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Wednesday, August 15, 2012

Operation Diagnose: The Best Soil for a Terrarium

Soil Mix by Nesting Ranch

Operation Diagnose:  This week I'll explore a variety of ailments that commonly afflict terrariums.  A reader contacted me a with a bevy of floundering terraria (with accompanying photos).  This affords a great opportunity for me to attempt to diagnose the problems and make suggestions - but I also encourage you to chime in and share suggestions or anecdotes in the comments!

Here is a brief summary of the questions from Nesting Ranch:

Question:  We could really use your advice as we're having just a dickens of a time with our terrariums. Some are open, others closed, some with ferns, some are just moss, some "wild" from the yard and some revived... none are doing quite well. Ferns are loosing leaves or turning black. Mosses are browning or yellowing. Just can't seem to pin-point why. 

Answer: Today I'd like to discuss soil mixes for terrariums.  

TL:DR Answer: I recommend a 50/50 potting soil and sand mix for terrariums.

For those of you that have some more time: read on...

If you are using a larger terrarium vessel like an aquarium, you may want to increase the ratio of soil you mix into your substrate.  This will ensure your larger plants get more nutrients as time goes on.

Nesting Ranch, it appears from your photos that you used a potting soil mixed with peat?  I use soil mixed with sand to ensure that I can water frequently and have it drain through the soil quickly.  As the environment is so small and enclosed, I do what I can to make sure roots aren't standing in overly dry soil or damp soil.  Here is my brief spiel on soil from my how to, Adding Soil Layers:
"Mix one part potting mix with one part sand to ensure proper drainage. Add distilled water to make damp but it is easier to transfer to terrarium when it is dry. Remember - you can always add more water to your final terrarium but it is difficult to remove!  You should have enough soil to fill about 1/4 to 1/3 of your terrarium container. I would recommend using about 1/2 of the portion you have set aside for your terrarium and then adding the rest after you have planted your largest terrarium plants. Add the soil by scooping spoonfuls into your paper cone and tamping down with a skewer.   Once you have layered your dirt into the terrarium you'll want to mist down the soil.  Soil will need to remain damp throughout the life of your terrarium to support your plant growth."

According to Wikipedia, "The most important property of peat is retaining moisture in soil when it is dry and yet preventing the excess of water from killing roots when it is wet."  That doesn't sound inherently bad for a terrarium, but the part about retaining moisture in the soil makes me hesitant of its application to terrarium life.  Yes, plants like moss and ferns need a lot of moisture.  However,  a healthy terrarium has an interior water cycle; water moves through the soil, evaporates into condensation on the sides of the vessel, which again spill down into the soil in a cycle.  

Has anyone had good luck using peat or sphagnum soil mix in terrariums?

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