My wicking bed experiments
2-2016 by GeorgeSchmid
I assume most members are familiar with the concept of wb and self-watering planters (swp) by now and again like all things gardening there are many different ways to produce the same outcome.
My definition of a wb is a container 200 litres or larger, may be a bathtub, fixed in one location, of a swp is usually a pot 10 L to 100 L in size, still mobile and can be moved according to season.
I like to start with my thoughts on choosing the tankfill material.
In the table below it is indicated how much water you approximately could drain out of a 10 litre tank filled with different materials.
Coal Ash, graded 5 litres
Hardwood chips 6 litres
5mm gravel 4 litres
Bedding sand 2 litres
Red Scoria 4 litres
Often plastic pipes and containers are also located in the tank area to increase water storage. So it is a matter of what is at hand or what is locally available. Commercial wb and swp have usually no tankfill but a nicely fitting slotted plastic bottom fitted with soil filled legs extending into the water tank to wick water.
Myself I used hardwood chips and coal ash with success in my six swp and wb. I prefer coal ash because of the price.
The depth of the tank in a wb may be around 150mm. This would store as much water as you would get from 80 mm of precipitation.
The depth of the tank in a swp is around 30 to 60mm
Incorporated in the tank is the fill pipe and overflow but this was illustrated and explained in previous articles.
In my first wb I covered the tankfill with geofabric. At that time I used 75% soil
25% compost as a growing medium. After two successful growing seasons I noticed that the wb was not draining freely any more. Investigation showed that clay particles from the soil collected on geofabric and stopped water draining through it.
So that’s where my learning curve about wb started.
At the same time I also used two swp filled with commercial potting mix using geofabric and this two planters draining well, up to now.
Why is it so…?
Geofabric works well with soil-less potting mixes and “soils” sold by landscape suppliers as all this products contain no soil, no soil no clay!
If you using garden soil in wb use a fabric with a more open weave such as shade cloth or plastic mesh to separate tank fill and soil. In swp insect screen will work as well. The small amount of clay will settle at the bottom of the tank and will not affect the function of the wb or swp.
All pot plants, container gardens, wb and swp benefit from having the potting mix or soils renewed or replaced every 2 to 4 years. The fabric or mesh is of much help when soil is changed in wb as it reduces the chance of disturbing the tankfill. Tankfills usually need not to be disturbed during soil change.
As indicated earlier I used a garden soil and compost mix in my first wb.
Initially the soil was loose and friable but after a period of months it started to become hard and I noticed considerable slump of the soil in the bathtub.
Subsequently, I started to experiment with different mixes for my containers. Some helpful information I got from the internet.
A very good description of components used in potting mixes on this site.
I try to substitute vermiculite with coal ash and use considerable amounts of composted forest mulch in my mixes.
I stay clear of perlite, vermiculite and peat as these are a long way from being sustainable.
At present I am using some soil-less mixes and some mixes including up 25% garden soil in my planters.
The section on porosity is worth reading as well.
“soil with good tilth has large pore spaces for air infiltration and water movement. Roots only grow where the soil tilth allows for adequate levels of soil oxygen.”
Also keep in mind the difference between a potting mix and a wb mix.
The potting mix is made to hold and store water the wb mix supports wicking of water.
This is definitely all work in progress for me…..
As many variations on wicking beds, bins and pots as there are people wanting to grow plants in them! They can be really complex (see Costa's ones on TV!) or very simple without extra hardware. They all seem to work! I've had a couple with stinky reservoirs out of the 2-3 dozens I have; I find most seem to dry out right down into the reservoir if they are not topped up enough. Weird but drainage is not really a problem. Probably an open mix plus the worms and there seems to be enough oxygen filtering through for the plants to grow well. Keeping the mix topped up as the organic matter compresses or is used and keeping the fertiliser up to the plants is the biggest challenge for me. I've only used a blanket in pots which were made that way by someone else. Either with nor without a blanket and the pots still work and the plants still grow!