what Mix to use in a Self Watering Container

jmhewitt(8a Coastal NC)January 9, 2011

I have been using Self Watering Containers (from Gardeners Supply, WalMart, Earthbox and Johnnies) since 2004. I have a very hot humid coastal environment.

I grow only tomatoes in thse containers.

I have been using Miracle Grow Moisture Control Potting Mix for some years. results have been pretty good, but I would hope for better.

What mix do you suggest that I use?

Michael in Hampstead, NC

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rnewste(8b NorCal)


I am sure there are folks here who will give you recommendations on mixing your own. As far as commercial products go, for a self-watering container (of which I have 31), upward wicking is a key factor. For this use, I find Sunshine #4 Mix is excellent, supplemented with a ratio of Bark Fines:

Also, I hear good things (but have never used) Pro-Mix HP (High Porosity):

Both are a bit pricy, but you should check out a few Hydroponics Shops for a deal. I am paying $27.95 for a 3.8 cu. ft. bag of Sunshine #4 Mix today.


    Bookmark   January 9, 2011 at 11:07PM
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jmhewitt(8a Coastal NC)

Raybo: what proportion of Bark should I add? should the small bits be sifted out?


P.S. I have been following your Earthtainer saga with great interest. as my present SWCs fail, I plan to build Earthtainers to replace them......BTW: what are you using for totes these days? I don't find any 31s anywhere near here....and what about the cages?

P.P.S. I am currently using 3/4 inch PVC cages for my SWCs. they get staked into the ground and the SWC in the case of the earthbox types sits on top of the bottom frame. I am sure this would work for the Earthtainers too.

    Bookmark   January 10, 2011 at 2:56AM
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rnewste(8b NorCal)


The Premiere folks recommend bark that is less than 1/2 inch in size in their Pro-Mix BRK blend. You will get lots of views on this Forum re: ratios, but for self-watering container applications, I have found a 3:2:1 ratio of Sunshine #4 Mix, Bark Fines, and Perlite are about optimal.

I have designed a new simplified (and less expensive) EarthTainer III with a removable folding cage system. The Construction Guide is now being written by me, and I hope to get it posted by the end of January. In the meantime, a few pictures:


    Bookmark   January 10, 2011 at 4:24AM
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I made two of raybo's Earthtainers last year, and wanted to use a mix I already have with some modification. With no SWC experience I knew it was a gamble. My regular potting mix I buy by the yard about $45 from a landscape supply, the same mix used by propagating nurseries. It contains no peat, mostly course sand, fir bark and pumice stone. I added 1/3 fir bark of my own grinding without screening. I added Dolomite about four cups per container and two cups time release fertilizer 18-6-12. The wicking action worked fine. Because of the lack of sunshine and heat the return was less than desired with lots of small tomatoes slow to ripen. A Burpees big boy had a BER problem that failed to respond to calcium treatment. Next year I will move the containers to (hopefully) a better location. Al

    Bookmark   January 10, 2011 at 9:38AM
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rnewste(8b NorCal)


Personally, I had bad results when I trialed Cactus Mix as an ingredient in one of my mixes. Porosity for aeration to the roots seems to be an important requirement for healthy plant development / production. Here is a clipping from an article on the Premier website regarding aeration and bark particle size:

"""The process of aeration is one of the most important factors of productivity. Plants absorb oxygen (O2) and release carbon dioxide (CO2) during the respiration process. "Respiring" roots use the sugars made during photosynthesis to provide the energy necessary to drive mineral salt absorption. For most plants, internal transfer of oxygen from the leaves and stems to the roots is too slow to supply enough oxygen to the roots. In order to have good plant growth, the growing media needs to be "aerated", meaning that air exchange between the growing medium and atmosphere has to provide the plant roots with sufficient oxygen and to remove excess carbon dioxide from the root zone. This air exchange takes place through a mechanism called diffusion.

Figure 1. The yellow line extending from a to b represents the path which gas travels through this growing medium. Notice the path is short. It has a low tortuosity due to the uniform size of the media particles.
Traditionally, only air-filled porosity (% air space) was used as a standard index to guide substrate manufacturing, but more recently scientists have found that air exchange properties (diffusion of gases), were more closely correlated with plant growth performance than was strict air-filled porosity. Air diffusion in a substrate will depend not only on the air space provided, but also on the way pores are interconnected and open to the atmosphere. Pore continuity and tortuosity (length of the path that the air must travel) will influence the movement of air through the substrate. Air (and water) will move faster from one point to another if the pores are well linked together to create a shorter, less tortuous path (fig.1).

Figure 2. The yellow line extending from a to b is longer. Air has to go through a more tortuous path due to the large bark pieces.
Some growing media components, like large-sized bark particles and large peat chunks, can form gas impermeable barriers that can increase tortuosity, which will make air diffusion more difficult (fig. 2). Studies conducted on both nursery and greenhouse plant species have shown that variations in tortuosity and gas relative diffusivity can decrease yield by up to 50%, despite the fact the growing medium had an adequate air-filled porosity (Caron, 1997). Having sufficient air space is important, but it appears to be even more important to make sure that our substrate components be of the right size to insure good pore continuity to maximize air circulation. Premier has recognized this and for these reasons has used smaller bark particles in our 2 bark mixes, PRO-MIX 'BRK' and PRO-MIX 'GSX'."""

Here is a link to the full Article:



Here is a link that might be useful: Aeration Study by Premier

    Bookmark   January 10, 2011 at 2:14PM
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Thanks raybo for the interesting article. Fir bark unlike redwood is not stringy and when run through a chipper/shredder no single piece is larger than a nickle. When I screen, it is to remove the fines less than 1/16 inch in size. In the SWC application I thought the fines might help in the wicking, which was my main concern. The great majority of the bark pieces measure between 1/8 and 1/4 inch without screening. I mentioned I would measure the water used daily in the SWC during the summer, as the vines filled out at about six feet. During the warmest days(which were not warm enough) two plants transpired about a gallon and a half a day. Al

    Bookmark   January 11, 2011 at 9:32AM
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I've always used whatever mix I can find on sale.

I empty my containers and mix with fresh/aged compost from my pile and an unusual organic slow release nitrogen source every spring, then refill the planters and add about a cup or so of pelletized lime and mix it into the top couple of inches of soil.

My planter design is different from any others I've seen since mine have a flow through design that allows aeration at the root level.

Gardenweb's Terms & Conditions prevents me from giving a link to my web site though.

    Bookmark   January 14, 2011 at 3:34PM
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