Outside garden beds for Cacti are a real challenge for some people depending what part of the world you live in. And a medium that works for one person, might not work as well for another. What species of cactus, or even different forms your growing has certain factors to consider also. So it's good to know as much knowledge about climate, soil, and habitat of each cactus your trying to grow. So we take into account all known factors, and the climate your trying to grow them in. Then we amend soils, build hills, and rockeries to manipulate things to best suit them for there survival. There are still limits though, to the point of building top covers, or even complete temporary structures, and still there are some we just can't grow outside.
So to start with, to get things ready for what we can grow, you want to figure out which growing Zone and climate your in. Then you can get started on what will work the best for you.
First we'll talk about "OXYGEN" in the growing medium. This is one of the most important aspects to keep in mind. And dealing with that aspect always remember, that fresh built beds, even with the best mediums, eventually collapse on them selves. What I mean is, as a soil medium ages, it condenses, getting tighter and finally filling in all the gaps. In term cutting off the oxygen to the roots. So in time, you may have to amend or fluff up your cactus beds again. Nothing is forever, but I'll try to tell you how to prolong your beds as long as you can.
When building a cactus bed I take into account what I've observed over the years, where cacti grow. Most of the time in higher moisture areas, there are some type of fast draining soil, consisting of high amounts of gravel, and sand. Or hanging off ledges, or cliffs in rockeries or crevices. So most of us, except in the real arid hot zones, need to duplicate something close to this effect.
Raised, or hilled beds are the best, which are usually built above the existing soil you have. I recommend at least 6 inches of pea gravel, and sand mix. Better yet is putting a layer of about 6 inches of sandy loam down first, then adding the gravel sand mix over it. Depending on the amount of moisture you have, will give you the amounts of pea gravel, and sand in combination to use. High moisture areas I would recommend 75 percent pea gravel, and 25 percent sand. Where in my drier climate here, I use about a 50 50 mix.
The next thing I do is to take some fine powered mineral clay, and dust it around real lightly on the top surface. But sometimes this is not needed because some gravel, or sand will have a certain amount of this dirty clay in it already. This clay supplies critical mineral nutrients to your plants.
Now you can start laying rocks and deciding where to place your plants.
One other thing, do not put peat or any other organic material in your mix. And try to keep any dead plant material, or leaves cleaned up also. You can still use leaves or straw for added insulation in winter, but get it cleaned up in the spring. Cause any of these organic compounds braking down, are the fastest way to clog up and condense your soil mix. In term cutting off your valuable oxygen to your healthy roots. Plus these organic bases sometimes are the fastest way to spread fungus, and bacteria.
On a last note, there is one other component that will fight you all the time, and there's not much you can do about it. All depends on what mother nature throws at you. Wind blown silt, or air dust. A little is alright, but with time, this is another factor that will eventually catch up on you and clog your medium. But this usually takes years, and then it is time to amend and do it all over again.
Long Term, and Short Term