TL;DR for below:1. Self-watering containers (SIPs) need a different mix than regular containers.
2. SIPs DO need dolomite.
3. Use one of the potting mixes recommended by Earthbox, or a mix that is at least 75%-80% peat with perlite and vermiculite.
4. If you want to chance a cheaper mix that may not have the right composition, I suggest a side-by-side test with at least one SIP using the test mix and at least one SIP using a recommended potting mix. Same plants, same time, same fertilizer, identical SIP containers, same spot.
Full post:Earthboxes and SIPs are different than in-ground gardening, and are different than conventional container gardening as well. Certain practices and soil compositions that work great for in-ground or even conventional container gardening
can actually cause failure when used in an Earthbox or other SIP.
SIPs need a different soil mix because water gets to the plant roots by wicking up through the soil from the bottom water reservoir. Water is added to the system by refilling the bottom reservoir via a fill tube. The reservoir is not supposed to ever be allowed to empty, so wicking continues constantly. With the right soil mix, this means moisture levels are maintained at all times. Most veggies LOVE the consistent moisture.
Most of the things that you would ordinarily add to a potting mix for nutrition (e.g. manure, humus, or compost) make the mix stay too wet in a SIP, setting you up for root rots and sick or dead plants. Adding sand messes up the wicking action and your plants can die of thirst even with a full reservoir. So for an Earthbox or SIP, you use a peat-based mix, about 70%-80% peat, which also contains perlite or vermiculite. You don't add any sand, manure, humus, or compost...ever.
Also, in a SIP the water added to the system generally only leaves through transpiration and minimal evaporation. Water doesn't wash through the soil and out. In these conditions, the peat-based potting mix will get more acidic with time. Eventually the pH will drop low enough to interfere with nutrient uptake. So the lime is
absolutely necessary when growing most veggies, particularly any veggie that is prone to Blossom End Rot (BER).
Dolomite or dolomitic lime that contains about half as much magnesium as calcium is preferred over regular garden lime or dolomitic lime that has smaller amounts of magnesium. But if you can't find lime that has enough magnesium, get the most magnesium you can. It's better to use a lime with too little magnesium than to skip the lime, especially with veggies that can get BER.
Nutrition is provided by setting up a fertilizer "band" of granular fertilizer. This can be synthetic or organic, but timed-release formulas do
not release properly in the SIP environment, so don't use any timed-release fertilizer. The band is dug out and replaced each growing season. (You can use the old fertilizer from the band on your lawn, flower beds, or in-ground garden so it doesn't go to waste.)
If you use a synthetic fertilizer in the fertilizer band, you
must cover the top of the soil with plastic so rain doesn't wash the concentrated fertilizer into the soil under it or into the reservoir. The plastic cover is technically optional if you use organic fertilizer, but it also keeps out weeds and helps slow evaporation so you don't have to water as often. You can tuck the cover into the edges of the container to let rain come in at the perimeter only. That will cut down on the need to water. Just be sure that water
never washes down through
any part of the fertilizer band.
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The EB folks have a page for recommended potting mixes, dolomite, and fertilizers:
earthbox.com/approved-for-earthbox Of course can make your own mix, if you get the proportions right. But with the cost of the ingredients, it hasn't been cost-effective for me to try it.
My favorite pre-made mix is Pro-Mix BX. You'll might be able to order it at Ace Hardware or Tractor Supply, but you won't ever get it at Lowe's, Home Depot or Walmart. Also call your local nurseries and see if they can order it for you. Usually they will have the best price. When you find the best price, remember that the 3.8 cu ft compressed bale expands to about 7 cu ft when moistened and uncompressed. So if you can get the Pro-Mix for under $47 for the 3.8 cu ft compressed bale, it's actually cheaper than the 2 cu ft uncompressed bags of Miracle Grow.
Miracle-Grow mix works fine if you can't find or can't afford the Pro-Mix. Get the regular kind, since there is no point in paying extra for the Moisture Control for use in a SIP where it will always be moist. Jungle Growth is also well-regarded for use in SIPs.
Sta-Green potting mix is not on the list, as apparently the formula varies regionally. Read the bag and see if peat is the first ingredient. If not...don't get it. If it is you might try calling the company and see if you can find out the percentage. But apparently in at least some regions it contains a fair amount of compost. Also, you might look at the Lowe's reviews on this brand of potting mix. They are not encouraging!
If you still want to try the Sta-Green, I encourage you to try one bag as a side by side comparison with a bag of Pro-Mix BX, Miracle Grow, or Jungle Mix. Be sure to plant the same type of plants at the same time, with the same fertilizer, in identical containers. This would also be a good way to test any homemade potting mix that you whip up.