critique my potting mix recipe (for sub-irrigated planter)

ydfz(deep south texas 9)July 23, 2012

hello and happy gardening to all.

i've done my homework (though, as a gardening amateur, my understanding is likely far more limited than yours) and purchased all the raw ingredients necessary to make a well-draining but moisture-wicking potting mix suitable for a sub irrigated (a.k.a. self-watering) earthbox-type container garden. i'll be mixing en masse (i see myself in knee-pads on a tarp in the near future) to the tune of a cubic yard or so. the time for fall planting is fast approaching in my area, and before i screw up several cubic feet of mix, i wanted to get some critique on the best ratios of ingredients either based on your own experience or simply by your educated opinion.

i have the following in my stockpile:

12 cu ft sphagnum peat moss

12 cu ft pine bark fines

8 cu ft coarse perlite

4 cu ft coarse vermiculite

other amendments and fertilizers:

5 lb worm castings

5 lb dolomite lime

12 oz mycorrhizal fungi

20 lb garden-tone granular fertilizer 3-4-4

1 gal liquid fish blend 2-3-2 (for supplemental feeding as necessary)

NOW here's where i need the most help. i was thinking the best balance of moisture retention, proper drainage, and reasonable cost would be a formula similar to the following:

(by volume):

40% sphagnum peat moss

25% pine bark fines

25% coarse perlite

10% coarse vermiculite

plus incidentals (lime for ph, some castings for good measure, etc.)

what do you think? am i cheaping out by not including enough vermiculite (it is quite pricy here)? or, conversely, do you think i could get away with increasing the peat moss or bark fines to bulk up the mix (as those are the cheapest ingredients and in plentiful supply)?

PLEASE HELP!!! i only get one chance to make this right before i'm stuck with it the whole season.

Thank you for reporting this comment. Undo
ydfz(deep south texas 9)

oh, i forgot. my fall/winter garden will include a variety of heirloom carrots, beets, radishes, brassicas, salad greens, favas, and creole garlic, as well as common herbs, if that is in any way helpful to know in determining the best mix composition.

thank you all again...i am ALWAYS eager to learn from the BEST!

    Bookmark   July 23, 2012 at 4:59AM
Thank you for reporting this comment. Undo
redshirtcat(6a MO StL)

I have tomatoes in a 3-1-1 and 5-1-1 (bark-peat-perlite) mix in earthtainers and earth boxes. 2 Tomatoes per box and they are over 10 feet tall at this point. So far those mixes are still wicking fast enough to support the tomatoes.

I guess I'm saying I don't think you will have a problem no matter how you combine your ingredients. What you have will hold and wick more water than the mixes I use which are working with tomatoes in 100 degree heat. I might reduce the perlite proportion but that's me..

    Bookmark   July 23, 2012 at 6:21AM
Thank you for reporting this comment. Undo

I find vermiculite to be very fragile in a mix. Even if it's sifted beforehand, the weight of the mix around it and the force of the mixing is enough to break it into much smaller pieces, which will make it act in a different way than expected, right off the bat. All my mixes with vermiculite ended up completely anaerobic, and I've stopped using it entirely.

I'd say if you wanted to use less vermiculite you could probably replace it with more bark fines. Also, both bark and peat repel water if they completely dry out, so I'd start with wet or damp materials, and keep an eye on it to make sure it doesn't get too dry.

Good luck, have fun, keep notes, report back! :) Also, take pictures once you get some seedlings!

    Bookmark   July 23, 2012 at 1:20PM
Thank you for reporting this comment. Undo
ydfz(deep south texas 9)

ah. thank you for the advice. so, pixel pepper, when vermiculite is weighed down by the surrounding mix, it inhibits the penetration of oxygen through the media? i am wondering if i can get away with leaving it out of my recipe. it cost $30 for 4 cubic feet, and if it's not going to do much good anyway, then i'd be best returning it to the store before i tear the bag open. it is by far the priciest ingredient in the mix.

redshirtcat, if i can get away with scrapping the vermiculite, i may try something like the 3-1-1 formula you use. if 3-1-1 works for you in missouri, then might it work for me in south texas?

pixel pepper, you say i can use bark fines instead of vermiculite, but i heard that bark fines are more for drainage than retention? or is that incorrect? anyway, it's not much of a concern in winter here, but i do want the mix to last for a few seasons (amending as necessary as the bark breaks down), and summers here are brutally hot (100+ degrees F) and often quite dry. i am planning on tomatoes for next summer, and i can only imagine how thirsty they will get.

i will have to remember to take some pictures, and i will certainly report back with them. here's hoping for a bountiful harvest!

    Bookmark   July 23, 2012 at 9:55PM
Thank you for reporting this comment. Undo
dickiefickle(5B Dousman,Wi.)

As much as mix is important ,the size of the materials has the greatest impact . Thew largest portion should be pine bark less than 3/8 inch. Pine bark 50 % peat 30 % then 20 % Perlite . No worm castings at all

    Bookmark   July 23, 2012 at 11:41PM
Thank you for reporting this comment. Undo
ydfz(deep south texas 9)

ah. dickie, the pieces are about a 1/4" in size, so i should be set there. but why no worm castings? i thought they were a good all-purpose soil additive? especially since i can't add heavier composts (holds too much moisture in a sub-irrigated system).

    Bookmark   July 24, 2012 at 2:39AM
Thank you for reporting this comment. Undo


Vermiculite by itself doesn't inhibit aeration, but broken, very small pieces of vermiculite will create the dreaded perched water table. That goes along with what dickieficke says. Since the worm castings are also small particles(and break into smaller pieces over time), that will influence how long the mix has good aeration.

Bark is multi-purpose in a mix. It's much better for drainage than peat, and also holds water (maybe not as much as vermiculite or peat, but definitely more than perlite).

I think as long as the mix has good aeration and is durable enough to last for a while, the exact ratio of components doesn't matter so much. And don't worry if it doesn't work exactly the way you think it should the first time. It may take a couple of tries to find a mix that's good year-round in your climate.

    Bookmark   July 25, 2012 at 5:25PM
Thank you for reporting this comment. Undo
ydfz(deep south texas 9)

UPDATE! i wanted to report back with some preliminary results if anyone is interested.

per everyone's advice, i ditched the vermiculite and went with a strictly bark-peat-perlite mix. last weekend, i whipped up 16.4 cu ft of mix, which filled 5 big sterilite tubs. it worked out to:

36.5% (6 cu ft, 3 bags) bark fines
36.5% (6 cu ft, 1 bale) peat moss (expanded)
27% (4.4 cu ft, 1 bag) coarse perlite

(plus 1 gallon of dolomite lime (1 cup/cu ft) for ph balance and 2 gallons (2 cups/cu ft) of espoma garden-tone fertilizer).

THIS RECIPE HAS ISSUES. BEWARE: after making the mix, wetting it down, filling the containers, sowing my seeds, and observing for a week, it seems the proportion of perlite is much too high. the mix is supremely light and virtually impossible to compact (i tried, it just stays super loose), and there is plenty of oxygen-containing airspace. i realize that these all sound like good things. HOWEVER, the soil is so fluffy from being full of perlite that it's not wicking water up all that well. the base of the containers are soupy bogs, while the top inch is bone dry. this has made starting seeds difficult because the seedlings are too tiny to reach the vast water reserves (this would not be a problem with larger transplants).

in the meantime, i am top-watering the seedlings until they are large enough to reach the moister mix a couple of inches down. next season, i will cut the mix with additional peat and bark to reduce the proportion of perlite.

i have much more bulk ingredients and more containers to fill this season, and so i am revising the above recipe by using only half of the 4.4 cu ft bag of perlite per batch, which puts me at

42% bark
42% peat
16% perlite

if the little veggie babies survive, i will report back with pictures of the setup. happy gardening!

    Bookmark   September 28, 2012 at 8:21PM
Sign Up to comment
More Discussions
Best glue for terracotta pots?
Yes I did. Left some of my pots outside and now they...
Reusing infested soil
Last year, my outdoor container-grown kale suffered...
Root bound jasmine separation
HI.. It is a first time I disturbed a rootball of...
Help, my tap water is not good for soil
Its been a while since i decided to garden vegetables...
Era Alicka
Replacing potting mix
How often should old potting mix be washed entirely...
Sponsored Products
David Trubridge Design | Coral Pendant - White
$450.00 | YLighting
Kani Bronze Four-Light Square Canopy Bath Fixture with Garnet Glass
$544.50 | Bellacor
Black 24-inch Plant Dolly
Terracotta Non-sanded Tile Grout
$25.00 | TileBar
Custom Hemmed Table Runner
Home Decorators Collection
Moooi | Common Comrades Tailor
Jadeite Hanging Vase
$23.99 | Dot & Bo
Stacy Garcia Chevron Pride Pink 7 1/2"W Steel Mini Pendant
$99.99 | Lamps Plus
People viewed this after searching for:
© 2015 Houzz Inc. Houzz® The new way to design your home™