they both say “Mix #4.” That said, either should be OK and there’s not really a “right” thing to use. The high porosity is going to have more perlite in it, meaning it’ll drain water a bit better…but, the HP version also doesn’t have mycorrhizae according to SunGro. This is good stuff to get into your soil. (It helps with micro-soil structure, bacteria and otherwise creates a thriving biologic environment.) We’re less concerned about that since we use fertilizers that have it as well. We debated between the two and settled on the HP version as we like introducing perlite into our soils…but it’s not a mistake to use the other version, either.

In the FWIW department, we are in the process of planning an update post in the next few days. We all ready know that we wouldn’t recommend the SunGro product for seed germination. Ours has all ready started to develop moss, we have soil-induced seeds germinating and worse yet, we’ve had our first fungal infection with it. We’re likely going to shift over to a DIY soil build for actual germination to get something a bit more sterile. There’s serious risks (especially from fungal infection) in the germination environment. But the SunGro stuff is fine for more established plants (i.e. post transplant) where they can resist these issues more easily.

I am going to look for the pro-mix you are going to try this year. I haven’t found that one around here. I’ll be interested to hear your experience.
Also curious – you have another post on how you reuse your potting soil from year to year – so are you buying this soil you are speaking of in your post for seed starting and adding some to your soil compost pile? Wondering if you reuse, why you are buying more.

Thanks!