Q&A: Wicking beds

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Q

I was very interested in your article on wicking beds in ReNew 135. I have had good success with the Gardening Australia wicking boxes made from polystyrene broccoli boxes. These have PVC tubes filled with soil which go right down to the bottom of the water reservoir. Even if the water level is very low, they are still wicking up through the tube of soil.

My question concerns the beds you showed that had gravel in the bottom. I can’t see how the water would wick up through gravel unless the water level was right up against the geotech. If it only wicks when the water level is near the top, it kind of defeats the purpose of having a wicking bed that is self-watering!

Maybe I’m missing something here. Can you clarify this for me please?

—Rosemary

A

To the best of our knowledge it is undesirable to submerge soil in water—even though it will wick very nicely. Organic matter (such as that found in healthy soil) is food for microbes, both good and bad. When submerged in water, the balance of microbes changes from mostly healthy ones that require oxygen (aerobic), to smelly unhealthy types that don’t (anaerobic—the same goes for your compost/worm farm!).

So by refining the wicking concept, or the materials involved, we’ve found a way to provide the soil with a good balance of moisture and air for healthy soil and happy plants, but prevent organic matter from being submerged in water. You’re right to say when the water reservoir is full, the water touches the geo-textile and is sucked up into the soil via capillary action.

And yes we do use a specific kind of gravel—7 mm bluestone screenings or ¼” minus in the old tongue—for three reasons: it is inert or close enough to it so won’t decompose, therefore it will hold up the soil (!) and still have enough room between the pieces for lots of water.

The screenings themselves actually wick water upward to a certain extent, thus contacting the geotextile fabric and wetting the blanket, even after the water level has dropped below full. In our research we have observed the wicking effect up the screenings themselves to be up to 10 cm.

To round it all out, the enclosed nature of the wicking reservoir, regardless of how much or how little water is retained, creates conditions for humidity and condensation to occur (especially in warm weather when the plants most need that moisture). We don’t know if this has been studied or not, but it does explain why the plants in our wicking beds remain happy even as the water level drops below the extent to which the screenings may wick water up to the geotextile fabric. We think water, even from the lower reaches of the reservoir, condenses on the underside of the geotextile fabric, then wicks up the standard height (30-40 cm) into soil. Hence the fact that the many beds we build in this way all work so well.

Hopefully some university or more scientific types from the ATA membership will set up clear-walled wicking beds and start quantifying this phenomenon so everyone can have a better understanding of what is and isn’t going on.

—Carey Priest, Very Edible Gardens

EOFY ReNew 2017

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