The water is wicked up from the reservoir. It is important that a soil mix is used that is efficient as a wick. With the right soil the roots will grow where the soil contains both air and moisture, and of course fertilizer. Al

Peacey, it's a physics thing. The soil itself has the ability to move the water upward from particle to particle. As Al noted, the type of soil mix used is important. Some do better than others at "wicking" the water upward.

The top inch or so does not need to be dry, though it can be. Again, it depends on the soil mix used.

The roots will eventually often reach the reservoir, but they do not need to as the soil is watered by itself, so to speak.

Does this make sense? Sometimes it's hard to remember all the things you didn't know before you knew, lol.

Hi Al and lathyrus, thanks for the info! It does make much more sense now lathyrus (to my scientifically deprived brain anyway).

What kind of soil would you recommend then? Would an organic potting mix be any good, as it is part-compost as well as soil? What should I stay away from?

It's a bit complicated, as science stuff tends to be. There are several physics concepts involved beyond its ability to wick upwards.

If made up of very small particles, it has a tendency to "hold" water between particles, which keeps it quite wet and doesn't allow for oxygen to reach the roots.

So, too large of particles, it's too dry. Too small, it holds too much water and is too wet. Compost tends to be too small.

There's also the issue of how fast the particles break down and if they break down. Granite, for example, breaks down very, very slowly, so it will stay the same size for a long time. Peat, on the other hand, starts to break down quickly.

Many people have success using a traditional bagged mix. Many of us start there. Some are happy there. Others of us start questioning how wet it is and how quickly it breaks down into something akin to sludge, rather than soil! I had problems because I'd grown in ground first and realized my plants weren't nearly as productive in the SWC using the mix I was using.

I came here, among other places, and started learning and eventually changed what I do. I learned that I could maximize the soil mix and get much better results than I was. If I'd been happy with the results, I never would have done that.

The bad news is there isn't really anything to buy off the shelf if you want to maximize everything. It's not a lot of work, but it's a LOT more work than going to a big box store and buying one bag. For me it's meant many trips to find the right components, storing it somewhere (I'm in a condo), and trying multiple things to get it to the point it worked well for me.

I guess the short answer is, it depends. How much do you want to learn? How much time do you want to spend? Are you the type that is just happy doing it or are you the type that needs to know as much as you can? All of things, and probably more, are part of the decision.

We can direct you to relatively easy, don't have to know much solutions or solutions where you need to learn a bit more and make your own decisions based on what you learn.

Hope that didn't complicate things too much...

Some of us use a soil mix we make our self and it would be easier for you to buy a mix available everywhere and proven to wick well. Look at the posts here from Raybo who has vast SWC experience and will give exact directions. Al