First wicking bed experiment results

20/3/2020

The preliminary results from my first wicking bed experiment are in. I have more analysis to do to understand the significance of these results and I have another experiment under way that will provide more data. So I don't regard this as the final answer as to what's the best way to construct a wicking bed but it does provide some insights.

This experiment tested four reservoir media: washed sand, 10mm gravel, WaterUps, and a mix of cocopeat, compost and sand. Two growing media were used

Cocopeat Mix: a mix of 60% cocopeat, 30% compost and 10% sand plus slow release fertiliser
Potting Mix: Scott's Osmocote Professional Premium Potting Mix.

Each wicking bed was set up with a reservoir media at the bottom. Geotextile was used to cover the gravel and sand reservoirs. Growing growing medium was placed on top.

The Potting Mix was only used with a sand-filled reservoir. The Cocopeat Mix was used with all four reservoir materials. Three replicates of each combination were used.

Spinach seedlings were planted on 17/10/19 and plants were harvested and weighed on 17/1/20. The water reservoirs were full at time of planting, topped up as needed through the experiment and refilled after harvest. The total amount of water added to each bed was recorded.

The average plant weight per bed is shown in the following graph. The best growth was from the bed completely filled with Cocopeat Mix. The beds with a sand reservoir and Cocopeat Mix growing medium (labelled sand.cp) grew slightly less. The beds with the Potting Mix (sand.pm) grew the least. A wicking bed with a gravel reservoir and Cocopeat Mix growing medium was the worst performing of the beds for plant growth.

The amount of water used was pretty much in line with the amount of growth, except for the Potting Mix wicking beds. The following graphs show the average amount of water used per bed and the correlation between the plant weight and the water used.

Based on these results, the simple design using the Cocopeat Mix for both the reservoir and growing layers is best.

I have some concerns about the long-term performance of this design since I have found in other wicking beds that I have built that potting mix/compost in the reservoir decomposes anaerobically and deteriorates into a black sludge. I have had better long-term results using a 50:50 mix of garden soil and compost through the whole wicking bed but I haven't done a controlled experiment to test this yet.

Washed sand in the reservoir layer appears to be a good choice and sand would not suffer from decomposition.

I am not drawing any conclusions on the WaterUps product from this experiment. I filled the hollow legs of the WaterUps with coarse perlite and have subsequently found that this does not wick as well as perlite with smaller particles. In my next experiment I have replaced the perlite in the WaterUps with the cocopeat mix that I know does wick well.

As I hypothesised before the start of this experiment, 10mm gravel did not perform well as a reservoir material. Gravel does not have good wicking properties and even though the reservoir was 200mm deep, only part of the water was able to wick up into the growing medium.

I currently have the same wicking beds planted with lettuce and am recording more detailed measurements of soil moisture at various depths that should allow me to draw more certain conclusions about the better materials for wicking beds. I'll report on those results in the future. At the moment the lettuce growth is spectacular.

15 Comments

Kate Laffer

26/9/2020 12:35:29 pm

What about crusher dust as the reservoir medium? Your earlier tests showed it had the best wicking ability. Any reason for not including it in your vege growing tests?

Chris Curtis

26/9/2020 12:59:34 pm

Hi Kate,
Thanks for your comment. Crusher dust seems to wick really well but holds a bit less water than sand. I was limited in the number of variations I could try. I was doing three replicates of each design so adding more increased the numbers greatly, so I thought I'd limit this round of experiments to what was commonly used.

My results so far lead me to think that using crusher dust as wicks into a reservoir that has lots of void space to fill with water (using upside down pots or ag pipe or similar) might be the best option. More experiments are planned to test this.

I'm in the final stages of writing my thesis so I'll publish more results soon.

Chris

Darby Bicheno

6/10/2020 12:10:27 pm

Hi there, Thank you so much for performing and publishing this information. I'm in the process of building my wicking beds, and it's great to see some definitive numbers on what media to use in the reservoir.

Based on your analysis, I will likely fill my reservoir with sand, and use a variety of different growth media, likely garden soil mixed with compost. I'd love to use coco peat, however my research leads me to believe that it is not such an ethical product. Certainly great for limited use in a nursery, but filling whole garden beds with it seems excessive.

Thanks again,
Darby

Chris Curtis

6/10/2020 12:32:30 pm

Hi Darby, Thanks for your comments. With sand in the reservoir I think there is a big advantage to also having something in there that makes empty spaces that fill with water - eg. ag pipe, upturned pots. Perhaps cover then with geotextile so they don't fill with sand, but it is important to have some areas of sand that go down between the spacers to the bottom of the wicking bed. This gives you more water storage in the reservoir but I have some evidence that it also improves the wicking (more experiments are about to start to prove this). Garden soil and compost should be fine. That's what I have used for years. I only used cocopeat in these experiments because I wanted something standardised and repeatable - there is soo much variation in soils. I'm doing the final edits on my honours thesis now so I should be publishing more results soon. Cheers! Chris

Darby Bicheno

6/10/2020 01:14:57 pm

Hey thanks for the reply!

Yes, I forgot to mention that I will include agi-pipe and geotextile as a barrier. I do wonder though about how the water moves up from the sand and into the soil through this material.

Chris Curtis

6/10/2020 01:41:28 pm

I've done an experiment comparing beds with and without geotextile between the reservoir and the soil and found it makes no difference to water movement within the wicking bed. Roots do go through the geotextile into the sand. The gaps in the sand are so fine that I don't think that the soil would mix with the sand (and even if it does it wouldn't be a problem). I'd put geotextile over the ag pipe to keep sand out of it, then level the top of the sand and put the soil directly on top of the sand. Cheers! Chris

Darby Bicheno

15/10/2020 10:00:43 am

Hi again Chris, Do you have any thoughts on type of sand to use? I'm hoping to use a construction sand, so I can order enough to use later on in a cob pizza oven.

Thanks!

Chris Curtis

15/10/2020 11:19:58 am

Darby, I've used what was sold as "washed sand". It had a variety of particle sizes. I also did capillary rise tests on sand sold as "river sand" which looked to have roughly the same range of particle sizes but was a different colour. The river sand didn't wick quite as well as the washed sand but would have been OK. I haven't tried bricks sand which has much more uniform particle sizes. Chris.

Phil

1/12/2020 11:28:43 am

Hello Roogulli,
Firstly, thanks for undertaking and publishing your research. It has helped me out a heap. Love the charts. Keep up the great work.
After sitting through your research, I've concluded to go with either a sand reservoir layer, or growing medium all the way through. Otherwise, it's IBC design to your standard. The only downside to the growing medium for all, so far as I can read, is a slightly lesser wicking ability and the potential for it to decompose anaerobically. I can manage the aerobic side of things, I hope. However, I think if I don't get the soil just right, it'll perform poorly.
So for that reason perhaps the sand layer is best. It seems more reliable. Assured wicking. Purer reservoir water. I'd like more soil for roots, but your plant weight graph showed similar performance across both methods.
I'm leaning towards sand...
But I want more soil!
I'm torn between the two!
If you would weigh in on this decision, that would be great. My soil mix so far is a lot of alpaca manure, cow manure, biochar and a heap of microbial goodies. Soil and the rest I'll have to buy. What might I add to ensure good wicking performance?
And what you think of sand vs growing medium reservoirs? Any advice would be greatly appreciated.
Thank you

Chris

1/12/2020 10:07:13 pm

Hi Phil,

Thanks for your comments. I think it's pretty clear that gavel is only good for reservoirs 50-100mm deep - sand or soil is a much better option. I've dug out 5 year old wicking beds that had 50:50 soil:compost and the soil in the bottom was beautifully structured. But in beds filled with a mix of potting mix and compost the bottom layer turned to sludge in a year or so. Have you seen the Waterups product (https://www.waterups.com.au)? In an experiment I'll post soon, I trialled these alongside gravel, sand and cocopeat filled reservoirs. The waterups with the wicking 'legs' filled with sand maintained a higher soil moisture than sand alone. Having sand columns surrounded by free water wicked better than just sand. So I think it's good to have something that makes voids within the sand reservoir. Waterups have about 12% of their surface area as wicks. I've made some beds with sand columns (in 100mm ag pipe) covering about 30% of the area. It's early days in this experiment but these are maintaining higher soil moisture than any other reservoir. There is much more to learn about wicking in growing mediums. From what I've seen, it is better with plenty of fine material (soil, sand or fine compost). I found that straight potting mix wicks very poorly. I've got more experiment results to back all this up that I will publish soon. I'm just waiting for the university to get around to assessing my Honours thesis before I make it public.

Cheers!
Chris

Phil

3/12/2020 06:01:50 pm

That's all great. Thanks for answering my questions. I'll be sure to follow your soil recommendations. Beautiful structure 5 years later is good to hear. I will aim to achieve this.

But now I shall contend with this sand tower idea. I've seen the waterups but I already have the 100mm ag pipe. Sand towers in a water reservoir.... I shall brainstorm!
Much more food for thought.

Thanks heaps again. I look forward to your thesis.

Kate Bartlett

22/2/2021 06:08:16 pm

Hi, thanks for publishing all this really useful research! I'm planning on making some wicking beds to grow veg very soon, but I'm currently making a wicking worm bed, having seen a video on Youtube about it, which says that worms survive the summer a lot better in hot climates using this technology (I live In Southern Spain and we often have days above 40C in July and August). But I don't have access to large amounts of sand, other than builders' sand, and I'm not sure about using it anyway. The medium should be pH neutral so as not to affect the worms. The video uses shale but I can't get that easily either. I was wondering about using vermiculite, as it's light, sterile, inert and neutral and it retains water well. What do you think its wicking capabilities might be? Another consideration is that we want to avoid anaerobic activity in the wicking layer, so that any nutrients from worm castings that are washed through to the bottom layer come out of the overflow as worm tea. So what would you advise in my case? Any other suggestions for the wicking layer or do you think vermiculite might work well?

Chris

22/2/2021 10:06:23 pm

Hi Kate,

I haven't used vermiculite but I have found that fine grade perlite wicks well so I would assume that vermiculite would work too - but get the smallest particle size that you can.

The only problems I have had with anaerobic decomposition was when I used a combination of potting mix and mushroom compost for both the growing and reservoir layers in a wicking bed. After a couple of years the media in the bottom had decomposed into a structureless mush. But I have also used a soil/compost mix in the whole depth of a wicking bed and have had no problems with the soil in the bottom that was continually saturated.

Kate Bartlett

23/2/2021 09:38:09 pm

Thanks so much, that's really helpful. One more question - when you use ag pipe, do you completely fill the bottom of the bed with it or just run one line along the bottom? Is you ag pipe attached to the unslotted pipe where the water goes in or separate from it? I've seen it done in so many different ways, I'm wondering whether that part of the process really makes much difference.

Chris Curtis

5/3/2021 11:49:41 am

Hi Kate,
Sorry I didn't see your comment earlier. I'd use as much slotted ag pipe in the base as you can, but leave some gaps so some of the wicking medium can go to the bottom of the reservoir. The reservoir will fill faster if some of the ag pipe is connected to your fill pipe, but the bits that aren't will still fill with water that goes through your reservoir medium.

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