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My local community gardening organization gave me plans and instructions for constructing and filling a raised bed. They said to use half top soil and half compost, in layers. I foolishly did not follow the instructions, and instead used a local "garden mix" which supposedly had half top soil and half compost and also a bit of sand.

However, this mix does not drain well. After watering gently for only 2 or 3 minutes, puddles already start to form. I think that supplier made their mulch by breaking up old pallets and then composting them.

Except for the drainage problem I've been pretty happy with that mix I bought.

I propose to mix in some perlite or vermiculite or both, to improve the drainage. (In the spring I could put some compost on top as well.)

What factors should I consider when choosing between perlite and vermiculite? I like to grow lettuce, baby spinach, kohlrabi, sugar snap peas, ground cherry, tomatoes, zucchini and sticky corn.

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  • Is the raised bed open to the soil beneath, or completely separate/sealed off?
    – Bamboo
    Nov 6, 2021 at 16:48
  • @Bamboo - We put cardboard on top of closely clipped grass. In time the cardboard will degrade but by that time the grass will presumably have died. Nov 6, 2021 at 17:32
  • Ok, thanks - a bit more info please; when did you construct and fill the bed, was it this year or longer ago? What depth is the bed? And if you pick up a handful of the soil in the raised bed and squeeze it hard in your hand, does it stick together or crumble when you open your hand again?
    – Bamboo
    Nov 7, 2021 at 10:56
  • @Bamboo - Built and filled in July, 2021. Depth: 16 inches. It sticks together until I poke it a little bit with my thumb. Please note that in general the soil in my area tends towards clayey. I think the "compost" component doesn't have enough microbiological and biochemical activity going on -- I haven't seen a single earthworm. (I do plan to add some better compost in the spring.) Nov 7, 2021 at 19:15

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From the extra information given, it may be that your bed does not drain easily because it is sitting on layers of cardboard on top of turf/sod. Turf or sod does drain, but it does it more slowly than open soil; with a layer or two of cardboard on top and 16 inches of soil, this would slow it down a bit more, especially given your 'natural' garden soil has a high clay content and may not be free draining anyway.

The soil you've used does not appear to be heavy clay, since it breaks apart easily after the 'squeeze' test. Had it been heavy clay, the addition of horticultural grit would have helped a lot, but if you want to use perlite or vermiculite, then perlite is the better option to improve drainage, see here https://www.gardeningchannel.com/perlite-versus-vermiculite/. The real question is whether it needs improving by adding anything at all - as the cardboard degrades and (hopefully) the turf beneath dies off, drainage might not be a big problem, but in the meantime, it won't do any harm to add perlite, and may be helpful.

The addition of composted materials, either as mulch or dug into the soil, is always useful, though it does not necessarily make a huge difference to how well it drains initially. It should increase the worm population and improve soil fertility, but bear in mind worms will disappear deep into the soil when the weather is cold or very dry, so you will not see them in the soil at those times. Once the cardboard degrades, they will also be able to access the soil in your bed from the turf beneath over time, and will also be able burrow down deeper when necessary.

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  • Thanks, this is helpful. I don't think the cardboard is the cause, because of the almost immediate pooling after initiating light watering. The article was helpful. I think I'm going to go with vermiculite for a couple reasons: 1- I won't have to water often because the vermiculite will provide moisture unless it's a prolonged dry period; I don't grow cacti, succulents or rhododendrons, and lettuce really doesn't like to dry out; the only perlite available locally has some Miracle Gro in it which I would not like to use, but when I was shopping online it was hard to choose a specific product. Nov 11, 2021 at 4:39
  • It seems hard to choose a weight or particle size, and I saw a lot of mention of large quantities of dust. Nov 11, 2021 at 4:41
  • There's always dust - larger quantities also means more dust, with both products. If you;re sure its the soil in the bed that's the problem, I'd be inclined to add horticultural sharp grit as well as some perlite, or vermiculite if that's your preference.
    – Bamboo
    Nov 11, 2021 at 10:37

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