Mark Anderson wrote:
This summer will be the 5th year for my rooftop container garden. Last
summer we had a pretty bad drought here in Chicago which I think
exasperated some mistakes I have been making with regards to my soil
recipes. So now looking forward to Spring I'm researching on how to
build container soils and its getting so confusing I feel like my head
is about to explode. Some say add sand for drainage but sand clogs
macro and micro pores. Other sites say you need good drainage but your
containers also need to retain water -- two completely opposite
requirements. Some say use compost in your mixture others say compost
does no good and micro-organisms in your container soil is not a good
thing.

So the more I read about container soils the less I know and the now I'm
completely confused. It now amazes me how anything grew in my garden
these last 4 years because I've been doing everything wrong.

I want to keep things simple so I found this one recipe that I might use
this summer:

1 part top soil
1 part peat
1 part perlite

That seems like a lot of perlite but I suppose it helps in aeration.
There's a nursery by me that sells perlite in 3 cu. ft. bags for not
that much so that's not a big deal. But even the potting soils I see in
the stores never have this much perlite. I'm also considering using
pine bark but am a little concerned about having woody stuff in the
soil. I thought wood leeches nitrogen. Last summer I used building
sand in my soil mix and apparently that was a *big* mistake. Although I
had a good pepper and cuke crop, my tomatoes didn't do so well even
though I watered them every day.

Does anyone have good (hopefully simple) recipes using material that can
be purchased at Home Depot or preferably Menards?

Is it bad to use compost in the soil for veggies? I read last year that
growing tomatoes in a container requires about 1/3 compost and that's
what I used last summer but now I'm reading that using compost is not
good. Compost supposedly breaks down the soil, reduces aeration, and
increases water retention leading to root rot. I suppose everything is
a tradeoff but some of these sites use absolutes.

I'll be digging out all my containers and recycling the soil as the top
soil component in any recipe. Some sites say not to use this recycled
soil because its broken down and get fresh soil. But how can this soil,
after I break it up in the soil mixing box, be any worse than those bags
of dirt that you get from Menards? How do I know if the Menards soil
isn't broken down either?

Also if anyone has good links to soil recipes that would be good too.
Thanks for any help.

See my recipe for home-made potting mix at
http://www.rossde.com/garden/garden_potting_mix.html. This requires
no pH adjustments (lime or sulfur) for most plants.

If you can't get a small amount of compost from a friend, use topsoil
where something was actually growing. You need only a small amount,
just enough to supply the beneficial soil bacteria that will release the
nutrients from the bone meal, etc. In a gallon of mix, I might use only
a handful of compost. With only a small amount of compost or topsoil,
local variations in texture and pH have little effect on the mix.

This mix will drain very well. But it will also remain moist. Yes, it
is moist without ever being soggy (unless the container does not have a
drain hole or is sitting in water). Unlike many mixes, the moisture
remains available to plant roots until the mix is almost totally dry.
In other mixes, existing moisture gradually becomes unavailable as they
dry.

In any case, either use a porous container that allows moisture to
escape through the sides and thus stay cool, put the container where it
is in the shade while the plants get sun, or put the container inside
another container. Many container-grown plants fail in the summer
because the roots cook. No amount of moisture can prevent this.

--
David E. Ross
Climate: California Mediterranean
Sunset Zone: 21 -- interior Santa Monica Mountains with some ocean
influence (USDA 10a, very close to Sunset Zone 19)
Gardening pages at http://www.rossde.com/garden/