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?
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.
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.