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How to....Create your Soil

The soil used in container gardening - and particularly in the self-watering GrowBarrels - is very special.  GrowBarrels, because they are always moist, must contain a soil that can retain water, but that also has components which will keep the soil from compacting and losing oxygen (because plants need it too).  The right amount of nutrients are found in the soil, and a good, fairly neutral pH (not too acidic, not too basic) should be achieved.  As always, sustainability is a main consideration for the Como Green Village.  Here are some things to think about as you design your own soil in future years!
GrowBarrel 2011: The Soil Choice

For the GrowBarrell 2011 program, Como Green Village worked with Outback Nursery and Landscaping.  We were really impressed with their Outback Nursery Mix because it contained many ingredients perfectly suited to GrowBarrels.  The soil was:

  • 15% regular compost (from a variety of organic materials) and some additional fertilizer, which provided a good balance of nutrients.
  • 40% Composted Pine Bark, which provided a light-weight source of nutrients.  By using this special type of compost, the soil remained relatively light (was not as dense as regular compost) and helped prevent compacting of the soil
  • 35% Sphagnum Peat Moss absorbs water and allows it to flow around - which helps maintain air in the soil.  There are currently concerns with the sustainability of sphagnum peat moss, so please see the center section for the Como Green Village's consideration of this ingredient.
  •  10% Coarse Sand - Again, helps promote drainage and the light weight nature of the soil

Sphagnum Peat Moss: The Controversy

Canadian Peat Bog

Spagnum Peat Moss is....complicated.  For our purposes, it is really the best material currently available to retain air and water in the GrowBarrels.  However, there is a great deal of controversy regarding whether it is really sustainable because it is harvested from peat bogs and is a product of carbon, similar to fossil fuels but not as ancient; it is questionable how quickly the bogs can regenerate after being harvested.  We decided to use it for this project despite the ongoing debate for several reasons:
  1. It is a fairly local product; Canada is our near neighbor, and most of the peat harvesting occurs in the southern provinces.
  2. The Canadian government regulates the sphagnum peat most harvesting industry. 
  3. Recent research indicates that peat bogs regenerate more quickly than expected.
Coconut husks, a popular alternative, are considered more sustainable because coconuts are renewable.  However, for our purposes, they come from a very long distance (Indonesia), the industry is not regulated (so we are unsure of how coconuts are grown or the working conditions), and there is no research on these topics or its effectiveness.

As always, though, SECIA's Green Village is on the lookout for more sustainable options.  In the mean time, here are some resources to help you determine whether you think sphagnum peat moss is sustainable or not.  Click on the title to see the website or Google Document.  The documents can also be downloaded for printing below in the Attachments section. 

Canadian Sphagnum Peat Moss Association
    Review regulations, research, and industry practices
    Of specific interest are their report on the peat moss
and the Code of Practice.

Sphagnum: A Keystone Genus in Habitat Restoration
    A research article exploring the ecological role of peat     bogs.

Sphagnum farming: local agricultural production of a horticultural peat substitute   
    A research article exploring the possibility of             
    sustainably farming peat

Greenhouse Gas Emissions
    An analysis of GHG emissions from peat harvesting
Resources to Help You Choose Your Soil

******The Main Soil Considerations****

Air and Water!

    Soil isn't the particles and matter in it - it's also the air between its components.  Plants need this air and space to make a sufficient root system and grow successfully.  Therefore, in container gardening it is especially important to design soil so that it does not compact, maintaining the space for the air and water.  Here is a suggested recipe and some tips that Ed Smith (author of Incredible Begetables from Self-Watering Containers) provides for designing your own soil:

Suggested Recipe:
  1. One bag (about 20 quarts) mature compost
  2. One bag (about 20 quarts) potting mix
  3. Optional nutrient
    sources: 1/3 cup blood meal (nitrogen source), 1/3 cup colloidal phosphate (phosphorus source), 1/3 cup greensand (for potassium and trace elements), and 1 tablespoon azomite.

Pictures: Greensand (upper right), collodial phosphate (left), blood meal (lower right)

Tips for Designing your own Soil:
  • Avoid using 100% top soil or compost - these are too dense in their pure form and will compact easily
  • Look for soil labeled potting mix/soil, starting mix, transplanting mix, or container mix/soil.
  • Aim for a mix that contains sphagnum peat for optimal air/water retention, but sedge peat, coco peat/coir, and bark/sawdust will also retain air and water.
  • Vermiculite, perlite, and limestone will help neutralize the acidity of sphagnum peat.
  • If your mix does not contain these additives, they can also be purchased separately and mixed in by hand.  Check with your local nursery or garden center for advise!
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