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
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.