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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.

aparente001
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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.

Bamboo
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