Hi there folks, there are many ways to build SIP's, aka wickingbeds and the following shows how I like to build them after having build about a dozen so far (for non cannabis related horticulture) and having learned from various mistakes made along the way.
First off a quick list of things I used:
1 bucket (15liters in this case)
about a 55cm section of 80mm drainage pipe
about a 40cm section of 40mm poly to use as fill tube
36x70cm wide section of geotextile
10 cm or so of 16mm poly + a 90 degree corner piece
nine 290mmx3.6mm zip ties
The reason I like to use drainage pipe wrapped in geotextile as my reservoir is simple, this holds by far the largest volume of water. I've seen plenty of builds where people use rocks to build their reservoir but rocks barely hold any water and are needlessly heavy as well. Rocks also don't have much of a capillary capacity to wick water upwards.
Wrapping the drainage pipe in geotextile prevents sand or soil from making it's way in there which means your bed will last much longer before needing maintenance.
As a fill pipe I like to use 40 or 50mm thick pipe, anything smaller becomes a pain in the ass when you have to top up your reservoir, on the bottom I always make some slots so the water can flow into the reservoir freely.
After you've jammed your drainage pipe reservoir and fill pipe in the bucket you can make the hole (16mm) for the overflow pipe. An overflow pipe isn't strictly necessary but this bucket will most likely find it's way to my indoor groom as well and having a proper overflow makes life much easier in that scenario.
I like to tie my overflow directly into my water reservoir because this way it is virtually impossible for the overflow to become blocked. An issue I've seen happen with people that just stick a pipe through the side which they covered with weed fabric/geotextile or shadecloth on the inside and over time a biofilm build up on this which prevented the overflow from freely draining excess water out of the system which then results into your system turning into a swamp
To do this I first make a tiny cut in the geotextile and then stretch the hole till it's big enough to get my overflow pipe in. I also cut a hole into the drainage pipe, this one doesn't have to be pretty or tight fitting.
The geotextile is strapped to the overflow pipe with a zip-tie, again preventing sand from being able to enter the reservoir pipe.
Ideally you would put the overflow on the opposite side of the fill pipe but considering this bucket is likely to be used indoors at some point it made more sense to have them on the same side.
The wick, this layer brings the water from the reservoir up to the soil layer and because the soil layer itself isn't sitting directly in the water the soil layer can't become more saturated than the wicking layer used below it (provided the mix has adequate drainage). A good analogy would be bain marie cooking, the flame heats the water and makes it boil, then the food in the container that is sitting inside the boiling water can never get hotter than the boiling water itself.
A good wick is course builders sand and using just that would be perfectly fine. However to increase the water holding capacity of the whole bucket I like to mix raw untreated biochar pellets into my wick layer and in this particular build I went with 2/3 biochar pellets and 1/3 coarse builders sand. The biochar being raw also helps keep the reservoir cleaner for a longer period because it will adsorb any nutrients that might leach down into the reservoir when it rains or if you are doing some watering from above after topdressing some amendments.
Always make sure your wick layer is at least an inch higher than your highest water level.
Finally the soil mix itself, it is very important that this mix allows proper aeration and drainage, if you would use just compost here and you have worms in there as well (which should be there in my opinion) then the worms will eat all your compost and turn your whole bucket into vermicompost sludge which isn't so great if you want to use your bucket for multiple rounds without using new soil all the time, we are in the organic growers section after all so we are aiming to build a living soil here.
Because I want this bucket to be a long term thing I went with a bonsai mix as my main ingredient because it has excellent drainage and various types of stone for long term mineral release.
3 little buckets of bonsai mix
2 little buckets of my own proprietary soil blend consisting of 25% biochar that has been soaking in biodigester effluent, 25% finely ground peat, 25% homemade vermicompost and 25% woody material from the inside of a local tree here
1 little bucket of coarse builders sand
a couple tea spoons of ground up (<1mm) oyster shell and lava rock
Then one additional little bucket of my proprietary blend on the top of the soil and leaving a bit of space for a mulch layer.
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