Clinker cinders, also called scoria or lava rock, is great for the base layer of a wicking bed.
I have been interested in aquaponics and hydroponics for a long time, and they are the reasons I built the nice personal greenhouse that I built at my home. Our growing season is fairly short here on the Southern Colorado River Plateau of Arizona, and winters can be bitter, bitter cold, so the greenhouse made sense for that reason as well. I spent years researching various methods and setups for both growing styles and one doesn't spend much time researching hydroponics or aquaponics without learning about a wicking bed sooner or later.
Popularized by Murray Hallam of Australia, the wicking bed works much like an oversized self-watering pot. The water, whether from the fish tanks as in aquaponics, or nutrient water, like in a hydroponics setup, sits under the growing medium and wicks itself upwards, keeping the growing medium moist, but not soggy, and growing the plants in the top.
The beds are very often fashioned out of large fish tanks, vinyl-lined wooden grow beds, or even old stock tanks, just something that will not leak and is at least 24 inches deep or so. A fill-pipe, usually like a 1.5-inch or 2-inch PVC pipe, is run down from the top of the bed to the bottom. Some designs have notches cut in the bottom end of the PVC to allow water to flow down, some have holes. Some designs, like the one I found, uses an elbow and a bottom pipe that runs the width or length of the bed, just for filling ease and to help the pipe stand upright.
The beds are filled to a level (at least 8”, but could be more) with some sort of very coarse scoria (also called lava rock or clinker cinders if you get them from a landscaping company, see photo above), then covered with a sheet of landscaping fabric or fine shade cloth, so moisture can wick through, but it will hold the planting media from going down into the water. An overflow pipe should exit the wicking bed just about the level of this landscape fabric, so that water doesn’t accumulate at the planting media level and leave everything soggy. The planting media, in my case a mixture of peat moss, good quality homemade compost, and some store-bought organic potting soil, is laid on top of the landscape fabric, at least 12 to 16 inches deep. The water then wicks up through the landscape fabric into the planting medium, supposedly keeping it moist. (See terrible drawing that I made, below)
As I said, the wicking bed seems to have gotten its popular start in Australia, and although some parts of the land Down Under are very jungle-y, some is as arid as my part of Arizona, so I thought I’d give it a try here. The first thing I did was research online (why reinvent the wheel, right?) and then I hit up all my hydroponics, aquaponics, and Kratky groups on Facebook, Instagram, and TikTok, and I learned there are no gardeners in those particular groups who have either done what I was attempting, or none that wanted to share, so here’s what I came up with.
**It is important to note at this point that I do understand not everyone has the same access to the same materials for the same prices. For me, this entire build was completed for the cost of a couple hinges, some 2 ½-inch wood screws, and a couple 2-by-4s. It cost be about $20 to make this entire thing, given MY particular situation and access to materials. For someone else who doesn’t have access to hundreds of linear feet of recycled lumber, free IBC containers from a nearby shampoo factory, basically unlimited amounts of homemade compost, and buckets and buckets of PVC fittings, bulkhead fittings, hinges, and hardware, this could cost several hundreds of dollars to replicate.**
I used two 275-gallon IBC containers, like this:
I cut off the top 14 inches, for the lid. IBC containers come with Aluminum cages. I cut the cages with an angle grinder, and cut the plastic container, also called a tote, with the jigsaw. My son is helping me cut this one.
I drilled a hole in the side of each tote, about 24 inches from the bottom, for the overflow pipe. I used 3/4" bulkhead fittings that I had laying around from a previous aquaponics project that never worked well. They are about $6 to 7 each on Amazon, if you buy them new.
After I had the cages and IBC totes cut, and the drain pipe hole drilled, I laid them out where I wanted them in the yard:
And then I used old 2x12 boards, from old raised beds, and made a nice cover to keep light away from the water reservoirs and make it look nicer:
I put the fill-pipe, which I used 2-inch PVC and an elbow, next. I did drill 1/2-inch holes in the bottom pipe to help water flow out in all directions.
I put the lids on before filling the beds, so I'd know how everything was going to fit and I could be ready to cover them as soon as I had soil mix in.
Cleaning out all the dirt and plastic shavings is important at this point.
Then I put in about 24 inches of coarse cinder. It is available for $20 a ton from my local landscape company. I understand that you can get small bags of it from big box home improvement stores, but I needed about a cubic yard for the two bins, so there was no way I was going to buy it in tiny bags.
After the cinder was in, I covered it with an old shade screen I had from a dog pen cover. I did wash it carefully and dry it in the sun, but it was HUGE, and it was laying around, so it worked. I know big box home improvement stores carry this, too, but they also carry landscape fabric and all sorts of other things that would work. I just happened to have this, so I used it. I had to cut a hole in the shade cloth, so it would fit over the top of the fill-pip and be snug around the cinder.
My little grandson decided he needed to try out the cinder layer before we put soil in. Such a good helper.
Then I filled with a mix of quality homemade compost, peat moss, store-bought potting soil, and a little soil from my garden piles.
Once I got the soil mix in, I went ahead and planted the beds. As of the writing on November 15, I have spinach and beets coming up in the raised beds!
Here are some more pictures of the build:
Regina Hitchock is a high school biology teacher in St. Johns, Arizona, where she co-founded the Gardeners with Altitude organic garden club and brings gardening, aquaponics, aeroponics, hydroponics, and seed starting into her classrooms. She serves as Secretary for White Mountain Community Cooperative to promote food- and economically secure self-sufficiency in Arizona. Connect with Regina on Facebook, and read all of her MOTHER EARTH NEWS posts here.
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