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I am repurposing a fire pit into a raised garden bed. From my readings, it seems like the popular soil mixes for raised garden beds include peat moss, compost, topsoil, and vermiculite. I have the first three ingredients, but I am lacking the vermiculite. However, I do have a bag of perlite laying around. Since perlite aids in aeration while peat moss aids in moisture retention, would mixing the two together cancel their respective attributes? Or would their attributes coexist to create a moist soil that drains well?

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2 Answers 2

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Perfectly fine to use both.

Everybody and his brother has a soil mix they prefer, even if it is just the bagged "topsoil" or "topsoil/compost blend" bags that one can get at the local big box stores.

I have a few different mixes that I use based on the purpose. Usually it involves a mixture of something that'll hold onto some moisture (peat, for instance), something to provide some aeration in the soil (typically vermiculite but perlite would be OK too - I buy vermiculite in large bags that hold several cubic feet as it is more economical), something that has actual nutrients in it such as compost (we've got 5 compost bins made from free pallets) or well aged horse/goat/chicken manure (I live on a small farm) and sometimes some topsoil if I happen to have it but often not. For seed starting I don't put much compost or manure in the mix for the seeds but I do add it later when the plants grow a bit.

Ultimately what you're looking for is a soil that

  • provides drainage
  • holds onto moisture
  • encourages good root growth
  • is weed-free, or nearly weed-free
  • has an average pH value that is appropriate for the plants

So with a pH of 7 being "neutral" - neither acidic nor alkaline - and many garden plants (but certainly not all) do quite well in a slightly acidic (pH values around 6.0 - 6.5 work well for many), it is important to watch how much peat ends up in the soil mix. It'll lower the pH value - the pH of peat varies but is maybe in the low 5's if I remember correctly. Some plants love a more acidic soil - blueberries, for instance, do much better in the range of maybe 4.0 - 5.0. Just wanted to add this bit about pH because the soil composition can affect how things grow (or don't) and pH is one of the considerations.

Personally I'd save the perlite for seed starting and use perlite outside. It looks better and blends better in the mixture.

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Initially, I'll be growing strawberries that I've already propagated from the mother plant. I hear strawberries like slightly acidic soil, so the peat moss would be good for it. Do you know how the math works for calculating final pH? If I mix 1/3 peat moss with 2/3 neutral soil, would the final pH be 0.33 * 4.5 + 0.66 * 7.0 = 6.1? ... I also got some cheap bags of composted steer manure from Home Depot for $1.50 each. Is it considered fully composted when it is cool to the touch? I hear if it is still warm, it can still have harmful bacteria. –  JoJo Jul 12 at 4:23
    
Trying to accurately nail the pH based on estimating the pH value of the parts is going to be somewhat inaccurate because all of the parts (the peat, the soil, the manure) are going to vary from bag to bag (assuming bags here). –  itsmatt Jul 12 at 13:00
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Also, you'll likely be "in the ballpark" on this but what I do instead is test the resulting mixture - there are different means of doing this. I've got both test strips and a meter and they generally agree pretty well. On the composted manure in bags - might bacteria in it but I think they heat the topsoil to sterilize it. They might do the same with the manure. I imagine ours has some bacteria in it but I don't get hung up on that. Soil is a living thing. –  itsmatt Jul 12 at 13:10

Perlite is used in many potting mixes that contain peat moss so I don't see a problem with mixing the two together.

Both perlite and vermiculite serve the same purpose. To help with aeration and moisture retention. Vermiculite holds water better than perlite though. For soils you want to retain moisture vermiculite would be better. In cases where you want the soil to dry out more, like for succulents, perlite would be better. Perlite also has a tendency to float up in the mix after a heavy rain.

This is an interesting video of vermiculite vs perlite. He grew bean plants in perlite and vermiculite together with compost in mixes of 100%, 50% and 0% of either perlite and vermiculite. The ones in vermiculite did better.

For a raised garden I would try to get vermiculite. It's recommended for Square Foot Gardening and the author doesn't like perlite in his soil mixes.

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+1 Pretty much what I've found. I usually use vermiculite, and find that it is also more attractive than perlite. –  J. Musser Jul 12 at 2:10
    
I just went to Home Depot and they were selling vermiculite for $20 (it looked like 1 cubic feet). It was too expensive, so I passed. If vermiculite's advantage over perlite is extra water retention, then why don't I just add more peat moss for the same effect? I got 3 cubic feet of peat moss for $12. Then I can add some perlite I already have to reduce the sogginess. –  JoJo Jul 12 at 4:18
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@jojo the $20 bag at the HD near me is 2 cubic feet if that makes a difference. Only about $3 more than the vermiculite they sell. Peat moss is usually acidic so you may need to adjust for that. –  OrganicLawnDIY Jul 12 at 6:25
    
oops. I meant only $3 more than the perlite they sell in 2 cu ft bags. –  OrganicLawnDIY Jul 12 at 22:22

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